diff --git a/pkg/_fe_analyzer_shared/lib/src/messages/codes_generated.dart b/pkg/_fe_analyzer_shared/lib/src/messages/codes_generated.dart
index c4ec8364e29..bfae6d03073 100644
--- a/pkg/_fe_analyzer_shared/lib/src/messages/codes_generated.dart
+++ b/pkg/_fe_analyzer_shared/lib/src/messages/codes_generated.dart
@@ -10613,6 +10613,37 @@ const MessageCode messagePatchNonExternal = const MessageCode(
         r"""Can't apply this patch as its origin declaration isn't external.""",
     correctionMessage: r"""Try adding 'external' to the origin declaration.""");
 
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const Template<
+    Message Function(
+        String
+            name)> templatePatternAssignmentDeclaresVariable = const Template<
+        Message Function(String name)>(
+    problemMessageTemplate:
+        r"""Variable '#name' can't be declared in a pattern assignment.""",
+    correctionMessageTemplate:
+        r"""Try using a preexisting variable or changing the assignment to a pattern variable declaration.""",
+    withArguments: _withArgumentsPatternAssignmentDeclaresVariable);
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const Code<Message Function(String name)>
+    codePatternAssignmentDeclaresVariable =
+    const Code<Message Function(String name)>(
+  "PatternAssignmentDeclaresVariable",
+);
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+Message _withArgumentsPatternAssignmentDeclaresVariable(String name) {
+  if (name.isEmpty) throw 'No name provided';
+  name = demangleMixinApplicationName(name);
+  return new Message(codePatternAssignmentDeclaresVariable,
+      problemMessage:
+          """Variable '${name}' can't be declared in a pattern assignment.""",
+      correctionMessage:
+          """Try using a preexisting variable or changing the assignment to a pattern variable declaration.""",
+      arguments: {'name': name});
+}
+
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
 const Code<Null> codePlatformPrivateLibraryAccess =
     messagePlatformPrivateLibraryAccess;
diff --git a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
index c842fd5d9d2..f51e3b1a53a 100644
--- a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
@@ -8889,21 +8889,27 @@ class BodyBuilder extends StackListenerImpl
 
     reportIfNotEnabled(
         libraryFeatures.patterns, variable.charOffset, variable.charCount);
-    // ignore: unused_local_variable
     TypeBuilder? type = pop(NullValues.TypeBuilder) as TypeBuilder?;
     DartType? patternType = type?.build(libraryBuilder, TypeUse.variableType);
     Pattern pattern;
     if (variable.lexeme == "_") {
       pattern = new WildcardPattern(patternType, variable.charOffset);
     } else if (inAssignmentPattern) {
-      Expression variableUse =
-          toValue(scopeLookup(scope, variable.lexeme, variable));
-      if (variableUse is VariableGet) {
-        pattern = new AssignedVariablePattern(variableUse.variable,
-            offset: variable.charOffset);
+      String name = variable.lexeme;
+      if (keyword != null || type != null) {
+        pattern = new InvalidPattern(buildProblem(
+            fasta.templatePatternAssignmentDeclaresVariable.withArguments(name),
+            variable.charOffset,
+            variable.charCount));
       } else {
-        // Recover by using [WildcardPattern] instead.
-        pattern = new WildcardPattern(patternType, variable.charOffset);
+        Expression variableUse = toValue(scopeLookup(scope, name, variable));
+        if (variableUse is VariableGet) {
+          pattern = new AssignedVariablePattern(variableUse.variable,
+              offset: variable.charOffset);
+        } else {
+          // Recover by using [WildcardPattern] instead.
