diff --git a/pkg/front_end/lib/src/type_inference/inference_visitor.dart b/pkg/front_end/lib/src/type_inference/inference_visitor.dart
index 4fc0b16c997..09af5a8a16a 100644
--- a/pkg/front_end/lib/src/type_inference/inference_visitor.dart
+++ b/pkg/front_end/lib/src/type_inference/inference_visitor.dart
@@ -15004,77 +15004,70 @@ class InferenceVisitorImpl extends InferenceVisitorBase
       requiredType: requiredType,
     );
 
-    DartType lookupType;
     if (node.needsCheck) {
-      lookupType = node.lookupType = requiredType;
+      node.lookupType = requiredType;
     } else {
-      lookupType = node.lookupType = matchedValueType;
+      node.lookupType = matchedValueType;
     }
 
     ObjectAccessTarget lengthTarget = findInterfaceMember(
-      lookupType,
+      requiredType,
       lengthName,
       node.fileOffset,
       includeExtensionMethods: true,
       isSetter: false,
     );
-    DartType lengthType;
-    if (lengthTarget.isNever) {
-      lengthType = const NeverType.nonNullable();
-      node.isNeverPattern = true;
-    } else {
-      assert(lengthTarget.isInstanceMember);
+    assert(lengthTarget.isInstanceMember);
 
-      lengthType = node.lengthType = lengthTarget.getGetterType(this);
-      node.lengthTarget = lengthTarget.classMember!;
+    DartType lengthType = node.lengthType = lengthTarget.getGetterType(this);
+    node.lengthTarget = lengthTarget.classMember!;
 
-      ObjectAccessTarget sublistInvokeTarget = findInterfaceMember(
-        lookupType,
-        sublistName,
-        node.fileOffset,
-        includeExtensionMethods: true,
-        isSetter: false,
-      );
-      assert(sublistInvokeTarget.isInstanceMember);
+    ObjectAccessTarget sublistInvokeTarget = findInterfaceMember(
+      requiredType,
+      sublistName,
+      node.fileOffset,
+      includeExtensionMethods: true,
+      isSetter: false,
+    );
+    assert(sublistInvokeTarget.isInstanceMember);
 
-      node.sublistTarget = sublistInvokeTarget.classMember as Procedure;
-      node.sublistType = sublistInvokeTarget
-          .getFunctionType(this)
-          .sublistFunctionType;
+    node.sublistTarget = sublistInvokeTarget.classMember as Procedure;
+    node.sublistType = sublistInvokeTarget
+        .getFunctionType(this)
+        .sublistFunctionType;
 
-      ObjectAccessTarget minusTarget = findInterfaceMember(
+    ObjectAccessTarget minusTarget = findInterfaceMember(
+      lengthType,
+      minusName,
+      node.fileOffset,
+      includeExtensionMethods: true,
+      isSetter: false,
+    );
+    assert(minusTarget.isInstanceMember);
+    assert(minusTarget.isSpecialCasedBinaryOperator(this));
+
+    node.minusTarget = minusTarget.classMember as Procedure;
+    node.minusType = replaceReturnType(
+      minusTarget.getFunctionType(this).minusFunctionType,
+      typeSchemaEnvironment.getTypeOfSpecialCasedBinaryOperator(
         lengthType,
-        minusName,
-        node.fileOffset,
-        includeExtensionMethods: true,
-        isSetter: false,
-      );
-      assert(minusTarget.isInstanceMember);
-      assert(minusTarget.isSpecialCasedBinaryOperator(this));
+        coreTypes.intNonNullableRawType,
+      ),
+    );
 
-      node.minusTarget = minusTarget.classMember as Procedure;
-      node.minusType = replaceReturnType(
-        minusTarget.getFunctionType(this).minusFunctionType,
-        typeSchemaEnvironment.getTypeOfSpecialCasedBinaryOperator(
-          lengthType,
-          coreTypes.intNonNullableRawType,
-        ),
-      );
+    ObjectAccessTarget indexGetTarget = findInterfaceMember(
+      requiredType,
+      indexGetName,
+      node.fileOffset,
+      includeExtensionMethods: true,
+      isSetter: false,
+    );
+    assert(indexGetTarget.isInstanceMember);
 
-      ObjectAccessTarget indexGetTarget = findInterfaceMember(
-        lookupType,
-        indexGetName,
-        node.fileOffset,
-        includeExtensionMethods: true,
-        isSetter: false,
-      );
-      assert(indexGetTarget.isInstanceMember);
-
-      node.indexGetTarget = indexGetTarget.classMember as Procedure;
-      node.indexGetType = indexGetTarget
-          .getFunctionType(this)
-          .indexGetFunctionType;
-    }
+    node.indexGetTarget = indexGetTarget.classMember as Procedure;
+    node.indexGetType = indexGetTarget
+        .getFunctionType(this)
+        .indexGetFunctionType;
 
     for (Pattern pattern in node.patterns) {
       if (pattern is RestPattern) {
@@ -15083,52 +15076,50 @@ class InferenceVisitorImpl extends InferenceVisitorBase
       }
     }
 
-    if (!node.isNeverPattern) {
-      if (node.hasRestPattern) {
-        ObjectAccessTarget greaterThanOrEqualTarget = findInterfaceMember(
-          lengthType,
-          greaterThanOrEqualsName,
-          node.fileOffset,
-          includeExtensionMethods: true,
-          isSetter: false,
-        );
-        assert(greaterThanOrEqualTarget.isInstanceMember);
+    if (node.hasRestPattern) {
+      ObjectAccessTarget greaterThanOrEqualTarget = findInterfaceMember(
+        lengthType,
+        greaterThanOrEqualsName,
+        node.fileOffset,
+        includeExtensionMethods: true,
+        isSetter: false,
+      );
+      assert(greaterThanOrEqualTarget.isInstanceMember);
 
-        node.lengthCheckTarget =
-            greaterThanOrEqualTarget.classMember as Procedure;
-        node.lengthCheckType = greaterThanOrEqualTarget
-            .getFunctionType(this)
-            .greaterThanOrEqualsFunctionType;
-      } else if (node.patterns.isEmpty) {
-        ObjectAccessTarget lessThanOrEqualsInvokeTarget = findInterfaceMember(
-          lengthType,
-          lessThanOrEqualsName,
-          node.fileOffset,
-          includeExtensionMethods: true,
-          isSetter: false,
-        );
-        assert(lessThanOrEqualsInvokeTarget.isInstanceMember);
+      node.lengthCheckTarget =
+          greaterThanOrEqualTarget.classMember as Procedure;
+      node.lengthCheckType = greaterThanOrEqualTarget
+          .getFunctionType(this)
+          .greaterThanOrEqualsFunctionType;
+    } else if (node.patterns.isEmpty) {
+      ObjectAccessTarget lessThanOrEqualsInvokeTarget = findInterfaceMember(
+        lengthType,
+        lessThanOrEqualsName,
+        node.fileOffset,
+        includeExtensionMethods: true,
+        isSetter: false,
+      );
+      assert(lessThanOrEqualsInvokeTarget.isInstanceMember);
 
-        node.lengthCheckTarget =
-            lessThanOrEqualsInvokeTarget.classMember as Procedure;
-        node.lengthCheckType = lessThanOrEqualsInvokeTarget
-            .getFunctionType(this)
-            .lessThanOrEqualsFunctionType;
-      } else {
-        ObjectAccessTarget equalsInvokeTarget = findInterfaceMember(
-          lengthType,
-          equalsName,
-          node.fileOffset,
-          includeExtensionMethods: true,
-          isSetter: false,
-        );
-        assert(equalsInvokeTarget.isInstanceMember);
+      node.lengthCheckTarget =
+          lessThanOrEqualsInvokeTarget.classMember as Procedure;
+      node.lengthCheckType = lessThanOrEqualsInvokeTarget
+          .getFunctionType(this)
+          .lessThanOrEqualsFunctionType;
+    } else {
+      ObjectAccessTarget equalsInvokeTarget = findInterfaceMember(
+        lengthType,
+        equalsName,
+        node.fileOffset,
+        includeExtensionMethods: true,
+        isSetter: false,
+      );
+      assert(equalsInvokeTarget.isInstanceMember);
 
-        node.lengthCheckTarget = equalsInvokeTarget.classMember as Procedure;
-        node.lengthCheckType = equalsInvokeTarget
-            .getFunctionType(this)
-            .equalsFunctionType;
-      }
+      node.lengthCheckTarget = equalsInvokeTarget.classMember as Procedure;
+      node.lengthCheckType = equalsInvokeTarget
+          .getFunctionType(this)
+          .equalsFunctionType;
     }
 
     pushRewrite(replacement ?? node);
@@ -15583,44 +15574,40 @@ class InferenceVisitorImpl extends InferenceVisitorBase
       requiredType: requiredType,
     );
 
-    DartType lookupType;
     if (node.needsCheck) {
-      lookupType = node.lookupType = requiredType;
+      node.lookupType = requiredType;
     } else {
-      lookupType = node.lookupType = matchedValueType;
+      node.lookupType = matchedValueType;
     }
 
     ObjectAccessTarget containsKeyTarget = findInterfaceMember(
-      lookupType,
+      requiredType,
       containsKeyName,
       node.fileOffset,
       includeExtensionMethods: true,
       isSetter: false,
     );
-    if (containsKeyTarget.isNever) {
-      node.isNeverPattern = true;
-    } else {
-      assert(containsKeyTarget.isInstanceMember);
 
-      node.containsKeyTarget = containsKeyTarget.classMember as Procedure;
-      node.containsKeyType = containsKeyTarget
-          .getFunctionType(this)
-          .containsKeyFunctionType;
+    assert(containsKeyTarget.isInstanceMember);
 
-      ObjectAccessTarget indexGetTarget = findInterfaceMember(
-        lookupType,
-        indexGetName,
-        node.fileOffset,
-        includeExtensionMethods: true,
-        isSetter: false,
-      );
-      assert(indexGetTarget.isInstanceMember);
+    node.containsKeyTarget = containsKeyTarget.classMember as Procedure;
+    node.containsKeyType = containsKeyTarget
+        .getFunctionType(this)
+        .containsKeyFunctionType;
 
-      node.indexGetTarget = indexGetTarget.classMember as Procedure;
-      node.indexGetType = indexGetTarget
-          .getFunctionType(this)
-          .indexGetFunctionType;
-    }
+    ObjectAccessTarget indexGetTarget = findInterfaceMember(
+      requiredType,
+      indexGetName,
+      node.fileOffset,
+      includeExtensionMethods: true,
+      isSetter: false,
+    );
+    assert(indexGetTarget.isInstanceMember);
+
+    node.indexGetTarget = indexGetTarget.classMember as Procedure;
+    node.indexGetType = indexGetTarget
+        .getFunctionType(this)
+        .indexGetFunctionType;
 
     assert(
       checkStack(node, stackBase, [
diff --git a/pkg/front_end/lib/src/type_inference/matching_expressions.dart b/pkg/front_end/lib/src/type_inference/matching_expressions.dart
index ee15fbcbfa7..0ae0f83370d 100644
--- a/pkg/front_end/lib/src/type_inference/matching_expressions.dart
+++ b/pkg/front_end/lib/src/type_inference/matching_expressions.dart
@@ -119,221 +119,212 @@ class MatchingExpressionVisitor
     ListPattern node,
     CacheableExpression matchedExpression,
   ) {
-    if (node.isNeverPattern) {
-      return matchedExpression;
+    matchedExpression = matchedExpression.promote(node.matchedValueType!);
+
+    CacheableExpression? isExpression;
+    CacheableExpression typedMatchedExpression;
+    if (node.needsCheck) {
+      isExpression = matchingCache.createIsExpression(
+        matchedExpression,
+        node.requiredType!,
+        fileOffset: node.fileOffset,
+      );
+      typedMatchedExpression = new PromotedCacheableExpression(
+        matchedExpression,
+        node.requiredType!,
+      );
     } else {
-      matchedExpression = matchedExpression.promote(node.matchedValueType!);
+      typedMatchedExpression = matchedExpression;
+    }
 
-      CacheableExpression? isExpression;
-      CacheableExpression typedMatchedExpression;
-      if (node.needsCheck) {
-        isExpression = matchingCache.createIsExpression(
-          matchedExpression,
-          node.requiredType!,
-          fileOffset: node.fileOffset,
-        );
-        typedMatchedExpression = new PromotedCacheableExpression(
-          matchedExpression,
-          node.requiredType!,
-        );
-      } else {
-        typedMatchedExpression = matchedExpression;
-      }
-
-      CacheableExpression lengthGet = matchingCache.createPropertyGetExpression(
+    CacheableExpression lengthGet = matchingCache.createPropertyGetExpression(
+      typedMatchedExpression,
+      lengthName.text,
+      new DelayedInstanceGet(
         typedMatchedExpression,
-        lengthName.text,
-        new DelayedInstanceGet(
-          typedMatchedExpression,
-          node.lengthTarget,
-          node.lengthType!,
+        node.lengthTarget,
+        node.lengthType!,
+        fileOffset: node.fileOffset,
+      ),
+      fileOffset: node.fileOffset,
+    );
+
+    CacheableExpression? lengthCheck;
+    if (node.hasRestPattern) {
+      int minLength = node.patterns.length - 1;
+      if (minLength > 0) {
+        CacheableExpression constExpression = matchingCache.createIntConstant(
+          minLength,
           fileOffset: node.fileOffset,
-        ),
+        );
+
+        lengthCheck = matchingCache.createComparisonExpression(
+          lengthGet,
+          greaterThanOrEqualsName.text,
+          constExpression,
+          new DelayedInstanceInvocation(
+            lengthGet,
+            node.lengthCheckTarget,
+            node.lengthCheckType!,
+            [constExpression],
+            fileOffset: node.fileOffset,
+          ),
+          fileOffset: node.fileOffset,
+        );
+      }
+    } else {
+      int length = node.patterns.length;
+      CacheableExpression constExpression = matchingCache.createIntConstant(
+        length,
         fileOffset: node.fileOffset,
       );
 
-      CacheableExpression? lengthCheck;
-      if (node.hasRestPattern) {
-        int minLength = node.patterns.length - 1;
-        if (minLength > 0) {
-          CacheableExpression constExpression = matchingCache.createIntConstant(
-            minLength,
+      if (length == 0) {
+        lengthCheck = matchingCache.createComparisonExpression(
+          lengthGet,
+          lessThanOrEqualsName.text,
+          constExpression,
+          new DelayedInstanceInvocation(
+            lengthGet,
+            node.lengthCheckTarget,
+            node.lengthCheckType!,
+            [constExpression],
+            fileOffset: node.fileOffset,
+          ),
+          fileOffset: node.fileOffset,
+        );
+      } else {
+        lengthCheck = matchingCache.createEqualsExpression(
+          lengthGet,
+          constExpression,
+          new DelayedEqualsExpression(
+            lengthGet,
+            constExpression,
+            node.lengthCheckTarget,
+            node.lengthCheckType!,
+            fileOffset: node.fileOffset,
+          ),
+          fileOffset: node.fileOffset,
+        );
+      }
+    }
+
+    DelayedExpression? matchingExpression;
+    if (isExpression != null && lengthCheck != null) {
+      matchingExpression = matchingCache.createAndExpression(
+        isExpression,
+        lengthCheck,
+        fileOffset: node.fileOffset,
+      );
+    } else if (isExpression != null) {
+      matchingExpression = isExpression;
+    } else if (lengthCheck != null) {
+      matchingExpression = lengthCheck;
+    }
+
+    bool hasSeenRestPattern = false;
+    for (int i = 0; i < node.patterns.length; i++) {
+      CacheableExpression elementExpression;
+      Pattern elementPattern = node.patterns[i];
+      if (elementPattern is RestPattern) {
+        hasSeenRestPattern = true;
+        Pattern? subPattern = elementPattern.subPattern;
+        if (subPattern == null) {
+          continue;
+        }
+
+        int nextIndex = i + 1;
+        int headSize = i;
+        int tailSize = node.patterns.length - nextIndex;
+
+        DelayedExpression expression;
+        if (tailSize > 0) {
+          expression = new DelayedInstanceInvocation(
+            typedMatchedExpression,
+            node.sublistTarget,
+            node.sublistType!,
+            [
+              new IntegerExpression(headSize, fileOffset: node.fileOffset),
+              new DelayedInstanceInvocation(
+                lengthGet,
+                node.minusTarget,
+                node.minusType!,
+                [new IntegerExpression(tailSize, fileOffset: node.fileOffset)],
+                fileOffset: node.fileOffset,
+              ),
+            ],
             fileOffset: node.fileOffset,
           );
-
-          lengthCheck = matchingCache.createComparisonExpression(
-            lengthGet,
-            greaterThanOrEqualsName.text,
-            constExpression,
-            new DelayedInstanceInvocation(
-              lengthGet,
-              node.lengthCheckTarget,
-              node.lengthCheckType!,
-              [constExpression],
-              fileOffset: node.fileOffset,
-            ),
+        } else {
+          expression = new DelayedInstanceInvocation(
+            typedMatchedExpression,
+            node.sublistTarget,
+            node.sublistType!,
+            [new IntegerExpression(headSize, fileOffset: node.fileOffset)],
             fileOffset: node.fileOffset,
           );
         }
-      } else {
-        int length = node.patterns.length;
-        CacheableExpression constExpression = matchingCache.createIntConstant(
-          length,
+        elementExpression = matchingCache.createSublistExpression(
+          typedMatchedExpression,
+          lengthGet,
+          headSize,
+          tailSize,
+          expression,
           fileOffset: node.fileOffset,
         );
-
-        if (length == 0) {
-          lengthCheck = matchingCache.createComparisonExpression(
-            lengthGet,
-            lessThanOrEqualsName.text,
-            constExpression,
+      } else {
+        if (!hasSeenRestPattern) {
+          int index = i;
+          elementExpression = matchingCache.createHeadIndexExpression(
+            typedMatchedExpression,
+            index,
             new DelayedInstanceInvocation(
-              lengthGet,
-              node.lengthCheckTarget,
-              node.lengthCheckType!,
-              [constExpression],
+              typedMatchedExpression,
+              node.indexGetTarget,
+              node.indexGetType!,
+              [new IntegerExpression(index, fileOffset: node.fileOffset)],
               fileOffset: node.fileOffset,
             ),
             fileOffset: node.fileOffset,
           );
         } else {
-          lengthCheck = matchingCache.createEqualsExpression(
+          int index = node.patterns.length - i;
+          elementExpression = matchingCache.createTailIndexExpression(
+            typedMatchedExpression,
             lengthGet,
-            constExpression,
-            new DelayedEqualsExpression(
-              lengthGet,
-              constExpression,
-              node.lengthCheckTarget,
-              node.lengthCheckType!,
-              fileOffset: node.fileOffset,
-            ),
-            fileOffset: node.fileOffset,
-          );
-        }
-      }
-
-      DelayedExpression? matchingExpression;
-      if (isExpression != null && lengthCheck != null) {
-        matchingExpression = matchingCache.createAndExpression(
-          isExpression,
-          lengthCheck,
-          fileOffset: node.fileOffset,
-        );
-      } else if (isExpression != null) {
-        matchingExpression = isExpression;
-      } else if (lengthCheck != null) {
-        matchingExpression = lengthCheck;
-      }
-
-      bool hasSeenRestPattern = false;
-      for (int i = 0; i < node.patterns.length; i++) {
-        CacheableExpression elementExpression;
-        Pattern elementPattern = node.patterns[i];
-        if (elementPattern is RestPattern) {
-          hasSeenRestPattern = true;
-          Pattern? subPattern = elementPattern.subPattern;
-          if (subPattern == null) {
-            continue;
-          }
-
-          int nextIndex = i + 1;
-          int headSize = i;
-          int tailSize = node.patterns.length - nextIndex;
-
-          DelayedExpression expression;
-          if (tailSize > 0) {
-            expression = new DelayedInstanceInvocation(
+            index,
+            new DelayedInstanceInvocation(
               typedMatchedExpression,
-              node.sublistTarget,
-              node.sublistType!,
+              node.indexGetTarget,
+              node.indexGetType!,
               [
-                new IntegerExpression(headSize, fileOffset: node.fileOffset),
                 new DelayedInstanceInvocation(
                   lengthGet,
                   node.minusTarget,
                   node.minusType!,
-                  [
-                    new IntegerExpression(
-                      tailSize,
-                      fileOffset: node.fileOffset,
-                    ),
-                  ],
+                  [new IntegerExpression(index, fileOffset: node.fileOffset)],
                   fileOffset: node.fileOffset,
                 ),
               ],
               fileOffset: node.fileOffset,
-            );
-          } else {
-            expression = new DelayedInstanceInvocation(
-              typedMatchedExpression,
-              node.sublistTarget,
-              node.sublistType!,
-              [new IntegerExpression(headSize, fileOffset: node.fileOffset)],
-              fileOffset: node.fileOffset,
-            );
-          }
-          elementExpression = matchingCache.createSublistExpression(
-            typedMatchedExpression,
-            lengthGet,
-            headSize,
-            tailSize,
-            expression,
+            ),
             fileOffset: node.fileOffset,
           );
-        } else {
-          if (!hasSeenRestPattern) {
-            int index = i;
-            elementExpression = matchingCache.createHeadIndexExpression(
-              typedMatchedExpression,
-              index,
-              new DelayedInstanceInvocation(
-                typedMatchedExpression,
-                node.indexGetTarget,
-                node.indexGetType!,
-                [new IntegerExpression(index, fileOffset: node.fileOffset)],
-                fileOffset: node.fileOffset,
-              ),
-              fileOffset: node.fileOffset,
-            );
-          } else {
-            int index = node.patterns.length - i;
-            elementExpression = matchingCache.createTailIndexExpression(
-              typedMatchedExpression,
-              lengthGet,
-              index,
-              new DelayedInstanceInvocation(
-                typedMatchedExpression,
-                node.indexGetTarget,
-                node.indexGetType!,
-                [
-                  new DelayedInstanceInvocation(
-                    lengthGet,
-                    node.minusTarget,
-                    node.minusType!,
-                    [new IntegerExpression(index, fileOffset: node.fileOffset)],
-                    fileOffset: node.fileOffset,
-                  ),
-                ],
-                fileOffset: node.fileOffset,
-              ),
-              fileOffset: node.fileOffset,
-            );
-          }
         }
-
-        DelayedExpression elementMatcher = visitPattern(
-          elementPattern,
-          elementExpression,
-        );
-        matchingExpression = DelayedAndExpression.merge(
-          matchingExpression,
-          elementMatcher,
-          fileOffset: node.fileOffset,
-        );
       }
-      return matchingExpression ??
-          new BooleanExpression(true, fileOffset: node.fileOffset);
+
+      DelayedExpression elementMatcher = visitPattern(
+        elementPattern,
+        elementExpression,
+      );
+      matchingExpression = DelayedAndExpression.merge(
+        matchingExpression,
+        elementMatcher,
+        fileOffset: node.fileOffset,
+      );
     }
+    return matchingExpression ??
+        new BooleanExpression(true, fileOffset: node.fileOffset);
   }
 
   @override
@@ -341,123 +332,118 @@ class MatchingExpressionVisitor
     MapPattern node,
     CacheableExpression matchedExpression,
   ) {
-    if (node.isNeverPattern) {
-      return matchedExpression;
-    } else {
-      matchedExpression = matchedExpression.promote(node.matchedValueType!);
+    matchedExpression = matchedExpression.promote(node.matchedValueType!);
 
-      CacheableExpression? isExpression;
-      CacheableExpression typedMatchedExpression;
-      if (node.needsCheck) {
-        isExpression = matchingCache.createIsExpression(
-          matchedExpression,
-          node.requiredType!,
-          fileOffset: node.fileOffset,
-        );
-        typedMatchedExpression = new PromotedCacheableExpression(
-          matchedExpression,
-          node.requiredType!,
-        );
-      } else {
-        typedMatchedExpression = matchedExpression;
-      }
-
-      DelayedExpression? matchingExpression;
-      if (isExpression != null) {
-        matchingExpression = isExpression;
-      }
-
-      InterfaceType requiredType = node.requiredType as InterfaceType;
-      assert(
-        requiredType.classNode == coreTypes.mapClass &&
-            requiredType.typeArguments.length == 2,
+    CacheableExpression? isExpression;
+    CacheableExpression typedMatchedExpression;
+    if (node.needsCheck) {
+      isExpression = matchingCache.createIsExpression(
+        matchedExpression,
+        node.requiredType!,
+        fileOffset: node.fileOffset,
       );
-      DartType valueType = requiredType.typeArguments[1];
-      for (MapPatternEntry entry in node.entries) {
-        if (entry is MapPatternRestEntry) continue;
-        CacheableExpression keyExpression = matchingCache
-            .createConstantExpression(
-              entry.keyValue!,
-              entry.keyType!,
-              fileOffset: entry.key.fileOffset,
-            );
-        CacheableExpression containsExpression = matchingCache
-            .createContainsKeyExpression(
+      typedMatchedExpression = new PromotedCacheableExpression(
+        matchedExpression,
+        node.requiredType!,
+      );
+    } else {
+      typedMatchedExpression = matchedExpression;
+    }
+
+    DelayedExpression? matchingExpression;
+    if (isExpression != null) {
+      matchingExpression = isExpression;
+    }
+
+    InterfaceType requiredType = node.requiredType as InterfaceType;
+    assert(
+      requiredType.classNode == coreTypes.mapClass &&
+          requiredType.typeArguments.length == 2,
+    );
+    DartType valueType = requiredType.typeArguments[1];
+    for (MapPatternEntry entry in node.entries) {
+      if (entry is MapPatternRestEntry) continue;
+      CacheableExpression keyExpression = matchingCache
+          .createConstantExpression(
+            entry.keyValue!,
+            entry.keyType!,
+            fileOffset: entry.key.fileOffset,
+          );
+      CacheableExpression containsExpression = matchingCache
+          .createContainsKeyExpression(
+            typedMatchedExpression,
+            keyExpression,
+            new DelayedInstanceInvocation(
               typedMatchedExpression,
-              keyExpression,
-              new DelayedInstanceInvocation(
-                typedMatchedExpression,
-                node.containsKeyTarget,
-                node.containsKeyType!,
-                [keyExpression],
-                fileOffset: entry.fileOffset,
-              ),
-              fileOffset: entry.fileOffset,
-            );
-        CacheableExpression valueExpression = matchingCache
-            .createIndexExpression(
-              typedMatchedExpression,
-              keyExpression,
-              new DelayedInstanceInvocation(
-                typedMatchedExpression,
-                node.indexGetTarget,
-                node.indexGetType!,
-                [keyExpression],
-                fileOffset: entry.fileOffset,
-              ),
-              fileOffset: entry.fileOffset,
-            );
-        matchingExpression = DelayedAndExpression.merge(
-          matchingExpression,
-          new DelayedOrExpression(
-            new DelayedNullCheckExpression(
-              valueExpression,
-              fileOffset: entry.fileOffset,
-            ),
-            new DelayedAndExpression(
-              new DelayedIsExpression(
-                new FixedExpression(
-                  createNullLiteral(fileOffset: entry.fileOffset),
-                  const NullType(),
-                ),
-                valueType,
-                fileOffset: entry.fileOffset,
-              ),
-              containsExpression,
+              node.containsKeyTarget,
+              node.containsKeyType!,
+              [keyExpression],
               fileOffset: entry.fileOffset,
             ),
             fileOffset: entry.fileOffset,
+          );
+      CacheableExpression valueExpression = matchingCache.createIndexExpression(
+        typedMatchedExpression,
+        keyExpression,
+        new DelayedInstanceInvocation(
+          typedMatchedExpression,
+          node.indexGetTarget,
+          node.indexGetType!,
+          [keyExpression],
+          fileOffset: entry.fileOffset,
+        ),
+        fileOffset: entry.fileOffset,
+      );
+      matchingExpression = DelayedAndExpression.merge(
+        matchingExpression,
+        new DelayedOrExpression(
+          new DelayedNullCheckExpression(
+            valueExpression,
+            fileOffset: entry.fileOffset,
+          ),
+          new DelayedAndExpression(
+            new DelayedIsExpression(
+              new FixedExpression(
+                createNullLiteral(fileOffset: entry.fileOffset),
+                const NullType(),
+              ),
+              valueType,
+              fileOffset: entry.fileOffset,
+            ),
+            containsExpression,
+            fileOffset: entry.fileOffset,
           ),
           fileOffset: entry.fileOffset,
-        );
-        valueExpression = new PromotedCacheableExpression(
-          valueExpression,
-          valueType,
-        );
+        ),
+        fileOffset: entry.fileOffset,
+      );
+      valueExpression = new PromotedCacheableExpression(
+        valueExpression,
+        valueType,
+      );
 
-        DelayedExpression subExpression = visitPattern(
-          entry.value,
-          valueExpression,
+      DelayedExpression subExpression = visitPattern(
+        entry.value,
+        valueExpression,
+      );
+      if (!subExpression.uses(valueExpression)) {
+        // Ensure that we perform the lookup even if we don't use the result.
+        matchingExpression = DelayedAndExpression.merge(
+          matchingExpression,
+          new EffectExpression(valueExpression, subExpression),
+          fileOffset: node.fileOffset,
+        );
+      } else {
+        matchingExpression = DelayedAndExpression.merge(
+          matchingExpression,
+          subExpression,
+          fileOffset: node.fileOffset,
         );
-        if (!subExpression.uses(valueExpression)) {
-          // Ensure that we perform the lookup even if we don't use the result.
-          matchingExpression = DelayedAndExpression.merge(
-            matchingExpression,
-            new EffectExpression(valueExpression, subExpression),
-            fileOffset: node.fileOffset,
-          );
-        } else {
-          matchingExpression = DelayedAndExpression.merge(
-            matchingExpression,
-            subExpression,
-            fileOffset: node.fileOffset,
-          );
-        }
       }
-      // Coverage-ignore(suite): Not run.
-      return matchingExpression ??
-          new BooleanExpression(true, fileOffset: node.fileOffset);
     }
+    // Coverage-ignore(suite): Not run.
+    return matchingExpression ??
+        new BooleanExpression(true, fileOffset: node.fileOffset);
   }
 