+          pattern = new WildcardPattern(patternType, variable.charOffset);
+        }
       }
     } else {
       VariableDeclaration declaredVariable = forest.createVariableDeclaration(
diff --git a/pkg/front_end/messages.status b/pkg/front_end/messages.status
index 9e30d9155dc..f349046c7c3 100644
--- a/pkg/front_end/messages.status
+++ b/pkg/front_end/messages.status
@@ -836,6 +836,7 @@ PatchInjectionFailed/analyzerCode: Fail
 PatchInjectionFailed/example: Fail
 PatchNonExternal/analyzerCode: Fail
 PatchNonExternal/example: Fail
+PatternAssignmentDeclaresVariable/analyzerCode: Fail
 PlatformPrivateLibraryAccess/example: Fail
 PositionalAfterNamedArgument/example: Fail
 PositionalSuperParametersAndArguments/analyzerCode: Fail
diff --git a/pkg/front_end/messages.yaml b/pkg/front_end/messages.yaml
index 2d65fd99134..cd4f9e4fe52 100644
--- a/pkg/front_end/messages.yaml
+++ b/pkg/front_end/messages.yaml
@@ -6308,3 +6308,13 @@ UnnamedObjectPatternField:
     method(x) {
       if (x case int(5)) {}
     }
+
+PatternAssignmentDeclaresVariable:
+  problemMessage: "Variable '#name' can't be declared in a pattern assignment."
+  correctionMessage: "Try using a preexisting variable or changing the assignment to a pattern variable declaration."
+  experiments: patterns
+  script: |
+    method(x) {
+      var y;
+      [y, var z] = x;
+    }
diff --git a/pkg/front_end/test/spell_checking_list_messages.txt b/pkg/front_end/test/spell_checking_list_messages.txt
index 93eb66694a9..778a117be55 100644
--- a/pkg/front_end/test/spell_checking_list_messages.txt
+++ b/pkg/front_end/test/spell_checking_list_messages.txt
@@ -79,6 +79,7 @@ part(s)
 patch(es)
 patterns
 placing
+preexisting
 pubspec.yaml
 r
 re
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart
new file mode 100644
index 00000000000..702dda26534
--- /dev/null
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart
@@ -0,0 +1,15 @@
+// Copyright (c) 2023, 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.
+
+class Class {
+  dynamic field1;
+  dynamic field2;
+}
+
+method(x, y, z) {
+  var a;
+  (a, int b, final int c) = x;
+  [a, final d] = y;
+  Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+}
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.strong.expect b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.strong.expect
new file mode 100644
index 00000000000..0a9bf3a1692
--- /dev/null
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.strong.expect
@@ -0,0 +1,81 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:11: Error: Variable 'b' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   (a, int b, final int c) = x;
+//           ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:24: Error: Variable 'c' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   (a, int b, final int c) = x;
+//                        ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:13:13: Error: Variable 'd' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   [a, final d] = y;
+//             ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:34: Error: Variable 'e' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+//                                  ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:49: Error: Variable 'f' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+//                                                 ^
+//
+import self as self;
+import "dart:core" as core;
+import "dart:_internal" as _in;
+
+class Class extends core::Object {
+  field dynamic field1 = null;
+  field dynamic field2 = null;
+  synthetic constructor •() → self::Class
+    : super core::Object::•()
+    ;
+}
+static method method(dynamic x, dynamic y, dynamic z) → dynamic {
+  dynamic a;
+  block {
+    final dynamic #0#0 = x;
+    if(!(#0#0 is{ForNonNullableByDefault} (core::Object?, core::Object?, core::Object?) && (let final dynamic #t1 = a = #0#0{(core::Object?, core::Object?, core::Object?)}.$1{core::Object?} in true) && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:11: Error: Variable 'b' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  (a, int b, final int c) = x;
+          ^" && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:24: Error: Variable 'c' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  (a, int b, final int c) = x;
+                       ^"))
+      throw new _in::ReachabilityError::•();
+  } =>#0#0;
+  block {
+    final dynamic #1#0 = y;
+    if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t2 = a = #1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} in true) && (let final dynamic #t3 = #1#0{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} in invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:13:13: Error: Variable 'd' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  [a, final d] = y;
+            ^")))
+      throw new _in::ReachabilityError::•();
+  } =>#1#0;
+  block {
+    final dynamic #2#0 = z;
+    late final dynamic #2#3 = #2#0{self::Class}.{self::Class::field2}{dynamic};
+    late final dynamic #2#9 = #2#3{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic};
+    late final dynamic #2#16 = #2#3{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic};
+    if(!(#2#0 is{ForNonNullableByDefault} self::Class && (let final dynamic #t4 = a = #2#0{self::Class}.{self::Class::field1}{dynamic} in true) && (#2#3 is{ForNonNullableByDefault} core::List<dynamic> && #2#3{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (#2#9 is{ForNonNullableByDefault} core::List<dynamic> && #2#9{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t5 = #2#9{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} in invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:34: Error: Variable 'e' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+                                 ^") && (let final dynamic #t6 = #2#9{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} in true)) && (#2#16 is{ForNonNullableByDefault} core::List<dynamic> && #2#16{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && #C2 =={core::num::==}{(core::Object) → core::bool} #2#16{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} && (let final dynamic #t7 = #2#16{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} in invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:49: Error: Variable 'f' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+                                                ^")))))