   @override
diff --git a/pkg/front_end/testcases/patterns/effect_only.dart.strong.expect b/pkg/front_end/testcases/patterns/effect_only.dart.strong.expect
index 62ad878e754..812cb36470b 100644
--- a/pkg/front_end/testcases/patterns/effect_only.dart.strong.expect
+++ b/pkg/front_end/testcases/patterns/effect_only.dart.strong.expect
@@ -7,7 +7,7 @@ static method test(core::List<core::int> list) → dynamic {
   {
     final synthesized core::List<core::int> #0#0 = list;
     final const synthesized core::int #0#3 = #C1;
-    if(!(let final dynamic #t1 = #0#0 as core::List<core::int> in (#0#0 as core::List<core::int>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t2 = v = (#0#0 as core::List<core::int>).{core::List::[]}(0){(core::int) → core::int} in true)))
+    if(!(let final dynamic #t1 = #0#0 as core::List<core::int> in (#0#0 as core::List<core::int>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t2 = v = (#0#0 as core::List<core::int>).{core::List::[]}(0){(core::int) → core::num} in true)))
       throw{for-error-handling} new core::StateError::•("Pattern matching error");
   }
   self::expect(42, v);
diff --git a/pkg/front_end/testcases/patterns/effect_only.dart.strong.modular.expect b/pkg/front_end/testcases/patterns/effect_only.dart.strong.modular.expect
index 62ad878e754..812cb36470b 100644
--- a/pkg/front_end/testcases/patterns/effect_only.dart.strong.modular.expect
+++ b/pkg/front_end/testcases/patterns/effect_only.dart.strong.modular.expect
@@ -7,7 +7,7 @@ static method test(core::List<core::int> list) → dynamic {
   {
     final synthesized core::List<core::int> #0#0 = list;
     final const synthesized core::int #0#3 = #C1;
-    if(!(let final dynamic #t1 = #0#0 as core::List<core::int> in (#0#0 as core::List<core::int>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t2 = v = (#0#0 as core::List<core::int>).{core::List::[]}(0){(core::int) → core::int} in true)))
+    if(!(let final dynamic #t1 = #0#0 as core::List<core::int> in (#0#0 as core::List<core::int>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t2 = v = (#0#0 as core::List<core::int>).{core::List::[]}(0){(core::int) → core::num} in true)))
       throw{for-error-handling} new core::StateError::•("Pattern matching error");
   }
   self::expect(42, v);
diff --git a/pkg/front_end/testcases/patterns/effect_only.dart.strong.transformed.expect b/pkg/front_end/testcases/patterns/effect_only.dart.strong.transformed.expect
index f16a28e138f..60c364a1846 100644
--- a/pkg/front_end/testcases/patterns/effect_only.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/patterns/effect_only.dart.strong.transformed.expect
@@ -7,7 +7,7 @@ static method test(core::List<core::int> list) → dynamic {
   {
     final synthesized core::List<core::int> #0#0 = list;
     final const synthesized core::int #0#3 = #C1;
-    if(!(let final dynamic #t1 = #0#0 as{Unchecked} core::List<core::int> in (#0#0 as{Unchecked} core::List<core::int>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t2 = v = (#0#0 as{Unchecked} core::List<core::int>).{core::List::[]}(0){(core::int) → core::int} in true)))
+    if(!(let final dynamic #t1 = #0#0 as{Unchecked} core::List<core::int> in (#0#0 as{Unchecked} core::List<core::int>).{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t2 = v = (#0#0 as{Unchecked} core::List<core::int>).{core::List::[]}(0){(core::int) → core::num} in true)))
       throw{for-error-handling} new core::StateError::•("Pattern matching error");
   }
   self::expect(42, v);
diff --git a/pkg/front_end/testcases/patterns/issue56138.dart.strong.expect b/pkg/front_end/testcases/patterns/issue56138.dart.strong.expect
index 6235e5a8da6..368fa0c16d8 100644
--- a/pkg/front_end/testcases/patterns/issue56138.dart.strong.expect
+++ b/pkg/front_end/testcases/patterns/issue56138.dart.strong.expect
@@ -6,7 +6,8 @@ static method main() → void {
   try {
     {
       final synthesized Never #0#0 = throw "A";
-      if(!#0#0)
+      final const synthesized core::int #0#2 = #C1;
+      if(!#0#0.{core::List::length}{core::int}.{core::num::<=}(#C1){(core::num) → core::bool})
         throw{for-error-handling} new core::StateError::•("Pattern matching error");
     }
   }
@@ -16,3 +17,7 @@ static method main() → void {
   }
   throw "Missing exception";
 }
+
+constants  {
+  #C1 = 0
+}
diff --git a/pkg/front_end/testcases/patterns/issue56138.dart.strong.modular.expect b/pkg/front_end/testcases/patterns/issue56138.dart.strong.modular.expect
index 6235e5a8da6..368fa0c16d8 100644
--- a/pkg/front_end/testcases/patterns/issue56138.dart.strong.modular.expect
+++ b/pkg/front_end/testcases/patterns/issue56138.dart.strong.modular.expect
@@ -6,7 +6,8 @@ static method main() → void {
   try {
     {
       final synthesized Never #0#0 = throw "A";
-      if(!#0#0)
+      final const synthesized core::int #0#2 = #C1;
+      if(!#0#0.{core::List::length}{core::int}.{core::num::<=}(#C1){(core::num) → core::bool})
         throw{for-error-handling} new core::StateError::•("Pattern matching error");
     }
   }
@@ -16,3 +17,7 @@ static method main() → void {
   }
   throw "Missing exception";
 }
+
+constants  {
+  #C1 = 0
+}
diff --git a/pkg/front_end/testcases/patterns/issue56138.dart.strong.transformed.expect b/pkg/front_end/testcases/patterns/issue56138.dart.strong.transformed.expect
index 6235e5a8da6..368fa0c16d8 100644
--- a/pkg/front_end/testcases/patterns/issue56138.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/patterns/issue56138.dart.strong.transformed.expect
@@ -6,7 +6,8 @@ static method main() → void {
   try {
     {
       final synthesized Never #0#0 = throw "A";
-      if(!#0#0)
+      final const synthesized core::int #0#2 = #C1;
+      if(!#0#0.{core::List::length}{core::int}.{core::num::<=}(#C1){(core::num) → core::bool})
         throw{for-error-handling} new core::StateError::•("Pattern matching error");
     }
   }
@@ -16,3 +17,7 @@ static method main() → void {
   }
   throw "Missing exception";
 }
+
+constants  {
+  #C1 = 0
+}
diff --git a/pkg/front_end/testcases/patterns/issue63315.dart b/pkg/front_end/testcases/patterns/issue63315.dart
new file mode 100644
index 00000000000..34fd065a67c
--- /dev/null
+++ b/pkg/front_end/testcases/patterns/issue63315.dart
@@ -0,0 +1,54 @@
+// Copyright (c) 2026, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+extension type JsonMap1<T>(Map<String, Object?> _)
+    implements Map<String, Object?> {
+  T operator [](String key) => _[key] as T;
+}
+
+void parseJson1(JsonMap1 map) {
+  if (map case {"key": "value"}) print("ok");
+}
+
+extension type JsonMap2<T>(Map<String, Object?> _)
+    implements Map<String, Object?> {
+  bool containsKey(String key) => _.containsKey(key);
+}
+
+void parseJson2(JsonMap2 map) {
+  if (map case {"key": "value"}) print("ok");
+}
+
+extension type JsonMap3<T>(Map<String, Object?> _)
+    implements Map<String, Object?> {
+  int get length => _.length;
+}
+
+void parseJson3(JsonMap3 map) {
+  if (map case {"key": "value"}) print("ok");
+}
+
+extension type JsonList1<T>(List<Object?> _) implements List<Object?> {
+  T operator [](int index) => _[index] as T;
+}
+
+void parseJson4(JsonList1 list) {
+  if (list case ["value"]) print("ok");
+}
+
+extension type JsonList2<T>(List<Object?> _) implements List<Object?> {
+  int get length => _.length;
+}
+
+void parseJson5(JsonList2 list) {
+  if (list case ["value"]) print("ok");
+}
+
+extension type JsonList3<T>(List<Object?> _) implements List<Object?> {
+  List<T> sublist(int start, [int? end]) => _.sublist(start, end).cast<T>();
+}
+
+void parseJson6(JsonList3 list) {
+  if (list case ["value"]) print("ok");
+}
diff --git a/pkg/front_end/testcases/patterns/issue63315.dart.strong.expect b/pkg/front_end/testcases/patterns/issue63315.dart.strong.expect
new file mode 100644
index 00000000000..8d5f0f2e7e9
--- /dev/null
+++ b/pkg/front_end/testcases/patterns/issue63315.dart.strong.expect
@@ -0,0 +1,161 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+extension type JsonMap1<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _) implements core::Map<core::String, core::Object?> {
+  abstract extension-type-member representation-field get _() → core::Map<core::String, core::Object?>;
+  operator [] = self::JsonMap1|[];
+  constructor • = self::JsonMap1|constructor#;
+  constructor tearoff • = self::JsonMap1|constructor#_#new#tearOff;
+}
+extension type JsonMap2<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _) implements core::Map<core::String, core::Object?> {
+  abstract extension-type-member representation-field get _() → core::Map<core::String, core::Object?>;
+  method containsKey = self::JsonMap2|containsKey;
+  method tearoff containsKey = self::JsonMap2|get#containsKey;
+  constructor • = self::JsonMap2|constructor#;
+  constructor tearoff • = self::JsonMap2|constructor#_#new#tearOff;
+}
+extension type JsonMap3<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _) implements core::Map<core::String, core::Object?> {
+  abstract extension-type-member representation-field get _() → core::Map<core::String, core::Object?>;
+  get length = self::JsonMap3|get#length;
+  constructor • = self::JsonMap3|constructor#;
+  constructor tearoff • = self::JsonMap3|constructor#_#new#tearOff;
+}
+extension type JsonList1<T extends core::Object? = dynamic>(core::List<core::Object?> _) implements core::List<core::Object?> {
+  abstract extension-type-member representation-field get _() → core::List<core::Object?>;
+  operator [] = self::JsonList1|[];
+  constructor • = self::JsonList1|constructor#;
+  constructor tearoff • = self::JsonList1|constructor#_#new#tearOff;
+}
+extension type JsonList2<T extends core::Object? = dynamic>(core::List<core::Object?> _) implements core::List<core::Object?> {
+  abstract extension-type-member representation-field get _() → core::List<core::Object?>;
+  get length = self::JsonList2|get#length;
+  constructor • = self::JsonList2|constructor#;
+  constructor tearoff • = self::JsonList2|constructor#_#new#tearOff;
+}
+extension type JsonList3<T extends core::Object? = dynamic>(core::List<core::Object?> _) implements core::List<core::Object?> {
+  abstract extension-type-member representation-field get _() → core::List<core::Object?>;
+  method sublist = self::JsonList3|sublist;
+  method tearoff sublist = self::JsonList3|get#sublist;
+  constructor • = self::JsonList3|constructor#;
+  constructor tearoff • = self::JsonList3|constructor#_#new#tearOff;
+}
+static extension-type-member method JsonMap1|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::Map<core::String, core::Object?> _#wc0#formal) → self::JsonMap1<self::JsonMap1|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */ {
+  lowered final self::JsonMap1<self::JsonMap1|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */ #this = _#wc0#formal;
+  return #this;
+}
+static extension-type-member synthetic method JsonMap1|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _#wc0#formal) → self::JsonMap1<self::JsonMap1|constructor#_#new#tearOff::T%> /* erasure=core::Map<core::String, core::Object?> */
+  return self::JsonMap1|constructor#<self::JsonMap1|constructor#_#new#tearOff::T%>(_#wc0#formal);
+static extension-type-member method JsonMap1|[]<T extends core::Object? = dynamic>(lowered final self::JsonMap1<self::JsonMap1|[]::T%> /* erasure=core::Map<core::String, core::Object?> */ #this, core::String key) → self::JsonMap1|[]::T%
+  return (#this as{Unchecked} core::Map<core::String, core::Object?>).{core::Map::[]}(key){(core::Object?) → core::Object?} as self::JsonMap1|[]::T%;
+static method parseJson1(self::JsonMap1<dynamic> /* erasure=core::Map<core::String, core::Object?> */ map) → void {
+  {
+    final synthesized self::JsonMap1<dynamic> /* erasure=core::Map<core::String, core::Object?> */ #0#0 = map;
+    synthesized core::Object? #0#3;
+    synthesized core::bool #0#3#isSet = false;
+    final const synthesized core::String #0#1 = #C1;
+    final const synthesized core::String #0#4 = #C2;
+    if((!((#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t1 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}) == null) || null is core::Object? && #0#0.{core::Map::containsKey}(#C1){(core::Object?) → core::bool}) && #C2 =={core::String::==}{(core::Object) → core::bool} (#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t2 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}))
+      core::print("ok");
+  }
+}
+static extension-type-member method JsonMap2|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::Map<core::String, core::Object?> _#wc1#formal) → self::JsonMap2<self::JsonMap2|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */ {
+  lowered final self::JsonMap2<self::JsonMap2|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */ #this = _#wc1#formal;
+  return #this;
+}
+static extension-type-member synthetic method JsonMap2|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _#wc1#formal) → self::JsonMap2<self::JsonMap2|constructor#_#new#tearOff::T%> /* erasure=core::Map<core::String, core::Object?> */
+  return self::JsonMap2|constructor#<self::JsonMap2|constructor#_#new#tearOff::T%>(_#wc1#formal);
+static extension-type-member method JsonMap2|containsKey<T extends core::Object? = dynamic>(lowered final self::JsonMap2<self::JsonMap2|containsKey::T%> /* erasure=core::Map<core::String, core::Object?> */ #this, core::String key) → core::bool
+  return (#this as{Unchecked} core::Map<core::String, core::Object?>).{core::Map::containsKey}(key){(core::Object?) → core::bool};
+static extension-type-member method JsonMap2|get#containsKey<T extends core::Object? = dynamic>(lowered final self::JsonMap2<self::JsonMap2|get#containsKey::T%> /* erasure=core::Map<core::String, core::Object?> */ #this) → (core::String) → core::bool
+  return (core::String key) → core::bool => self::JsonMap2|containsKey<self::JsonMap2|get#containsKey::T%>(#this, key);
+static method parseJson2(self::JsonMap2<dynamic> /* erasure=core::Map<core::String, core::Object?> */ map) → void {
+  {
+    final synthesized self::JsonMap2<dynamic> /* erasure=core::Map<core::String, core::Object?> */ #0#0 = map;
+    synthesized core::Object? #0#3;
+    synthesized core::bool #0#3#isSet = false;
+    final const synthesized core::String #0#1 = #C1;
+    final const synthesized core::String #0#4 = #C2;
+    if((!((#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t3 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}) == null) || null is core::Object? && #0#0.{core::Map::containsKey}(#C1){(core::Object?) → core::bool}) && #C2 =={core::String::==}{(core::Object) → core::bool} (#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t4 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}))
+      core::print("ok");
+  }
+}
+static extension-type-member method JsonMap3|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::Map<core::String, core::Object?> _#wc2#formal) → self::JsonMap3<self::JsonMap3|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */ {
+  lowered final self::JsonMap3<self::JsonMap3|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */ #this = _#wc2#formal;
+  return #this;
+}
+static extension-type-member synthetic method JsonMap3|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _#wc2#formal) → self::JsonMap3<self::JsonMap3|constructor#_#new#tearOff::T%> /* erasure=core::Map<core::String, core::Object?> */
+  return self::JsonMap3|constructor#<self::JsonMap3|constructor#_#new#tearOff::T%>(_#wc2#formal);
+static extension-type-member method JsonMap3|get#length<T extends core::Object? = dynamic>(lowered final self::JsonMap3<self::JsonMap3|get#length::T%> /* erasure=core::Map<core::String, core::Object?> */ #this) → core::int
+  return (#this as{Unchecked} core::Map<core::String, core::Object?>).{core::Map::length}{core::int};
+static method parseJson3(self::JsonMap3<dynamic> /* erasure=core::Map<core::String, core::Object?> */ map) → void {
+  {
+    final synthesized self::JsonMap3<dynamic> /* erasure=core::Map<core::String, core::Object?> */ #0#0 = map;
+    synthesized core::Object? #0#3;
+    synthesized core::bool #0#3#isSet = false;
+    final const synthesized core::String #0#1 = #C1;
+    final const synthesized core::String #0#4 = #C2;
+    if((!((#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t5 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}) == null) || null is core::Object? && #0#0.{core::Map::containsKey}(#C1){(core::Object?) → core::bool}) && #C2 =={core::String::==}{(core::Object) → core::bool} (#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t6 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}))
+      core::print("ok");
+  }
+}
+static extension-type-member method JsonList1|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::List<core::Object?> _#wc3#formal) → self::JsonList1<self::JsonList1|constructor#::T%> /* erasure=core::List<core::Object?> */ {
+  lowered final self::JsonList1<self::JsonList1|constructor#::T%> /* erasure=core::List<core::Object?> */ #this = _#wc3#formal;
+  return #this;
+}
+static extension-type-member synthetic method JsonList1|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::List<core::Object?> _#wc3#formal) → self::JsonList1<self::JsonList1|constructor#_#new#tearOff::T%> /* erasure=core::List<core::Object?> */
+  return self::JsonList1|constructor#<self::JsonList1|constructor#_#new#tearOff::T%>(_#wc3#formal);
+static extension-type-member method JsonList1|[]<T extends core::Object? = dynamic>(lowered final self::JsonList1<self::JsonList1|[]::T%> /* erasure=core::List<core::Object?> */ #this, core::int index) → self::JsonList1|[]::T%
+  return (#this as{Unchecked} core::List<core::Object?>).{core::List::[]}(index){(core::int) → core::Object?} as self::JsonList1|[]::T%;
+static method parseJson4(self::JsonList1<dynamic> /* erasure=core::List<core::Object?> */ list) → void {
+  {
+    final synthesized self::JsonList1<dynamic> /* erasure=core::List<core::Object?> */ #0#0 = list;
+    final const synthesized core::int #0#2 = #C3;
+    final const synthesized core::String #0#5 = #C2;
+    if(#0#0.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C3 && #C2 =={core::String::==}{(core::Object) → core::bool} #0#0.{core::List::[]}(0){(core::int) → core::Object?})
+      core::print("ok");
+  }
+}
+static extension-type-member method JsonList2|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::List<core::Object?> _#wc4#formal) → self::JsonList2<self::JsonList2|constructor#::T%> /* erasure=core::List<core::Object?> */ {
+  lowered final self::JsonList2<self::JsonList2|constructor#::T%> /* erasure=core::List<core::Object?> */ #this = _#wc4#formal;
+  return #this;
+}
+static extension-type-member synthetic method JsonList2|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::List<core::Object?> _#wc4#formal) → self::JsonList2<self::JsonList2|constructor#_#new#tearOff::T%> /* erasure=core::List<core::Object?> */
+  return self::JsonList2|constructor#<self::JsonList2|constructor#_#new#tearOff::T%>(_#wc4#formal);
+static extension-type-member method JsonList2|get#length<T extends core::Object? = dynamic>(lowered final self::JsonList2<self::JsonList2|get#length::T%> /* erasure=core::List<core::Object?> */ #this) → core::int
+  return (#this as{Unchecked} core::List<core::Object?>).{core::List::length}{core::int};
+static method parseJson5(self::JsonList2<dynamic> /* erasure=core::List<core::Object?> */ list) → void {
+  {
+    final synthesized self::JsonList2<dynamic> /* erasure=core::List<core::Object?> */ #0#0 = list;
+    final const synthesized core::int #0#2 = #C3;
+    final const synthesized core::String #0#5 = #C2;
+    if(#0#0.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C3 && #C2 =={core::String::==}{(core::Object) → core::bool} #0#0.{core::List::[]}(0){(core::int) → core::Object?})
+      core::print("ok");
+  }
+}
+static extension-type-member method JsonList3|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::List<core::Object?> _#wc5#formal) → self::JsonList3<self::JsonList3|constructor#::T%> /* erasure=core::List<core::Object?> */ {
+  lowered final self::JsonList3<self::JsonList3|constructor#::T%> /* erasure=core::List<core::Object?> */ #this = _#wc5#formal;
+  return #this;
+}
+static extension-type-member synthetic method JsonList3|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::List<core::Object?> _#wc5#formal) → self::JsonList3<self::JsonList3|constructor#_#new#tearOff::T%> /* erasure=core::List<core::Object?> */
+  return self::JsonList3|constructor#<self::JsonList3|constructor#_#new#tearOff::T%>(_#wc5#formal);
+static extension-type-member method JsonList3|sublist<T extends core::Object? = dynamic>(lowered final self::JsonList3<self::JsonList3|sublist::T%> /* erasure=core::List<core::Object?> */ #this, core::int start, [core::int? end = #C4]) → core::List<self::JsonList3|sublist::T%>
+  return (#this as{Unchecked} core::List<core::Object?>).{core::List::sublist}(start, end){(core::int, [core::int?]) → core::List<core::Object?>}.{core::List::cast}<self::JsonList3|sublist::T%>(){() → core::List<self::JsonList3|sublist::T%>};
+static extension-type-member method JsonList3|get#sublist<T extends core::Object? = dynamic>(lowered final self::JsonList3<self::JsonList3|get#sublist::T%> /* erasure=core::List<core::Object?> */ #this) → (core::int, [core::int?]) → core::List<self::JsonList3|get#sublist::T%>
+  return (core::int start, [core::int? end = #C4]) → core::List<self::JsonList3|get#sublist::T%> => self::JsonList3|sublist<self::JsonList3|get#sublist::T%>(#this, start, end);
+static method parseJson6(self::JsonList3<dynamic> /* erasure=core::List<core::Object?> */ list) → void {
+  {
+    final synthesized self::JsonList3<dynamic> /* erasure=core::List<core::Object?> */ #0#0 = list;
+    final const synthesized core::int #0#2 = #C3;
+    final const synthesized core::String #0#5 = #C2;
+    if(#0#0.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C3 && #C2 =={core::String::==}{(core::Object) → core::bool} #0#0.{core::List::[]}(0){(core::int) → core::Object?})
+      core::print("ok");
+  }
+}
+
+constants  {
+  #C1 = "key"
+  #C2 = "value"
+  #C3 = 1
+  #C4 = null
+}
diff --git a/pkg/front_end/testcases/patterns/issue63315.dart.strong.modular.expect b/pkg/front_end/testcases/patterns/issue63315.dart.strong.modular.expect
new file mode 100644
index 00000000000..8d5f0f2e7e9
--- /dev/null
+++ b/pkg/front_end/testcases/patterns/issue63315.dart.strong.modular.expect
@@ -0,0 +1,161 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+extension type JsonMap1<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _) implements core::Map<core::String, core::Object?> {
+  abstract extension-type-member representation-field get _() → core::Map<core::String, core::Object?>;
+  operator [] = self::JsonMap1|[];
+  constructor • = self::JsonMap1|constructor#;
+  constructor tearoff • = self::JsonMap1|constructor#_#new#tearOff;
+}
+extension type JsonMap2<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _) implements core::Map<core::String, core::Object?> {
+  abstract extension-type-member representation-field get _() → core::Map<core::String, core::Object?>;
+  method containsKey = self::JsonMap2|containsKey;
+  method tearoff containsKey = self::JsonMap2|get#containsKey;
+  constructor • = self::JsonMap2|constructor#;
+  constructor tearoff • = self::JsonMap2|constructor#_#new#tearOff;
+}
+extension type JsonMap3<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _) implements core::Map<core::String, core::Object?> {
+  abstract extension-type-member representation-field get _() → core::Map<core::String, core::Object?>;
+  get length = self::JsonMap3|get#length;
+  constructor • = self::JsonMap3|constructor#;
+  constructor tearoff • = self::JsonMap3|constructor#_#new#tearOff;
+}
+extension type JsonList1<T extends core::Object? = dynamic>(core::List<core::Object?> _) implements core::List<core::Object?> {
+  abstract extension-type-member representation-field get _() → core::List<core::Object?>;
+  operator [] = self::JsonList1|[];
+  constructor • = self::JsonList1|constructor#;
+  constructor tearoff • = self::JsonList1|constructor#_#new#tearOff;
+}
+extension type JsonList2<T extends core::Object? = dynamic>(core::List<core::Object?> _) implements core::List<core::Object?> {
+  abstract extension-type-member representation-field get _() → core::List<core::Object?>;
+  get length = self::JsonList2|get#length;
+  constructor • = self::JsonList2|constructor#;
+  constructor tearoff • = self::JsonList2|constructor#_#new#tearOff;
+}
+extension type JsonList3<T extends core::Object? = dynamic>(core::List<core::Object?> _) implements core::List<core::Object?> {
+  abstract extension-type-member representation-field get _() → core::List<core::Object?>;
+  method sublist = self::JsonList3|sublist;
+  method tearoff sublist = self::JsonList3|get#sublist;
+  constructor • = self::JsonList3|constructor#;
+  constructor tearoff • = self::JsonList3|constructor#_#new#tearOff;
+}
+static extension-type-member method JsonMap1|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::Map<core::String, core::Object?> _#wc0#formal) → self::JsonMap1<self::JsonMap1|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */ {
+  lowered final self::JsonMap1<self::JsonMap1|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */ #this = _#wc0#formal;
+  return #this;
+}
+static extension-type-member synthetic method JsonMap1|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _#wc0#formal) → self::JsonMap1<self::JsonMap1|constructor#_#new#tearOff::T%> /* erasure=core::Map<core::String, core::Object?> */
+  return self::JsonMap1|constructor#<self::JsonMap1|constructor#_#new#tearOff::T%>(_#wc0#formal);
+static extension-type-member method JsonMap1|[]<T extends core::Object? = dynamic>(lowered final self::JsonMap1<self::JsonMap1|[]::T%> /* erasure=core::Map<core::String, core::Object?> */ #this, core::String key) → self::JsonMap1|[]::T%
+  return (#this as{Unchecked} core::Map<core::String, core::Object?>).{core::Map::[]}(key){(core::Object?) → core::Object?} as self::JsonMap1|[]::T%;
+static method parseJson1(self::JsonMap1<dynamic> /* erasure=core::Map<core::String, core::Object?> */ map) → void {
+  {
+    final synthesized self::JsonMap1<dynamic> /* erasure=core::Map<core::String, core::Object?> */ #0#0 = map;
+    synthesized core::Object? #0#3;
+    synthesized core::bool #0#3#isSet = false;
+    final const synthesized core::String #0#1 = #C1;
+    final const synthesized core::String #0#4 = #C2;
+    if((!((#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t1 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}) == null) || null is core::Object? && #0#0.{core::Map::containsKey}(#C1){(core::Object?) → core::bool}) && #C2 =={core::String::==}{(core::Object) → core::bool} (#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t2 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}))
+      core::print("ok");
+  }
+}
+static extension-type-member method JsonMap2|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::Map<core::String, core::Object?> _#wc1#formal) → self::JsonMap2<self::JsonMap2|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */ {
+  lowered final self::JsonMap2<self::JsonMap2|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */ #this = _#wc1#formal;
+  return #this;
+}
+static extension-type-member synthetic method JsonMap2|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _#wc1#formal) → self::JsonMap2<self::JsonMap2|constructor#_#new#tearOff::T%> /* erasure=core::Map<core::String, core::Object?> */
+  return self::JsonMap2|constructor#<self::JsonMap2|constructor#_#new#tearOff::T%>(_#wc1#formal);
+static extension-type-member method JsonMap2|containsKey<T extends core::Object? = dynamic>(lowered final self::JsonMap2<self::JsonMap2|containsKey::T%> /* erasure=core::Map<core::String, core::Object?> */ #this, core::String key) → core::bool
+  return (#this as{Unchecked} core::Map<core::String, core::Object?>).{core::Map::containsKey}(key){(core::Object?) → core::bool};
+static extension-type-member method JsonMap2|get#containsKey<T extends core::Object? = dynamic>(lowered final self::JsonMap2<self::JsonMap2|get#containsKey::T%> /* erasure=core::Map<core::String, core::Object?> */ #this) → (core::String) → core::bool
+  return (core::String key) → core::bool => self::JsonMap2|containsKey<self::JsonMap2|get#containsKey::T%>(#this, key);
+static method parseJson2(self::JsonMap2<dynamic> /* erasure=core::Map<core::String, core::Object?> */ map) → void {
+  {