+      throw new _in::ReachabilityError::•();
+  } =>#2#0;
+}
+
+constants  {
+  #C1 = 2
+  #C2 = 1
+}
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.strong.transformed.expect b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.strong.transformed.expect
new file mode 100644
index 00000000000..f132dcf0b38
--- /dev/null
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.strong.transformed.expect
@@ -0,0 +1,87 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:11: Error: Variable 'b' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   (a, int b, final int c) = x;
+//           ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:24: Error: Variable 'c' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   (a, int b, final int c) = x;
+//                        ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:13:13: Error: Variable 'd' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   [a, final d] = y;
+//             ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:34: Error: Variable 'e' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+//                                  ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:49: Error: Variable 'f' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+//                                                 ^
+//
+import self as self;
+import "dart:core" as core;
+import "dart:_internal" as _in;
+
+class Class extends core::Object {
+  field dynamic field1 = null;
+  field dynamic field2 = null;
+  synthetic constructor •() → self::Class
+    : super core::Object::•()
+    ;
+}
+static method method(dynamic x, dynamic y, dynamic z) → dynamic {
+  dynamic a;
+  block {
+    final dynamic #0#0 = x;
+    if(!(#0#0 is{ForNonNullableByDefault} (core::Object?, core::Object?, core::Object?) && (let final core::Object? #t1 = a = #0#0{(core::Object?, core::Object?, core::Object?)}.$1{core::Object?} in true) && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:11: Error: Variable 'b' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  (a, int b, final int c) = x;
+          ^" && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:24: Error: Variable 'c' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  (a, int b, final int c) = x;
+                       ^"))
+      throw new _in::ReachabilityError::•();
+  } =>#0#0;
+  block {
+    final dynamic #1#0 = y;
+    if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t2 = a = #1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} in true) && (let final dynamic #t3 = #1#0{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} in invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:13:13: Error: Variable 'd' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  [a, final d] = y;
+            ^")))
+      throw new _in::ReachabilityError::•();
+  } =>#1#0;
+  block {
+    final dynamic #2#0 = z;
+    function ##2#3#initializer() → dynamic
+      return #2#0{self::Class}.{self::Class::field2}{dynamic};
+    late final dynamic #2#3 = ##2#3#initializer(){() → dynamic};
+    function ##2#9#initializer() → dynamic
+      return #2#3{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic};
+    late final dynamic #2#9 = ##2#9#initializer(){() → dynamic};
+    function ##2#16#initializer() → dynamic
+      return #2#3{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic};
+    late final dynamic #2#16 = ##2#16#initializer(){() → dynamic};
+    if(!(#2#0 is{ForNonNullableByDefault} self::Class && (let final dynamic #t4 = a = #2#0{self::Class}.{self::Class::field1}{dynamic} in true) && (#2#3 is{ForNonNullableByDefault} core::List<dynamic> && #2#3{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (#2#9 is{ForNonNullableByDefault} core::List<dynamic> && #2#9{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t5 = #2#9{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} in invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:34: Error: Variable 'e' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+                                 ^") && (let final dynamic #t6 = #2#9{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} in true)) && (#2#16 is{ForNonNullableByDefault} core::List<dynamic> && #2#16{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && #C2 =={core::num::==}{(core::Object) → core::bool} #2#16{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} && (let final dynamic #t7 = #2#16{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} in invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:49: Error: Variable 'f' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+                                                ^")))))
+      throw new _in::ReachabilityError::•();
+  } =>#2#0;
+}
+
+constants  {
+  #C1 = 2
+  #C2 = 1
+}
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.textual_outline.expect b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.textual_outline.expect
new file mode 100644
index 00000000000..33f6d14ac9d
--- /dev/null
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.textual_outline.expect
@@ -0,0 +1,6 @@
+class Class {
+  dynamic field1;
+  dynamic field2;
+}
+
+method(x, y, z) {}