+    final synthesized self::JsonMap2<dynamic> /* erasure=core::Map<core::String, core::Object?> */ #0#0 = map;
+    synthesized core::Object? #0#3;
+    synthesized core::bool #0#3#isSet = false;
+    final const synthesized core::String #0#1 = #C1;
+    final const synthesized core::String #0#4 = #C2;
+    if((!((#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t3 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}) == null) || null is core::Object? && #0#0.{core::Map::containsKey}(#C1){(core::Object?) → core::bool}) && #C2 =={core::String::==}{(core::Object) → core::bool} (#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t4 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}))
+      core::print("ok");
+  }
+}
+static extension-type-member method JsonMap3|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::Map<core::String, core::Object?> _#wc2#formal) → self::JsonMap3<self::JsonMap3|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */ {
+  lowered final self::JsonMap3<self::JsonMap3|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */ #this = _#wc2#formal;
+  return #this;
+}
+static extension-type-member synthetic method JsonMap3|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _#wc2#formal) → self::JsonMap3<self::JsonMap3|constructor#_#new#tearOff::T%> /* erasure=core::Map<core::String, core::Object?> */
+  return self::JsonMap3|constructor#<self::JsonMap3|constructor#_#new#tearOff::T%>(_#wc2#formal);
+static extension-type-member method JsonMap3|get#length<T extends core::Object? = dynamic>(lowered final self::JsonMap3<self::JsonMap3|get#length::T%> /* erasure=core::Map<core::String, core::Object?> */ #this) → core::int
+  return (#this as{Unchecked} core::Map<core::String, core::Object?>).{core::Map::length}{core::int};
+static method parseJson3(self::JsonMap3<dynamic> /* erasure=core::Map<core::String, core::Object?> */ map) → void {
+  {
+    final synthesized self::JsonMap3<dynamic> /* erasure=core::Map<core::String, core::Object?> */ #0#0 = map;
+    synthesized core::Object? #0#3;
+    synthesized core::bool #0#3#isSet = false;
+    final const synthesized core::String #0#1 = #C1;
+    final const synthesized core::String #0#4 = #C2;
+    if((!((#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t5 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}) == null) || null is core::Object? && #0#0.{core::Map::containsKey}(#C1){(core::Object?) → core::bool}) && #C2 =={core::String::==}{(core::Object) → core::bool} (#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t6 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}))
+      core::print("ok");
+  }
+}
+static extension-type-member method JsonList1|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::List<core::Object?> _#wc3#formal) → self::JsonList1<self::JsonList1|constructor#::T%> /* erasure=core::List<core::Object?> */ {
+  lowered final self::JsonList1<self::JsonList1|constructor#::T%> /* erasure=core::List<core::Object?> */ #this = _#wc3#formal;
+  return #this;
+}
+static extension-type-member synthetic method JsonList1|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::List<core::Object?> _#wc3#formal) → self::JsonList1<self::JsonList1|constructor#_#new#tearOff::T%> /* erasure=core::List<core::Object?> */
+  return self::JsonList1|constructor#<self::JsonList1|constructor#_#new#tearOff::T%>(_#wc3#formal);
+static extension-type-member method JsonList1|[]<T extends core::Object? = dynamic>(lowered final self::JsonList1<self::JsonList1|[]::T%> /* erasure=core::List<core::Object?> */ #this, core::int index) → self::JsonList1|[]::T%
+  return (#this as{Unchecked} core::List<core::Object?>).{core::List::[]}(index){(core::int) → core::Object?} as self::JsonList1|[]::T%;
+static method parseJson4(self::JsonList1<dynamic> /* erasure=core::List<core::Object?> */ list) → void {
+  {
+    final synthesized self::JsonList1<dynamic> /* erasure=core::List<core::Object?> */ #0#0 = list;
+    final const synthesized core::int #0#2 = #C3;
+    final const synthesized core::String #0#5 = #C2;
+    if(#0#0.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C3 && #C2 =={core::String::==}{(core::Object) → core::bool} #0#0.{core::List::[]}(0){(core::int) → core::Object?})
+      core::print("ok");
+  }
+}
+static extension-type-member method JsonList2|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::List<core::Object?> _#wc4#formal) → self::JsonList2<self::JsonList2|constructor#::T%> /* erasure=core::List<core::Object?> */ {
+  lowered final self::JsonList2<self::JsonList2|constructor#::T%> /* erasure=core::List<core::Object?> */ #this = _#wc4#formal;
+  return #this;
+}
+static extension-type-member synthetic method JsonList2|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::List<core::Object?> _#wc4#formal) → self::JsonList2<self::JsonList2|constructor#_#new#tearOff::T%> /* erasure=core::List<core::Object?> */
+  return self::JsonList2|constructor#<self::JsonList2|constructor#_#new#tearOff::T%>(_#wc4#formal);
+static extension-type-member method JsonList2|get#length<T extends core::Object? = dynamic>(lowered final self::JsonList2<self::JsonList2|get#length::T%> /* erasure=core::List<core::Object?> */ #this) → core::int
+  return (#this as{Unchecked} core::List<core::Object?>).{core::List::length}{core::int};
+static method parseJson5(self::JsonList2<dynamic> /* erasure=core::List<core::Object?> */ list) → void {
+  {
+    final synthesized self::JsonList2<dynamic> /* erasure=core::List<core::Object?> */ #0#0 = list;
+    final const synthesized core::int #0#2 = #C3;
+    final const synthesized core::String #0#5 = #C2;
+    if(#0#0.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C3 && #C2 =={core::String::==}{(core::Object) → core::bool} #0#0.{core::List::[]}(0){(core::int) → core::Object?})
+      core::print("ok");
+  }
+}
+static extension-type-member method JsonList3|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::List<core::Object?> _#wc5#formal) → self::JsonList3<self::JsonList3|constructor#::T%> /* erasure=core::List<core::Object?> */ {
+  lowered final self::JsonList3<self::JsonList3|constructor#::T%> /* erasure=core::List<core::Object?> */ #this = _#wc5#formal;
+  return #this;
+}
+static extension-type-member synthetic method JsonList3|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::List<core::Object?> _#wc5#formal) → self::JsonList3<self::JsonList3|constructor#_#new#tearOff::T%> /* erasure=core::List<core::Object?> */
+  return self::JsonList3|constructor#<self::JsonList3|constructor#_#new#tearOff::T%>(_#wc5#formal);
+static extension-type-member method JsonList3|sublist<T extends core::Object? = dynamic>(lowered final self::JsonList3<self::JsonList3|sublist::T%> /* erasure=core::List<core::Object?> */ #this, core::int start, [core::int? end = #C4]) → core::List<self::JsonList3|sublist::T%>
+  return (#this as{Unchecked} core::List<core::Object?>).{core::List::sublist}(start, end){(core::int, [core::int?]) → core::List<core::Object?>}.{core::List::cast}<self::JsonList3|sublist::T%>(){() → core::List<self::JsonList3|sublist::T%>};
+static extension-type-member method JsonList3|get#sublist<T extends core::Object? = dynamic>(lowered final self::JsonList3<self::JsonList3|get#sublist::T%> /* erasure=core::List<core::Object?> */ #this) → (core::int, [core::int?]) → core::List<self::JsonList3|get#sublist::T%>
+  return (core::int start, [core::int? end = #C4]) → core::List<self::JsonList3|get#sublist::T%> => self::JsonList3|sublist<self::JsonList3|get#sublist::T%>(#this, start, end);
+static method parseJson6(self::JsonList3<dynamic> /* erasure=core::List<core::Object?> */ list) → void {
+  {
+    final synthesized self::JsonList3<dynamic> /* erasure=core::List<core::Object?> */ #0#0 = list;
+    final const synthesized core::int #0#2 = #C3;
+    final const synthesized core::String #0#5 = #C2;
+    if(#0#0.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C3 && #C2 =={core::String::==}{(core::Object) → core::bool} #0#0.{core::List::[]}(0){(core::int) → core::Object?})
+      core::print("ok");
+  }
+}
+
+constants  {
+  #C1 = "key"
+  #C2 = "value"
+  #C3 = 1
+  #C4 = null
+}
diff --git a/pkg/front_end/testcases/patterns/issue63315.dart.strong.outline.expect b/pkg/front_end/testcases/patterns/issue63315.dart.strong.outline.expect
new file mode 100644
index 00000000000..b94ed815c2f
--- /dev/null
+++ b/pkg/front_end/testcases/patterns/issue63315.dart.strong.outline.expect
@@ -0,0 +1,94 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+extension type JsonMap1<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _) implements core::Map<core::String, core::Object?> {
+  abstract extension-type-member representation-field get _() → core::Map<core::String, core::Object?>;
+  operator [] = self::JsonMap1|[];
+  constructor • = self::JsonMap1|constructor#;
+  constructor tearoff • = self::JsonMap1|constructor#_#new#tearOff;
+}
+extension type JsonMap2<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _) implements core::Map<core::String, core::Object?> {
+  abstract extension-type-member representation-field get _() → core::Map<core::String, core::Object?>;
+  method containsKey = self::JsonMap2|containsKey;
+  method tearoff containsKey = self::JsonMap2|get#containsKey;
+  constructor • = self::JsonMap2|constructor#;
+  constructor tearoff • = self::JsonMap2|constructor#_#new#tearOff;
+}
+extension type JsonMap3<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _) implements core::Map<core::String, core::Object?> {
+  abstract extension-type-member representation-field get _() → core::Map<core::String, core::Object?>;
+  get length = self::JsonMap3|get#length;
+  constructor • = self::JsonMap3|constructor#;
+  constructor tearoff • = self::JsonMap3|constructor#_#new#tearOff;
+}
+extension type JsonList1<T extends core::Object? = dynamic>(core::List<core::Object?> _) implements core::List<core::Object?> {
+  abstract extension-type-member representation-field get _() → core::List<core::Object?>;
+  operator [] = self::JsonList1|[];
+  constructor • = self::JsonList1|constructor#;
+  constructor tearoff • = self::JsonList1|constructor#_#new#tearOff;
+}
+extension type JsonList2<T extends core::Object? = dynamic>(core::List<core::Object?> _) implements core::List<core::Object?> {
+  abstract extension-type-member representation-field get _() → core::List<core::Object?>;
+  get length = self::JsonList2|get#length;
+  constructor • = self::JsonList2|constructor#;
+  constructor tearoff • = self::JsonList2|constructor#_#new#tearOff;
+}
+extension type JsonList3<T extends core::Object? = dynamic>(core::List<core::Object?> _) implements core::List<core::Object?> {
+  abstract extension-type-member representation-field get _() → core::List<core::Object?>;
+  method sublist = self::JsonList3|sublist;
+  method tearoff sublist = self::JsonList3|get#sublist;
+  constructor • = self::JsonList3|constructor#;
+  constructor tearoff • = self::JsonList3|constructor#_#new#tearOff;
+}
+static extension-type-member method JsonMap1|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::Map<core::String, core::Object?> _#wc0#formal) → self::JsonMap1<self::JsonMap1|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */
+  ;
+static extension-type-member synthetic method JsonMap1|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _#wc0#formal) → self::JsonMap1<self::JsonMap1|constructor#_#new#tearOff::T%> /* erasure=core::Map<core::String, core::Object?> */
+  return self::JsonMap1|constructor#<self::JsonMap1|constructor#_#new#tearOff::T%>(_#wc0#formal);
+static extension-type-member method JsonMap1|[]<T extends core::Object? = dynamic>(lowered final self::JsonMap1<self::JsonMap1|[]::T%> /* erasure=core::Map<core::String, core::Object?> */ #this, core::String key) → self::JsonMap1|[]::T%
+  ;
+static method parseJson1(self::JsonMap1<dynamic> /* erasure=core::Map<core::String, core::Object?> */ map) → void
+  ;
+static extension-type-member method JsonMap2|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::Map<core::String, core::Object?> _#wc1#formal) → self::JsonMap2<self::JsonMap2|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */
+  ;
+static extension-type-member synthetic method JsonMap2|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _#wc1#formal) → self::JsonMap2<self::JsonMap2|constructor#_#new#tearOff::T%> /* erasure=core::Map<core::String, core::Object?> */
+  return self::JsonMap2|constructor#<self::JsonMap2|constructor#_#new#tearOff::T%>(_#wc1#formal);
+static extension-type-member method JsonMap2|containsKey<T extends core::Object? = dynamic>(lowered final self::JsonMap2<self::JsonMap2|containsKey::T%> /* erasure=core::Map<core::String, core::Object?> */ #this, core::String key) → core::bool
+  ;
+static extension-type-member method JsonMap2|get#containsKey<T extends core::Object? = dynamic>(lowered final self::JsonMap2<self::JsonMap2|get#containsKey::T%> /* erasure=core::Map<core::String, core::Object?> */ #this) → (core::String) → core::bool
+  return (core::String key) → core::bool => self::JsonMap2|containsKey<self::JsonMap2|get#containsKey::T%>(#this, key);
+static method parseJson2(self::JsonMap2<dynamic> /* erasure=core::Map<core::String, core::Object?> */ map) → void
+  ;
+static extension-type-member method JsonMap3|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::Map<core::String, core::Object?> _#wc2#formal) → self::JsonMap3<self::JsonMap3|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */
+  ;
+static extension-type-member synthetic method JsonMap3|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _#wc2#formal) → self::JsonMap3<self::JsonMap3|constructor#_#new#tearOff::T%> /* erasure=core::Map<core::String, core::Object?> */
+  return self::JsonMap3|constructor#<self::JsonMap3|constructor#_#new#tearOff::T%>(_#wc2#formal);
+static extension-type-member method JsonMap3|get#length<T extends core::Object? = dynamic>(lowered final self::JsonMap3<self::JsonMap3|get#length::T%> /* erasure=core::Map<core::String, core::Object?> */ #this) → core::int
+  ;
+static method parseJson3(self::JsonMap3<dynamic> /* erasure=core::Map<core::String, core::Object?> */ map) → void
+  ;
+static extension-type-member method JsonList1|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::List<core::Object?> _#wc3#formal) → self::JsonList1<self::JsonList1|constructor#::T%> /* erasure=core::List<core::Object?> */
+  ;
+static extension-type-member synthetic method JsonList1|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::List<core::Object?> _#wc3#formal) → self::JsonList1<self::JsonList1|constructor#_#new#tearOff::T%> /* erasure=core::List<core::Object?> */
+  return self::JsonList1|constructor#<self::JsonList1|constructor#_#new#tearOff::T%>(_#wc3#formal);
+static extension-type-member method JsonList1|[]<T extends core::Object? = dynamic>(lowered final self::JsonList1<self::JsonList1|[]::T%> /* erasure=core::List<core::Object?> */ #this, core::int index) → self::JsonList1|[]::T%
+  ;
+static method parseJson4(self::JsonList1<dynamic> /* erasure=core::List<core::Object?> */ list) → void
+  ;
+static extension-type-member method JsonList2|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::List<core::Object?> _#wc4#formal) → self::JsonList2<self::JsonList2|constructor#::T%> /* erasure=core::List<core::Object?> */
+  ;
+static extension-type-member synthetic method JsonList2|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::List<core::Object?> _#wc4#formal) → self::JsonList2<self::JsonList2|constructor#_#new#tearOff::T%> /* erasure=core::List<core::Object?> */
+  return self::JsonList2|constructor#<self::JsonList2|constructor#_#new#tearOff::T%>(_#wc4#formal);
+static extension-type-member method JsonList2|get#length<T extends core::Object? = dynamic>(lowered final self::JsonList2<self::JsonList2|get#length::T%> /* erasure=core::List<core::Object?> */ #this) → core::int
+  ;
+static method parseJson5(self::JsonList2<dynamic> /* erasure=core::List<core::Object?> */ list) → void
+  ;
+static extension-type-member method JsonList3|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::List<core::Object?> _#wc5#formal) → self::JsonList3<self::JsonList3|constructor#::T%> /* erasure=core::List<core::Object?> */
+  ;
+static extension-type-member synthetic method JsonList3|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::List<core::Object?> _#wc5#formal) → self::JsonList3<self::JsonList3|constructor#_#new#tearOff::T%> /* erasure=core::List<core::Object?> */
+  return self::JsonList3|constructor#<self::JsonList3|constructor#_#new#tearOff::T%>(_#wc5#formal);
+static extension-type-member method JsonList3|sublist<T extends core::Object? = dynamic>(lowered final self::JsonList3<self::JsonList3|sublist::T%> /* erasure=core::List<core::Object?> */ #this, core::int start, [core::int? end]) → core::List<self::JsonList3|sublist::T%>
+  ;
+static extension-type-member method JsonList3|get#sublist<T extends core::Object? = dynamic>(lowered final self::JsonList3<self::JsonList3|get#sublist::T%> /* erasure=core::List<core::Object?> */ #this) → (core::int, [core::int?]) → core::List<self::JsonList3|get#sublist::T%>
+  return (core::int start, [core::int? end]) → core::List<self::JsonList3|get#sublist::T%> => self::JsonList3|sublist<self::JsonList3|get#sublist::T%>(#this, start, end);
+static method parseJson6(self::JsonList3<dynamic> /* erasure=core::List<core::Object?> */ list) → void
+  ;
diff --git a/pkg/front_end/testcases/patterns/issue63315.dart.strong.transformed.expect b/pkg/front_end/testcases/patterns/issue63315.dart.strong.transformed.expect
new file mode 100644
index 00000000000..2115f8de4d4
--- /dev/null
+++ b/pkg/front_end/testcases/patterns/issue63315.dart.strong.transformed.expect
@@ -0,0 +1,167 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+extension type JsonMap1<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _) implements core::Map<core::String, core::Object?> {
+  abstract extension-type-member representation-field get _() → core::Map<core::String, core::Object?>;
+  operator [] = self::JsonMap1|[];
+  constructor • = self::JsonMap1|constructor#;
+  constructor tearoff • = self::JsonMap1|constructor#_#new#tearOff;
+}
+extension type JsonMap2<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _) implements core::Map<core::String, core::Object?> {
+  abstract extension-type-member representation-field get _() → core::Map<core::String, core::Object?>;
+  method containsKey = self::JsonMap2|containsKey;
+  method tearoff containsKey = self::JsonMap2|get#containsKey;
+  constructor • = self::JsonMap2|constructor#;
+  constructor tearoff • = self::JsonMap2|constructor#_#new#tearOff;
+}
+extension type JsonMap3<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _) implements core::Map<core::String, core::Object?> {
+  abstract extension-type-member representation-field get _() → core::Map<core::String, core::Object?>;
+  get length = self::JsonMap3|get#length;
+  constructor • = self::JsonMap3|constructor#;
+  constructor tearoff • = self::JsonMap3|constructor#_#new#tearOff;
+}
+extension type JsonList1<T extends core::Object? = dynamic>(core::List<core::Object?> _) implements core::List<core::Object?> {
+  abstract extension-type-member representation-field get _() → core::List<core::Object?>;
+  operator [] = self::JsonList1|[];
+  constructor • = self::JsonList1|constructor#;
+  constructor tearoff • = self::JsonList1|constructor#_#new#tearOff;
+}
+extension type JsonList2<T extends core::Object? = dynamic>(core::List<core::Object?> _) implements core::List<core::Object?> {
+  abstract extension-type-member representation-field get _() → core::List<core::Object?>;
+  get length = self::JsonList2|get#length;
+  constructor • = self::JsonList2|constructor#;
+  constructor tearoff • = self::JsonList2|constructor#_#new#tearOff;
+}
+extension type JsonList3<T extends core::Object? = dynamic>(core::List<core::Object?> _) implements core::List<core::Object?> {
+  abstract extension-type-member representation-field get _() → core::List<core::Object?>;
+  method sublist = self::JsonList3|sublist;
+  method tearoff sublist = self::JsonList3|get#sublist;
+  constructor • = self::JsonList3|constructor#;
+  constructor tearoff • = self::JsonList3|constructor#_#new#tearOff;
+}
+static extension-type-member method JsonMap1|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::Map<core::String, core::Object?> _#wc0#formal) → self::JsonMap1<self::JsonMap1|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */ {
+  lowered final self::JsonMap1<self::JsonMap1|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */ #this = _#wc0#formal;
+  return #this;
+}
+static extension-type-member synthetic method JsonMap1|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _#wc0#formal) → self::JsonMap1<self::JsonMap1|constructor#_#new#tearOff::T%> /* erasure=core::Map<core::String, core::Object?> */
+  return self::JsonMap1|constructor#<self::JsonMap1|constructor#_#new#tearOff::T%>(_#wc0#formal);
+static extension-type-member method JsonMap1|[]<T extends core::Object? = dynamic>(lowered final self::JsonMap1<self::JsonMap1|[]::T%> /* erasure=core::Map<core::String, core::Object?> */ #this, core::String key) → self::JsonMap1|[]::T%
+  return (#this as{Unchecked} core::Map<core::String, core::Object?>).{core::Map::[]}(key){(core::Object?) → core::Object?} as self::JsonMap1|[]::T%;
+static method parseJson1(self::JsonMap1<dynamic> /* erasure=core::Map<core::String, core::Object?> */ map) → void {
+  {
+    final synthesized self::JsonMap1<dynamic> /* erasure=core::Map<core::String, core::Object?> */ #0#0 = map;
+    synthesized core::Object? #0#3;
+    synthesized core::bool #0#3#isSet = false;
+    final const synthesized core::String #0#1 = #C1;
+    final const synthesized core::String #0#4 = #C2;
+    if((!((#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t1 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}) == null) || null is core::Object? && #0#0.{core::Map::containsKey}(#C1){(core::Object?) → core::bool}) && #C2 =={core::String::==}{(core::Object) → core::bool} (#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t2 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}))
+      core::print("ok");
+  }
+}
+static extension-type-member method JsonMap2|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::Map<core::String, core::Object?> _#wc1#formal) → self::JsonMap2<self::JsonMap2|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */ {
+  lowered final self::JsonMap2<self::JsonMap2|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */ #this = _#wc1#formal;
+  return #this;
+}
+static extension-type-member synthetic method JsonMap2|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _#wc1#formal) → self::JsonMap2<self::JsonMap2|constructor#_#new#tearOff::T%> /* erasure=core::Map<core::String, core::Object?> */
+  return self::JsonMap2|constructor#<self::JsonMap2|constructor#_#new#tearOff::T%>(_#wc1#formal);
+static extension-type-member method JsonMap2|containsKey<T extends core::Object? = dynamic>(lowered final self::JsonMap2<self::JsonMap2|containsKey::T%> /* erasure=core::Map<core::String, core::Object?> */ #this, core::String key) → core::bool
+  return (#this as{Unchecked} core::Map<core::String, core::Object?>).{core::Map::containsKey}(key){(core::Object?) → core::bool};
+static extension-type-member method JsonMap2|get#containsKey<T extends core::Object? = dynamic>(lowered final self::JsonMap2<self::JsonMap2|get#containsKey::T%> /* erasure=core::Map<core::String, core::Object?> */ #this) → (core::String) → core::bool
+  return (core::String key) → core::bool => self::JsonMap2|containsKey<self::JsonMap2|get#containsKey::T%>(#this, key);
+static method parseJson2(self::JsonMap2<dynamic> /* erasure=core::Map<core::String, core::Object?> */ map) → void {
+  {
+    final synthesized self::JsonMap2<dynamic> /* erasure=core::Map<core::String, core::Object?> */ #0#0 = map;
+    synthesized core::Object? #0#3;
+    synthesized core::bool #0#3#isSet = false;
+    final const synthesized core::String #0#1 = #C1;
+    final const synthesized core::String #0#4 = #C2;
+    if((!((#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t3 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}) == null) || null is core::Object? && #0#0.{core::Map::containsKey}(#C1){(core::Object?) → core::bool}) && #C2 =={core::String::==}{(core::Object) → core::bool} (#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t4 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}))
+      core::print("ok");
+  }
+}
+static extension-type-member method JsonMap3|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::Map<core::String, core::Object?> _#wc2#formal) → self::JsonMap3<self::JsonMap3|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */ {
+  lowered final self::JsonMap3<self::JsonMap3|constructor#::T%> /* erasure=core::Map<core::String, core::Object?> */ #this = _#wc2#formal;
+  return #this;
+}
+static extension-type-member synthetic method JsonMap3|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::Map<core::String, core::Object?> _#wc2#formal) → self::JsonMap3<self::JsonMap3|constructor#_#new#tearOff::T%> /* erasure=core::Map<core::String, core::Object?> */
+  return self::JsonMap3|constructor#<self::JsonMap3|constructor#_#new#tearOff::T%>(_#wc2#formal);
+static extension-type-member method JsonMap3|get#length<T extends core::Object? = dynamic>(lowered final self::JsonMap3<self::JsonMap3|get#length::T%> /* erasure=core::Map<core::String, core::Object?> */ #this) → core::int
+  return (#this as{Unchecked} core::Map<core::String, core::Object?>).{core::Map::length}{core::int};
+static method parseJson3(self::JsonMap3<dynamic> /* erasure=core::Map<core::String, core::Object?> */ map) → void {
+  {
+    final synthesized self::JsonMap3<dynamic> /* erasure=core::Map<core::String, core::Object?> */ #0#0 = map;
+    synthesized core::Object? #0#3;
+    synthesized core::bool #0#3#isSet = false;
+    final const synthesized core::String #0#1 = #C1;
+    final const synthesized core::String #0#4 = #C2;
+    if((!((#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t5 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}) == null) || null is core::Object? && #0#0.{core::Map::containsKey}(#C1){(core::Object?) → core::bool}) && #C2 =={core::String::==}{(core::Object) → core::bool} (#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t6 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}))
+      core::print("ok");
+  }
+}
+static extension-type-member method JsonList1|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::List<core::Object?> _#wc3#formal) → self::JsonList1<self::JsonList1|constructor#::T%> /* erasure=core::List<core::Object?> */ {
+  lowered final self::JsonList1<self::JsonList1|constructor#::T%> /* erasure=core::List<core::Object?> */ #this = _#wc3#formal;
+  return #this;
+}
+static extension-type-member synthetic method JsonList1|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::List<core::Object?> _#wc3#formal) → self::JsonList1<self::JsonList1|constructor#_#new#tearOff::T%> /* erasure=core::List<core::Object?> */
+  return self::JsonList1|constructor#<self::JsonList1|constructor#_#new#tearOff::T%>(_#wc3#formal);
+static extension-type-member method JsonList1|[]<T extends core::Object? = dynamic>(lowered final self::JsonList1<self::JsonList1|[]::T%> /* erasure=core::List<core::Object?> */ #this, core::int index) → self::JsonList1|[]::T%
+  return (#this as{Unchecked} core::List<core::Object?>).{core::List::[]}(index){(core::int) → core::Object?} as self::JsonList1|[]::T%;
+static method parseJson4(self::JsonList1<dynamic> /* erasure=core::List<core::Object?> */ list) → void {
+  {
+    final synthesized self::JsonList1<dynamic> /* erasure=core::List<core::Object?> */ #0#0 = list;
+    final const synthesized core::int #0#2 = #C3;
+    final const synthesized core::String #0#5 = #C2;
+    if(#0#0.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C3 && #C2 =={core::String::==}{(core::Object) → core::bool} #0#0.{core::List::[]}(0){(core::int) → core::Object?})
+      core::print("ok");
+  }
+}
+static extension-type-member method JsonList2|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::List<core::Object?> _#wc4#formal) → self::JsonList2<self::JsonList2|constructor#::T%> /* erasure=core::List<core::Object?> */ {
+  lowered final self::JsonList2<self::JsonList2|constructor#::T%> /* erasure=core::List<core::Object?> */ #this = _#wc4#formal;
+  return #this;
+}
+static extension-type-member synthetic method JsonList2|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::List<core::Object?> _#wc4#formal) → self::JsonList2<self::JsonList2|constructor#_#new#tearOff::T%> /* erasure=core::List<core::Object?> */
+  return self::JsonList2|constructor#<self::JsonList2|constructor#_#new#tearOff::T%>(_#wc4#formal);
+static extension-type-member method JsonList2|get#length<T extends core::Object? = dynamic>(lowered final self::JsonList2<self::JsonList2|get#length::T%> /* erasure=core::List<core::Object?> */ #this) → core::int
+  return (#this as{Unchecked} core::List<core::Object?>).{core::List::length}{core::int};
+static method parseJson5(self::JsonList2<dynamic> /* erasure=core::List<core::Object?> */ list) → void {
+  {
+    final synthesized self::JsonList2<dynamic> /* erasure=core::List<core::Object?> */ #0#0 = list;
+    final const synthesized core::int #0#2 = #C3;
+    final const synthesized core::String #0#5 = #C2;
+    if(#0#0.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C3 && #C2 =={core::String::==}{(core::Object) → core::bool} #0#0.{core::List::[]}(0){(core::int) → core::Object?})
+      core::print("ok");
+  }
+}
+static extension-type-member method JsonList3|constructor#<T extends core::Object? = dynamic>(wildcard initializing-formal core::List<core::Object?> _#wc5#formal) → self::JsonList3<self::JsonList3|constructor#::T%> /* erasure=core::List<core::Object?> */ {
+  lowered final self::JsonList3<self::JsonList3|constructor#::T%> /* erasure=core::List<core::Object?> */ #this = _#wc5#formal;
+  return #this;
+}
+static extension-type-member synthetic method JsonList3|constructor#_#new#tearOff<T extends core::Object? = dynamic>(core::List<core::Object?> _#wc5#formal) → self::JsonList3<self::JsonList3|constructor#_#new#tearOff::T%> /* erasure=core::List<core::Object?> */
+  return self::JsonList3|constructor#<self::JsonList3|constructor#_#new#tearOff::T%>(_#wc5#formal);
+static extension-type-member method JsonList3|sublist<T extends core::Object? = dynamic>(lowered final self::JsonList3<self::JsonList3|sublist::T%> /* erasure=core::List<core::Object?> */ #this, core::int start, [core::int? end = #C4]) → core::List<self::JsonList3|sublist::T%>
+  return (#this as{Unchecked} core::List<core::Object?>).{core::List::sublist}(start, end){(core::int, [core::int?]) → core::List<core::Object?>}.{core::List::cast}<self::JsonList3|sublist::T%>(){() → core::List<self::JsonList3|sublist::T%>};
+static extension-type-member method JsonList3|get#sublist<T extends core::Object? = dynamic>(lowered final self::JsonList3<self::JsonList3|get#sublist::T%> /* erasure=core::List<core::Object?> */ #this) → (core::int, [core::int?]) → core::List<self::JsonList3|get#sublist::T%>
+  return (core::int start, [core::int? end = #C4]) → core::List<self::JsonList3|get#sublist::T%> => self::JsonList3|sublist<self::JsonList3|get#sublist::T%>(#this, start, end);
+static method parseJson6(self::JsonList3<dynamic> /* erasure=core::List<core::Object?> */ list) → void {
+  {
+    final synthesized self::JsonList3<dynamic> /* erasure=core::List<core::Object?> */ #0#0 = list;
+    final const synthesized core::int #0#2 = #C3;
+    final const synthesized core::String #0#5 = #C2;
+    if(#0#0.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C3 && #C2 =={core::String::==}{(core::Object) → core::bool} #0#0.{core::List::[]}(0){(core::int) → core::Object?})
+      core::print("ok");
+  }
+}
+
+constants  {
+  #C1 = "key"
+  #C2 = "value"
+  #C3 = 1
+  #C4 = null
+}
+
+Extra constant evaluation status:
+Evaluated: IsExpression @ org-dartlang-testcase:///issue63315.dart:11:22 -> BoolConstant(true)
+Evaluated: IsExpression @ org-dartlang-testcase:///issue63315.dart:20:22 -> BoolConstant(true)
+Evaluated: IsExpression @ org-dartlang-testcase:///issue63315.dart:29:22 -> BoolConstant(true)
+Extra constant evaluation: evaluated: 167, effectively constant: 3
diff --git a/pkg/front_end/testcases/patterns/issue63315.dart.textual_outline.expect b/pkg/front_end/testcases/patterns/issue63315.dart.textual_outline.expect
new file mode 100644
index 00000000000..8374d3b0d9f
--- /dev/null
+++ b/pkg/front_end/testcases/patterns/issue63315.dart.textual_outline.expect
@@ -0,0 +1,38 @@
+extension type JsonMap1<T>(Map<String, Object?> _)
+    implements Map<String, Object?> {
+  T operator [](String key) => _[key] as T;
+}
+
+void parseJson1(JsonMap1 map) {}
+
+extension type JsonMap2<T>(Map<String, Object?> _)
+    implements Map<String, Object?> {
+  bool containsKey(String key) => _.containsKey(key);
+}
+
+void parseJson2(JsonMap2 map) {}
+
+extension type JsonMap3<T>(Map<String, Object?> _)
+    implements Map<String, Object?> {
+  int get length => _.length;
+}
+
+void parseJson3(JsonMap3 map) {}
+
+extension type JsonList1<T>(List<Object?> _) implements List<Object?> {
+  T operator [](int index) => _[index] as T;
+}
+
+void parseJson4(JsonList1 list) {}
+
+extension type JsonList2<T>(List<Object?> _) implements List<Object?> {
+  int get length => _.length;
+}
+
+void parseJson5(JsonList2 list) {}
+
+extension type JsonList3<T>(List<Object?> _) implements List<Object?> {
+  List<T> sublist(int start, [int? end]) => _.sublist(start, end).cast<T>();
+}
+
+void parseJson6(JsonList3 list) {}
diff --git a/pkg/front_end/testcases/patterns/issue63315.dart.textual_outline_modelled.expect b/pkg/front_end/testcases/patterns/issue63315.dart.textual_outline_modelled.expect
new file mode 100644
index 00000000000..c829442998d
--- /dev/null
+++ b/pkg/front_end/testcases/patterns/issue63315.dart.textual_outline_modelled.expect
@@ -0,0 +1,38 @@
+extension type JsonList1<T>(List<Object?> _) implements List<Object?> {
+  T operator [](int index) => _[index] as T;
+}
+
+extension type JsonList2<T>(List<Object?> _) implements List<Object?> {
+  int get length => _.length;
+}
+
+extension type JsonList3<T>(List<Object?> _) implements List<Object?> {
+  List<T> sublist(int start, [int? end]) => _.sublist(start, end).cast<T>();
+}
+
+extension type JsonMap1<T>(Map<String, Object?> _)
+    implements Map<String, Object?> {
+  T operator [](String key) => _[key] as T;
+}
+
+extension type JsonMap2<T>(Map<String, Object?> _)
+    implements Map<String, Object?> {
+  bool containsKey(String key) => _.containsKey(key);
+}
+
+extension type JsonMap3<T>(Map<String, Object?> _)
+    implements Map<String, Object?> {
+  int get length => _.length;
+}
+
+void parseJson1(JsonMap1 map) {}
+
+void parseJson2(JsonMap2 map) {}
+
+void parseJson3(JsonMap3 map) {}
+
+void parseJson4(JsonList1 list) {}
+
+void parseJson5(JsonList2 list) {}
+
+void parseJson6(JsonList3 list) {}
diff --git a/pkg/front_end/testcases/patterns/map_lookup.dart.strong.expect b/pkg/front_end/testcases/patterns/map_lookup.dart.strong.expect
index 09dd4f600ac..7189d77393c 100644
--- a/pkg/front_end/testcases/patterns/map_lookup.dart.strong.expect
+++ b/pkg/front_end/testcases/patterns/map_lookup.dart.strong.expect
@@ -155,7 +155,7 @@ static method main() → dynamic {
     synthesized core::int? #0#6;
     synthesized core::bool #0#6#isSet = false;
     final const synthesized core::String #0#4 = #C7;
-    if(!((!((#0#3#isSet ?{core::int?} #0#3 : let final dynamic #t30 = #0#3#isSet = true in #0#3 = #0#0.{self::MyMap::[]}(#C5){(core::Object?) → core::int?}) == null) || null is core::int && #0#0.{self::MyMap::containsKey}(#C5){(core::Object?) → core::bool}) && (let final dynamic #t31 = x1 = (#0#3#isSet ?{core::int?} #0#3 : let final dynamic #t32 = #0#3#isSet = true in #0#3 = #0#0.{self::MyMap::[]}(#C5){(core::Object?) → core::int?}) as{Unchecked} core::int in true) && (!((#0#6#isSet ?{core::int?} #0#6 : let final dynamic #t33 = #0#6#isSet = true in #0#6 = #0#0.{self::MyMap::[]}(#C7){(core::Object?) → core::int?}) == null) || null is core::int && #0#0.{self::MyMap::containsKey}(#C7){(core::Object?) → core::bool}) && (let final dynamic #t34 = x2 = (#0#6#isSet ?{core::int?} #0#6 : let final dynamic #t35 = #0#6#isSet = true in #0#6 = #0#0.{self::MyMap::[]}(#C7){(core::Object?) → core::int?}) as{Unchecked} core::int in true)))
+    if(!((!((#0#3#isSet ?{core::int?} #0#3 : let final dynamic #t30 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C5){(core::Object?) → core::int?}) == null) || null is core::int && #0#0.{core::Map::containsKey}(#C5){(core::Object?) → core::bool}) && (let final dynamic #t31 = x1 = (#0#3#isSet ?{core::int?} #0#3 : let final dynamic #t32 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C5){(core::Object?) → core::int?}) as{Unchecked} core::int in true) && (!((#0#6#isSet ?{core::int?} #0#6 : let final dynamic #t33 = #0#6#isSet = true in #0#6 = #0#0.{core::Map::[]}(#C7){(core::Object?) → core::int?}) == null) || null is core::int && #0#0.{core::Map::containsKey}(#C7){(core::Object?) → core::bool}) && (let final dynamic #t34 = x2 = (#0#6#isSet ?{core::int?} #0#6 : let final dynamic #t35 = #0#6#isSet = true in #0#6 = #0#0.{core::Map::[]}(#C7){(core::Object?) → core::int?}) as{Unchecked} core::int in true)))
       throw{for-error-handling} new core::StateError::•("Pattern matching error");
   }
   self::expect(self::hasUnsoundNullSafety ?{core::String} self::unsoundResult : self::soundResult, map.{self::MyMap::log}{core::String});
@@ -170,7 +170,7 @@ static method main() → dynamic {
     synthesized core::int? #1#6;
     synthesized core::bool #1#6#isSet = false;
     final const synthesized core::String #1#4 = #C7;
-    if(!((!((#1#3#isSet ?{core::int?} #1#3 : let final dynamic #t36 = #1#3#isSet = true in #1#3 = #1#0.{self::MyMap::[]}(#C5){(core::Object?) → core::int?}) == null) || null is core::int && #1#0.{self::MyMap::containsKey}(#C5){(core::Object?) → core::bool}) && (let final dynamic #t37 = y1 = (#1#3#isSet ?{core::int?} #1#3 : let final dynamic #t38 = #1#3#isSet = true in #1#3 = #1#0.{self::MyMap::[]}(#C5){(core::Object?) → core::int?}) as{Unchecked} core::int in true) && (!((#1#6#isSet ?{core::int?} #1#6 : let final dynamic #t39 = #1#6#isSet = true in #1#6 = #1#0.{self::MyMap::[]}(#C7){(core::Object?) → core::int?}) == null) || null is core::int && #1#0.{self::MyMap::containsKey}(#C7){(core::Object?) → core::bool}) && (let final dynamic #t40 = y2 = (#1#6#isSet ?{core::int?} #1#6 : let final dynamic #t41 = #1#6#isSet = true in #1#6 = #1#0.{self::MyMap::[]}(#C7){(core::Object?) → core::int?}) as{Unchecked} core::int in true)))
+    if(!((!((#1#3#isSet ?{core::int?} #1#3 : let final dynamic #t36 = #1#3#isSet = true in #1#3 = #1#0.{core::Map::[]}(#C5){(core::Object?) → core::int?}) == null) || null is core::int && #1#0.{core::Map::containsKey}(#C5){(core::Object?) → core::bool}) && (let final dynamic #t37 = y1 = (#1#3#isSet ?{core::int?} #1#3 : let final dynamic #t38 = #1#3#isSet = true in #1#3 = #1#0.{core::Map::[]}(#C5){(core::Object?) → core::int?}) as{Unchecked} core::int in true) && (!((#1#6#isSet ?{core::int?} #1#6 : let final dynamic #t39 = #1#6#isSet = true in #1#6 = #1#0.{core::Map::[]}(#C7){(core::Object?) → core::int?}) == null) || null is core::int && #1#0.{core::Map::containsKey}(#C7){(core::Object?) → core::bool}) && (let final dynamic #t40 = y2 = (#1#6#isSet ?{core::int?} #1#6 : let final dynamic #t41 = #1#6#isSet = true in #1#6 = #1#0.{core::Map::[]}(#C7){(core::Object?) → core::int?}) as{Unchecked} core::int in true)))
       throw{for-error-handling} new core::StateError::•("Pattern matching error");
   }
   self::expect(self::hasUnsoundNullSafety ?{core::String} self::unsoundResult : self::soundResult, map.{self::MyMap::log}{core::String});
diff --git a/pkg/front_end/testcases/patterns/map_lookup.dart.strong.modular.expect b/pkg/front_end/testcases/patterns/map_lookup.dart.strong.modular.expect
index 09dd4f600ac..7189d77393c 100644
--- a/pkg/front_end/testcases/patterns/map_lookup.dart.strong.modular.expect
+++ b/pkg/front_end/testcases/patterns/map_lookup.dart.strong.modular.expect
@@ -155,7 +155,7 @@ static method main() → dynamic {
     synthesized core::int? #0#6;
     synthesized core::bool #0#6#isSet = false;
     final const synthesized core::String #0#4 = #C7;
-    if(!((!((#0#3#isSet ?{core::int?} #0#3 : let final dynamic #t30 = #0#3#isSet = true in #0#3 = #0#0.{self::MyMap::[]}(#C5){(core::Object?) → core::int?}) == null) || null is core::int && #0#0.{self::MyMap::containsKey}(#C5){(core::Object?) → core::bool}) && (let final dynamic #t31 = x1 = (#0#3#isSet ?{core::int?} #0#3 : let final dynamic #t32 = #0#3#isSet = true in #0#3 = #0#0.{self::MyMap::[]}(#C5){(core::Object?) → core::int?}) as{Unchecked} core::int in true) && (!((#0#6#isSet ?{core::int?} #0#6 : let final dynamic #t33 = #0#6#isSet = true in #0#6 = #0#0.{self::MyMap::[]}(#C7){(core::Object?) → core::int?}) == null) || null is core::int && #0#0.{self::MyMap::containsKey}(#C7){(core::Object?) → core::bool}) && (let final dynamic #t34 = x2 = (#0#6#isSet ?{core::int?} #0#6 : let final dynamic #t35 = #0#6#isSet = true in #0#6 = #0#0.{self::MyMap::[]}(#C7){(core::Object?) → core::int?}) as{Unchecked} core::int in true)))
+    if(!((!((#0#3#isSet ?{core::int?} #0#3 : let final dynamic #t30 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C5){(core::Object?) → core::int?}) == null) || null is core::int && #0#0.{core::Map::containsKey}(#C5){(core::Object?) → core::bool}) && (let final dynamic #t31 = x1 = (#0#3#isSet ?{core::int?} #0#3 : let final dynamic #t32 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C5){(core::Object?) → core::int?}) as{Unchecked} core::int in true) && (!((#0#6#isSet ?{core::int?} #0#6 : let final dynamic #t33 = #0#6#isSet = true in #0#6 = #0#0.{core::Map::[]}(#C7){(core::Object?) → core::int?}) == null) || null is core::int && #0#0.{core::Map::containsKey}(#C7){(core::Object?) → core::bool}) && (let final dynamic #t34 = x2 = (#0#6#isSet ?{core::int?} #0#6 : let final dynamic #t35 = #0#6#isSet = true in #0#6 = #0#0.{core::Map::[]}(#C7){(core::Object?) → core::int?}) as{Unchecked} core::int in true)))
       throw{for-error-handling} new core::StateError::•("Pattern matching error");
   }
   self::expect(self::hasUnsoundNullSafety ?{core::String} self::unsoundResult : self::soundResult, map.{self::MyMap::log}{core::String});
@@ -170,7 +170,7 @@ static method main() → dynamic {
     synthesized core::int? #1#6;
     synthesized core::bool #1#6#isSet = false;
     final const synthesized core::String #1#4 = #C7;
-    if(!((!((#1#3#isSet ?{core::int?} #1#3 : let final dynamic #t36 = #1#3#isSet = true in #1#3 = #1#0.{self::MyMap::[]}(#C5){(core::Object?) → core::int?}) == null) || null is core::int && #1#0.{self::MyMap::containsKey}(#C5){(core::Object?) → core::bool}) && (let final dynamic #t37 = y1 = (#1#3#isSet ?{core::int?} #1#3 : let final dynamic #t38 = #1#3#isSet = true in #1#3 = #1#0.{self::MyMap::[]}(#C5){(core::Object?) → core::int?}) as{Unchecked} core::int in true) && (!((#1#6#isSet ?{core::int?} #1#6 : let final dynamic #t39 = #1#6#isSet = true in #1#6 = #1#0.{self::MyMap::[]}(#C7){(core::Object?) → core::int?}) == null) || null is core::int && #1#0.{self::MyMap::containsKey}(#C7){(core::Object?) → core::bool}) && (let final dynamic #t40 = y2 = (#1#6#isSet ?{core::int?} #1#6 : let final dynamic #t41 = #1#6#isSet = true in #1#6 = #1#0.{self::MyMap::[]}(#C7){(core::Object?) → core::int?}) as{Unchecked} core::int in true)))
+    if(!((!((#1#3#isSet ?{core::int?} #1#3 : let final dynamic #t36 = #1#3#isSet = true in #1#3 = #1#0.{core::Map::[]}(#C5){(core::Object?) → core::int?}) == null) || null is core::int && #1#0.{core::Map::containsKey}(#C5){(core::Object?) → core::bool}) && (let final dynamic #t37 = y1 = (#1#3#isSet ?{core::int?} #1#3 : let final dynamic #t38 = #1#3#isSet = true in #1#3 = #1#0.{core::Map::[]}(#C5){(core::Object?) → core::int?}) as{Unchecked} core::int in true) && (!((#1#6#isSet ?{core::int?} #1#6 : let final dynamic #t39 = #1#6#isSet = true in #1#6 = #1#0.{core::Map::[]}(#C7){(core::Object?) → core::int?}) == null) || null is core::int && #1#0.{core::Map::containsKey}(#C7){(core::Object?) → core::bool}) && (let final dynamic #t40 = y2 = (#1#6#isSet ?{core::int?} #1#6 : let final dynamic #t41 = #1#6#isSet = true in #1#6 = #1#0.{core::Map::[]}(#C7){(core::Object?) → core::int?}) as{Unchecked} core::int in true)))
       throw{for-error-handling} new core::StateError::•("Pattern matching error");
   }
   self::expect(self::hasUnsoundNullSafety ?{core::String} self::unsoundResult : self::soundResult, map.{self::MyMap::log}{core::String});
diff --git a/pkg/front_end/testcases/patterns/map_lookup.dart.strong.transformed.expect b/pkg/front_end/testcases/patterns/map_lookup.dart.strong.transformed.expect
index a8b6f858589..978064d0f93 100644
--- a/pkg/front_end/testcases/patterns/map_lookup.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/patterns/map_lookup.dart.strong.transformed.expect
@@ -155,7 +155,7 @@ static method main() → dynamic {
     synthesized core::int? #0#6;
     synthesized core::bool #0#6#isSet = false;
     final const synthesized core::String #0#4 = #C7;
-    if(!((!((#0#3#isSet ?{core::int?} #0#3 : let final dynamic #t38 = #0#3#isSet = true in #0#3 = #0#0.{self::MyMap::[]}(#C5){(core::Object?) → core::int?}) == null) || null is core::int && #0#0.{self::MyMap::containsKey}(#C5){(core::Object?) → core::bool}) && (let final dynamic #t39 = x1 = let core::int? #t40 = #0#3#isSet ?{core::int?} #0#3 : let final dynamic #t41 = #0#3#isSet = true in #0#3 = #0#0.{self::MyMap::[]}(#C5){(core::Object?) → core::int?} in #t40 == null ?{core::int} #t40 as{Unchecked} core::int : #t40{core::int} in true) && (!((#0#6#isSet ?{core::int?} #0#6 : let final dynamic #t42 = #0#6#isSet = true in #0#6 = #0#0.{self::MyMap::[]}(#C7){(core::Object?) → core::int?}) == null) || null is core::int && #0#0.{self::MyMap::containsKey}(#C7){(core::Object?) → core::bool}) && (let final dynamic #t43 = x2 = let core::int? #t44 = #0#6#isSet ?{core::int?} #0#6 : let final dynamic #t45 = #0#6#isSet = true in #0#6 = #0#0.{self::MyMap::[]}(#C7){(core::Object?) → core::int?} in #t44 == null ?{core::int} #t44 as{Unchecked} core::int : #t44{core::int} in true)))
+    if(!((!((#0#3#isSet ?{core::int?} #0#3 : let final dynamic #t38 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C5){(core::Object?) → core::int?}) == null) || null is core::int && #0#0.{core::Map::containsKey}(#C5){(core::Object?) → core::bool}) && (let final dynamic #t39 = x1 = let core::int? #t40 = #0#3#isSet ?{core::int?} #0#3 : let final dynamic #t41 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C5){(core::Object?) → core::int?} in #t40 == null ?{core::int} #t40 as{Unchecked} core::int : #t40{core::int} in true) && (!((#0#6#isSet ?{core::int?} #0#6 : let final dynamic #t42 = #0#6#isSet = true in #0#6 = #0#0.{core::Map::[]}(#C7){(core::Object?) → core::int?}) == null) || null is core::int && #0#0.{core::Map::containsKey}(#C7){(core::Object?) → core::bool}) && (let final dynamic #t43 = x2 = let core::int? #t44 = #0#6#isSet ?{core::int?} #0#6 : let final dynamic #t45 = #0#6#isSet = true in #0#6 = #0#0.{core::Map::[]}(#C7){(core::Object?) → core::int?} in #t44 == null ?{core::int} #t44 as{Unchecked} core::int : #t44{core::int} in true)))
       throw{for-error-handling} new core::StateError::•("Pattern matching error");
   }
   self::expect(self::hasUnsoundNullSafety ?{core::String} self::unsoundResult : self::soundResult, map.{self::MyMap::log}{core::String});
@@ -170,7 +170,7 @@ static method main() → dynamic {
     synthesized core::int? #1#6;
     synthesized core::bool #1#6#isSet = false;
     final const synthesized core::String #1#4 = #C7;
-    if(!((!((#1#3#isSet ?{core::int?} #1#3 : let final dynamic #t46 = #1#3#isSet = true in #1#3 = #1#0.{self::MyMap::[]}(#C5){(core::Object?) → core::int?}) == null) || null is core::int && #1#0.{self::MyMap::containsKey}(#C5){(core::Object?) → core::bool}) && (let final dynamic #t47 = y1 = let core::int? #t48 = #1#3#isSet ?{core::int?} #1#3 : let final dynamic #t49 = #1#3#isSet = true in #1#3 = #1#0.{self::MyMap::[]}(#C5){(core::Object?) → core::int?} in #t48 == null ?{core::int} #t48 as{Unchecked} core::int : #t48{core::int} in true) && (!((#1#6#isSet ?{core::int?} #1#6 : let final dynamic #t50 = #1#6#isSet = true in #1#6 = #1#0.{self::MyMap::[]}(#C7){(core::Object?) → core::int?}) == null) || null is core::int && #1#0.{self::MyMap::containsKey}(#C7){(core::Object?) → core::bool}) && (let final dynamic #t51 = y2 = let core::int? #t52 = #1#6#isSet ?{core::int?} #1#6 : let final dynamic #t53 = #1#6#isSet = true in #1#6 = #1#0.{self::MyMap::[]}(#C7){(core::Object?) → core::int?} in #t52 == null ?{core::int} #t52 as{Unchecked} core::int : #t52{core::int} in true)))
+    if(!((!((#1#3#isSet ?{core::int?} #1#3 : let final dynamic #t46 = #1#3#isSet = true in #1#3 = #1#0.{core::Map::[]}(#C5){(core::Object?) → core::int?}) == null) || null is core::int && #1#0.{core::Map::containsKey}(#C5){(core::Object?) → core::bool}) && (let final dynamic #t47 = y1 = let core::int? #t48 = #1#3#isSet ?{core::int?} #1#3 : let final dynamic #t49 = #1#3#isSet = true in #1#3 = #1#0.{core::Map::[]}(#C5){(core::Object?) → core::int?} in #t48 == null ?{core::int} #t48 as{Unchecked} core::int : #t48{core::int} in true) && (!((#1#6#isSet ?{core::int?} #1#6 : let final dynamic #t50 = #1#6#isSet = true in #1#6 = #1#0.{core::Map::[]}(#C7){(core::Object?) → core::int?}) == null) || null is core::int && #1#0.{core::Map::containsKey}(#C7){(core::Object?) → core::bool}) && (let final dynamic #t51 = y2 = let core::int? #t52 = #1#6#isSet ?{core::int?} #1#6 : let final dynamic #t53 = #1#6#isSet = true in #1#6 = #1#0.{core::Map::[]}(#C7){(core::Object?) → core::int?} in #t52 == null ?{core::int} #t52 as{Unchecked} core::int : #t52{core::int} in true)))
       throw{for-error-handling} new core::StateError::•("Pattern matching error");
   }
   self::expect(self::hasUnsoundNullSafety ?{core::String} self::unsoundResult : self::soundResult, map.{self::MyMap::log}{core::String});
diff --git a/pkg/front_end/testcases/patterns/matching_and_capturing_accesses_list_element_once.dart.strong.expect b/pkg/front_end/testcases/patterns/matching_and_capturing_accesses_list_element_once.dart.strong.expect
index 1adcbb0a62f..2610799ab1f 100644
--- a/pkg/front_end/testcases/patterns/matching_and_capturing_accesses_list_element_once.dart.strong.expect
+++ b/pkg/front_end/testcases/patterns/matching_and_capturing_accesses_list_element_once.dart.strong.expect
@@ -157,8 +157,8 @@ static method main() → dynamic {
     {
       hoisted core::int x;
       hoisted core::int y;
-      if(#0#0.{self::A::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C5 && ((#0#4#isSet ?{core::int} #0#4 : let final dynamic #t1 = #0#4#isSet = true in #0#4 = #0#0.{self::A::[]}(0){(core::int) → core::int}) is core::int && (let final dynamic #t2 = x = #0#4#isSet ?{core::int} #0#4 : let final dynamic #t3 = #0#4#isSet = true in #0#4 = #0#0.{self::A::[]}(0){(core::int) → core::int} in true)) && (#0#5#isSet ?{core::int} #0#5 : let final dynamic #t4 = #0#5#isSet = true in #0#5 = #0#0.{self::A::[]}(1){(core::int) → core::int}) is core::int) {
-        y = #0#5#isSet ?{core::int} #0#5 : let final dynamic #t5 = #0#5#isSet = true in #0#5 = #0#0.{self::A::[]}(1){(core::int) → core::int};
+      if(#0#0.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C5 && ((#0#4#isSet ?{core::int} #0#4 : let final dynamic #t1 = #0#4#isSet = true in #0#4 = #0#0.{core::List::[]}(0){(core::int) → core::int}) is core::int && (let final dynamic #t2 = x = #0#4#isSet ?{core::int} #0#4 : let final dynamic #t3 = #0#4#isSet = true in #0#4 = #0#0.{core::List::[]}(0){(core::int) → core::int} in true)) && (#0#5#isSet ?{core::int} #0#5 : let final dynamic #t4 = #0#5#isSet = true in #0#5 = #0#0.{core::List::[]}(1){(core::int) → core::int}) is core::int) {
+        y = #0#5#isSet ?{core::int} #0#5 : let final dynamic #t5 = #0#5#isSet = true in #0#5 = #0#0.{core::List::[]}(1){(core::int) → core::int};
         {
           self::expectEquals(x, 0);
           self::expectEquals(y, 0);
diff --git a/pkg/front_end/testcases/patterns/matching_and_capturing_accesses_list_element_once.dart.strong.modular.expect b/pkg/front_end/testcases/patterns/matching_and_capturing_accesses_list_element_once.dart.strong.modular.expect
index 1adcbb0a62f..2610799ab1f 100644
--- a/pkg/front_end/testcases/patterns/matching_and_capturing_accesses_list_element_once.dart.strong.modular.expect
+++ b/pkg/front_end/testcases/patterns/matching_and_capturing_accesses_list_element_once.dart.strong.modular.expect
@@ -157,8 +157,8 @@ static method main() → dynamic {
     {
       hoisted core::int x;
       hoisted core::int y;
-      if(#0#0.{self::A::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C5 && ((#0#4#isSet ?{core::int} #0#4 : let final dynamic #t1 = #0#4#isSet = true in #0#4 = #0#0.{self::A::[]}(0){(core::int) → core::int}) is core::int && (let final dynamic #t2 = x = #0#4#isSet ?{core::int} #0#4 : let final dynamic #t3 = #0#4#isSet = true in #0#4 = #0#0.{self::A::[]}(0){(core::int) → core::int} in true)) && (#0#5#isSet ?{core::int} #0#5 : let final dynamic #t4 = #0#5#isSet = true in #0#5 = #0#0.{self::A::[]}(1){(core::int) → core::int}) is core::int) {
-        y = #0#5#isSet ?{core::int} #0#5 : let final dynamic #t5 = #0#5#isSet = true in #0#5 = #0#0.{self::A::[]}(1){(core::int) → core::int};
+      if(#0#0.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C5 && ((#0#4#isSet ?{core::int} #0#4 : let final dynamic #t1 = #0#4#isSet = true in #0#4 = #0#0.{core::List::[]}(0){(core::int) → core::int}) is core::int && (let final dynamic #t2 = x = #0#4#isSet ?{core::int} #0#4 : let final dynamic #t3 = #0#4#isSet = true in #0#4 = #0#0.{core::List::[]}(0){(core::int) → core::int} in true)) && (#0#5#isSet ?{core::int} #0#5 : let final dynamic #t4 = #0#5#isSet = true in #0#5 = #0#0.{core::List::[]}(1){(core::int) → core::int}) is core::int) {
+        y = #0#5#isSet ?{core::int} #0#5 : let final dynamic #t5 = #0#5#isSet = true in #0#5 = #0#0.{core::List::[]}(1){(core::int) → core::int};
         {
           self::expectEquals(x, 0);
           self::expectEquals(y, 0);
diff --git a/pkg/front_end/testcases/patterns/matching_and_capturing_accesses_list_element_once.dart.strong.transformed.expect b/pkg/front_end/testcases/patterns/matching_and_capturing_accesses_list_element_once.dart.strong.transformed.expect
index 4fc3aa2708c..ab9e2c9b799 100644
--- a/pkg/front_end/testcases/patterns/matching_and_capturing_accesses_list_element_once.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/patterns/matching_and_capturing_accesses_list_element_once.dart.strong.transformed.expect
@@ -534,8 +534,8 @@ static method main() → dynamic {
     {
       hoisted core::int x;
       hoisted core::int y;
-      if(#0#0.{self::A::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C8 && ((#0#4#isSet ?{core::int} #0#4 : let final dynamic #t10 = #0#4#isSet = true in #0#4 = #0#0.{self::A::[]}(0){(core::int) → core::int}) is core::int && (let final dynamic #t11 = x = #0#4#isSet ?{core::int} #0#4 : let final dynamic #t12 = #0#4#isSet = true in #0#4 = #0#0.{self::A::[]}(0){(core::int) → core::int} in true)) && (#0#5#isSet ?{core::int} #0#5 : let final dynamic #t13 = #0#5#isSet = true in #0#5 = #0#0.{self::A::[]}(1){(core::int) → core::int}) is core::int) {
-        y = #0#5#isSet ?{core::int} #0#5 : let final dynamic #t14 = #0#5#isSet = true in #0#5 = #0#0.{self::A::[]}(1){(core::int) → core::int};
+      if(#0#0.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C8 && ((#0#4#isSet ?{core::int} #0#4 : let final dynamic #t10 = #0#4#isSet = true in #0#4 = #0#0.{core::List::[]}(0){(core::int) → core::int}) is core::int && (let final dynamic #t11 = x = #0#4#isSet ?{core::int} #0#4 : let final dynamic #t12 = #0#4#isSet = true in #0#4 = #0#0.{core::List::[]}(0){(core::int) → core::int} in true)) && (#0#5#isSet ?{core::int} #0#5 : let final dynamic #t13 = #0#5#isSet = true in #0#5 = #0#0.{core::List::[]}(1){(core::int) → core::int}) is core::int) {
+        y = #0#5#isSet ?{core::int} #0#5 : let final dynamic #t14 = #0#5#isSet = true in #0#5 = #0#0.{core::List::[]}(1){(core::int) → core::int};
         {
           self::expectEquals(x, 0);
           self::expectEquals(y, 0);
diff --git a/pkg/front_end/testcases/patterns/never_list.dart.strong.expect b/pkg/front_end/testcases/patterns/never_list.dart.strong.expect
index 0d9088951b9..819fc2d9c47 100644
--- a/pkg/front_end/testcases/patterns/never_list.dart.strong.expect
+++ b/pkg/front_end/testcases/patterns/never_list.dart.strong.expect
@@ -4,10 +4,11 @@ import "dart:core" as core;
 