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.textual_outline_modelled.expect b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.textual_outline_modelled.expect
new file mode 100644
index 00000000000..33f6d14ac9d
--- /dev/null
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.textual_outline_modelled.expect
@@ -0,0 +1,6 @@
+class Class {
+  dynamic field1;
+  dynamic field2;
+}
+
+method(x, y, z) {}
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.expect b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.expect
new file mode 100644
index 00000000000..0a9bf3a1692
--- /dev/null
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.expect
@@ -0,0 +1,81 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:11: Error: Variable 'b' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   (a, int b, final int c) = x;
+//           ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:24: Error: Variable 'c' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   (a, int b, final int c) = x;
+//                        ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:13:13: Error: Variable 'd' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   [a, final d] = y;
+//             ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:34: Error: Variable 'e' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+//                                  ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:49: Error: Variable 'f' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+//                                                 ^
+//
+import self as self;
+import "dart:core" as core;
+import "dart:_internal" as _in;
+
+class Class extends core::Object {
+  field dynamic field1 = null;
+  field dynamic field2 = null;
+  synthetic constructor •() → self::Class
+    : super core::Object::•()
+    ;
+}
+static method method(dynamic x, dynamic y, dynamic z) → dynamic {
+  dynamic a;
+  block {
+    final dynamic #0#0 = x;
+    if(!(#0#0 is{ForNonNullableByDefault} (core::Object?, core::Object?, core::Object?) && (let final dynamic #t1 = a = #0#0{(core::Object?, core::Object?, core::Object?)}.$1{core::Object?} in true) && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:11: Error: Variable 'b' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  (a, int b, final int c) = x;
+          ^" && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:24: Error: Variable 'c' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  (a, int b, final int c) = x;
+                       ^"))
+      throw new _in::ReachabilityError::•();
+  } =>#0#0;
+  block {
+    final dynamic #1#0 = y;
+    if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t2 = a = #1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} in true) && (let final dynamic #t3 = #1#0{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} in invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:13:13: Error: Variable 'd' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  [a, final d] = y;
+            ^")))
+      throw new _in::ReachabilityError::•();
+  } =>#1#0;
+  block {
+    final dynamic #2#0 = z;
+    late final dynamic #2#3 = #2#0{self::Class}.{self::Class::field2}{dynamic};
+    late final dynamic #2#9 = #2#3{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic};
+    late final dynamic #2#16 = #2#3{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic};
+    if(!(#2#0 is{ForNonNullableByDefault} self::Class && (let final dynamic #t4 = a = #2#0{self::Class}.{self::Class::field1}{dynamic} in true) && (#2#3 is{ForNonNullableByDefault} core::List<dynamic> && #2#3{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (#2#9 is{ForNonNullableByDefault} core::List<dynamic> && #2#9{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t5 = #2#9{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} in invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:34: Error: Variable 'e' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+                                 ^") && (let final dynamic #t6 = #2#9{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} in true)) && (#2#16 is{ForNonNullableByDefault} core::List<dynamic> && #2#16{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && #C2 =={core::num::==}{(core::Object) → core::bool} #2#16{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} && (let final dynamic #t7 = #2#16{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} in invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:49: Error: Variable 'f' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+                                                ^")))))
+      throw new _in::ReachabilityError::•();
+  } =>#2#0;
+}
+
+constants  {
+  #C1 = 2
+  #C2 = 1
+}
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.modular.expect b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.modular.expect
new file mode 100644
index 00000000000..0a9bf3a1692
--- /dev/null
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.modular.expect
@@ -0,0 +1,81 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:11: Error: Variable 'b' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   (a, int b, final int c) = x;
+//           ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:24: Error: Variable 'c' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   (a, int b, final int c) = x;
+//                        ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:13:13: Error: Variable 'd' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   [a, final d] = y;
+//             ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:34: Error: Variable 'e' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+//                                  ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:49: Error: Variable 'f' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+//                                                 ^