 static method main() → void {
   try {
-    has-declared-initializer core::Object? y;
+    hoisted has-declared-initializer core::Object? y;
     {
       final synthesized Never #0#0 = throw 42;
-      if(!#0#0)
+      final const synthesized core::int #0#2 = #C1;
+      if(!(#0#0.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t1 = y = #0#0.{core::List::[]}(0){(core::int) → core::Object?} in true)))
         throw{for-error-handling} new core::StateError::•("Pattern matching error");
     }
   }
@@ -17,3 +18,7 @@ static method main() → void {
   }
   throw "Missing exception";
 }
+
+constants  {
+  #C1 = 1
+}
diff --git a/pkg/front_end/testcases/patterns/never_list.dart.strong.modular.expect b/pkg/front_end/testcases/patterns/never_list.dart.strong.modular.expect
index 0d9088951b9..819fc2d9c47 100644
--- a/pkg/front_end/testcases/patterns/never_list.dart.strong.modular.expect
+++ b/pkg/front_end/testcases/patterns/never_list.dart.strong.modular.expect
@@ -4,10 +4,11 @@ import "dart:core" as core;
 
 static method main() → void {
   try {
-    has-declared-initializer core::Object? y;
+    hoisted has-declared-initializer core::Object? y;
     {
       final synthesized Never #0#0 = throw 42;
-      if(!#0#0)
+      final const synthesized core::int #0#2 = #C1;
+      if(!(#0#0.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t1 = y = #0#0.{core::List::[]}(0){(core::int) → core::Object?} in true)))
         throw{for-error-handling} new core::StateError::•("Pattern matching error");
     }
   }
@@ -17,3 +18,7 @@ static method main() → void {
   }
   throw "Missing exception";
 }
+
+constants  {
+  #C1 = 1
+}
diff --git a/pkg/front_end/testcases/patterns/never_list.dart.strong.transformed.expect b/pkg/front_end/testcases/patterns/never_list.dart.strong.transformed.expect
index 0d9088951b9..819fc2d9c47 100644
--- a/pkg/front_end/testcases/patterns/never_list.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/patterns/never_list.dart.strong.transformed.expect
@@ -4,10 +4,11 @@ import "dart:core" as core;
 
 static method main() → void {
   try {
-    has-declared-initializer core::Object? y;
+    hoisted has-declared-initializer core::Object? y;
     {
       final synthesized Never #0#0 = throw 42;
-      if(!#0#0)
+      final const synthesized core::int #0#2 = #C1;
+      if(!(#0#0.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t1 = y = #0#0.{core::List::[]}(0){(core::int) → core::Object?} in true)))
         throw{for-error-handling} new core::StateError::•("Pattern matching error");
     }
   }
@@ -17,3 +18,7 @@ static method main() → void {
   }
   throw "Missing exception";
 }
+
+constants  {
+  #C1 = 1
+}
diff --git a/pkg/front_end/testcases/patterns/never_map.dart.strong.expect b/pkg/front_end/testcases/patterns/never_map.dart.strong.expect
index 0ccd2063a24..88c75a0fcca 100644
--- a/pkg/front_end/testcases/patterns/never_map.dart.strong.expect
+++ b/pkg/front_end/testcases/patterns/never_map.dart.strong.expect
@@ -4,10 +4,13 @@ import "dart:core" as core;
 
 static method main() → void {
   try {
-    has-declared-initializer core::Object? b;
+    hoisted has-declared-initializer core::Object? b;
     {
       final synthesized Never #0#0 = throw <core::int, core::int>{42: 42};
-      if(!#0#0)
+      synthesized core::Object? #0#3;
+      synthesized core::bool #0#3#isSet = false;
+      final const synthesized core::int #0#1 = #C1;
+      if(!((!((#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t1 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}) == null) || null is core::Object? && #0#0.{core::Map::containsKey}(#C1){(core::Object?) → core::bool}) && (let final dynamic #t2 = b = #0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t3 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?} in true)))
         throw{for-error-handling} new core::StateError::•("Pattern matching error");
     }
   }
@@ -17,3 +20,7 @@ static method main() → void {
   }
   throw "Missing exception";
 }
+
+constants  {
+  #C1 = 42
+}
diff --git a/pkg/front_end/testcases/patterns/never_map.dart.strong.modular.expect b/pkg/front_end/testcases/patterns/never_map.dart.strong.modular.expect
index 0ccd2063a24..88c75a0fcca 100644
--- a/pkg/front_end/testcases/patterns/never_map.dart.strong.modular.expect
+++ b/pkg/front_end/testcases/patterns/never_map.dart.strong.modular.expect
@@ -4,10 +4,13 @@ import "dart:core" as core;
 
 static method main() → void {
   try {
-    has-declared-initializer core::Object? b;
+    hoisted has-declared-initializer core::Object? b;
     {
       final synthesized Never #0#0 = throw <core::int, core::int>{42: 42};
-      if(!#0#0)
+      synthesized core::Object? #0#3;
+      synthesized core::bool #0#3#isSet = false;
+      final const synthesized core::int #0#1 = #C1;
+      if(!((!((#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t1 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}) == null) || null is core::Object? && #0#0.{core::Map::containsKey}(#C1){(core::Object?) → core::bool}) && (let final dynamic #t2 = b = #0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t3 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?} in true)))
         throw{for-error-handling} new core::StateError::•("Pattern matching error");
     }
   }
@@ -17,3 +20,7 @@ static method main() → void {
   }
   throw "Missing exception";
 }
+
+constants  {
+  #C1 = 42
+}
diff --git a/pkg/front_end/testcases/patterns/never_map.dart.strong.transformed.expect b/pkg/front_end/testcases/patterns/never_map.dart.strong.transformed.expect
index 0ccd2063a24..7ef4d318c82 100644
--- a/pkg/front_end/testcases/patterns/never_map.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/patterns/never_map.dart.strong.transformed.expect
@@ -4,10 +4,13 @@ import "dart:core" as core;
 
 static method main() → void {
   try {
-    has-declared-initializer core::Object? b;
+    hoisted has-declared-initializer core::Object? b;
     {
       final synthesized Never #0#0 = throw <core::int, core::int>{42: 42};
-      if(!#0#0)
+      synthesized core::Object? #0#3;
+      synthesized core::bool #0#3#isSet = false;
+      final const synthesized core::int #0#1 = #C1;
+      if(!((!((#0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t1 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?}) == null) || null is core::Object? && #0#0.{core::Map::containsKey}(#C1){(core::Object?) → core::bool}) && (let final dynamic #t2 = b = #0#3#isSet ?{core::Object?} #0#3 : let final dynamic #t3 = #0#3#isSet = true in #0#3 = #0#0.{core::Map::[]}(#C1){(core::Object?) → core::Object?} in true)))
         throw{for-error-handling} new core::StateError::•("Pattern matching error");
     }
   }
@@ -17,3 +20,11 @@ static method main() → void {
   }
   throw "Missing exception";
 }
+
+constants  {
+  #C1 = 42
+}
+
+Extra constant evaluation status:
+Evaluated: IsExpression @ org-dartlang-testcase:///never_map.dart:7:12 -> BoolConstant(true)
+Extra constant evaluation: evaluated: 34, effectively constant: 1
diff --git a/pkg/kernel/lib/src/ast/patterns.dart b/pkg/kernel/lib/src/ast/patterns.dart
index 00d070525b9..0a196d4087c 100644
--- a/pkg/kernel/lib/src/ast/patterns.dart
+++ b/pkg/kernel/lib/src/ast/patterns.dart
@@ -364,7 +364,6 @@ class NullCheckPattern extends Pattern {
 class ListPattern extends Pattern {
   static const int FlagNeedsCheck = 1 << 0;
   static const int FlagHasRestPattern = 1 << 1;
-  static const int FlagIsNeverPattern = 1 << 2;
 
   int flags = 0;
 
@@ -403,17 +402,6 @@ class ListPattern extends Pattern {
   /// This is set during inference.
   DartType? lookupType;
 
-  /// If `true`, this list pattern is performed on an expression of type
-  /// `Never`.
-  ///
-  /// This is set during inference.
-  bool get isNeverPattern => flags & FlagIsNeverPattern != 0;
-  void set isNeverPattern(bool value) {
-    flags = value
-        ? (flags | FlagIsNeverPattern)
-        : (flags & ~FlagIsNeverPattern);
-  }
-
   /// If `true`, this list pattern contains a rest pattern.
   ///
   /// This is set during inference.
@@ -966,7 +954,6 @@ class AssignedVariablePattern extends Pattern {
 
 class MapPattern extends Pattern {
   static const int FlagNeedsCheck = 1 << 0;
-  static const int FlagIsNeverPattern = 1 << 1;
 
   int flags = 0;
 
@@ -1008,16 +995,6 @@ class MapPattern extends Pattern {
   /// This is set during inference.
   DartType? lookupType;
 
-  /// If `true`, this map pattern is performed on an expression of type `Never`.
-  ///
-  /// This is set during inference.
-  bool get isNeverPattern => flags & FlagIsNeverPattern != 0;
-  void set isNeverPattern(bool value) {
-    flags = value
-        ? (flags | FlagIsNeverPattern)
-        : (flags & ~FlagIsNeverPattern);
-  }
-
   /// Reference to the target of the `containsKey` method of the map.
   ///
   /// This is set during inference.