+//
+import self as self;
+import "dart:core" as core;
+import "dart:_internal" as _in;
+
+class Class extends core::Object {
+  field dynamic field1 = null;
+  field dynamic field2 = null;
+  synthetic constructor •() → self::Class
+    : super core::Object::•()
+    ;
+}
+static method method(dynamic x, dynamic y, dynamic z) → dynamic {
+  dynamic a;
+  block {
+    final dynamic #0#0 = x;
+    if(!(#0#0 is{ForNonNullableByDefault} (core::Object?, core::Object?, core::Object?) && (let final dynamic #t1 = a = #0#0{(core::Object?, core::Object?, core::Object?)}.$1{core::Object?} in true) && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:11: Error: Variable 'b' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  (a, int b, final int c) = x;
+          ^" && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:24: Error: Variable 'c' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  (a, int b, final int c) = x;
+                       ^"))
+      throw new _in::ReachabilityError::•();
+  } =>#0#0;
+  block {
+    final dynamic #1#0 = y;
+    if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t2 = a = #1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} in true) && (let final dynamic #t3 = #1#0{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} in invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:13:13: Error: Variable 'd' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  [a, final d] = y;
+            ^")))
+      throw new _in::ReachabilityError::•();
+  } =>#1#0;
+  block {
+    final dynamic #2#0 = z;
+    late final dynamic #2#3 = #2#0{self::Class}.{self::Class::field2}{dynamic};
+    late final dynamic #2#9 = #2#3{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic};
+    late final dynamic #2#16 = #2#3{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic};
+    if(!(#2#0 is{ForNonNullableByDefault} self::Class && (let final dynamic #t4 = a = #2#0{self::Class}.{self::Class::field1}{dynamic} in true) && (#2#3 is{ForNonNullableByDefault} core::List<dynamic> && #2#3{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (#2#9 is{ForNonNullableByDefault} core::List<dynamic> && #2#9{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t5 = #2#9{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} in invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:34: Error: Variable 'e' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+                                 ^") && (let final dynamic #t6 = #2#9{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} in true)) && (#2#16 is{ForNonNullableByDefault} core::List<dynamic> && #2#16{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && #C2 =={core::num::==}{(core::Object) → core::bool} #2#16{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} && (let final dynamic #t7 = #2#16{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} in invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:49: Error: Variable 'f' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+                                                ^")))))
+      throw new _in::ReachabilityError::•();
+  } =>#2#0;
+}
+
+constants  {
+  #C1 = 2
+  #C2 = 1
+}
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.outline.expect b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.outline.expect
new file mode 100644
index 00000000000..f68690daf5e
--- /dev/null
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.outline.expect
@@ -0,0 +1,12 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+class Class extends core::Object {
+  field dynamic field1;
+  field dynamic field2;
+  synthetic constructor •() → self::Class
+    ;
+}
+static method method(dynamic x, dynamic y, dynamic z) → dynamic
+  ;
diff --git a/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.transformed.expect b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.transformed.expect
new file mode 100644
index 00000000000..f132dcf0b38
--- /dev/null
+++ b/pkg/front_end/testcases/patterns/pattern_assignment_declares.dart.weak.transformed.expect
@@ -0,0 +1,87 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:11: Error: Variable 'b' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   (a, int b, final int c) = x;
+//           ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:24: Error: Variable 'c' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   (a, int b, final int c) = x;
+//                        ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:13:13: Error: Variable 'd' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   [a, final d] = y;
+//             ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:34: Error: Variable 'e' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+//                                  ^
+//
+// pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:49: Error: Variable 'f' can't be declared in a pattern assignment.
+// Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+//   Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+//                                                 ^
+//
+import self as self;
+import "dart:core" as core;
+import "dart:_internal" as _in;
+
+class Class extends core::Object {
+  field dynamic field1 = null;
+  field dynamic field2 = null;
+  synthetic constructor •() → self::Class
+    : super core::Object::•()
+    ;
+}
+static method method(dynamic x, dynamic y, dynamic z) → dynamic {
+  dynamic a;
+  block {
+    final dynamic #0#0 = x;
+    if(!(#0#0 is{ForNonNullableByDefault} (core::Object?, core::Object?, core::Object?) && (let final core::Object? #t1 = a = #0#0{(core::Object?, core::Object?, core::Object?)}.$1{core::Object?} in true) && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:11: Error: Variable 'b' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  (a, int b, final int c) = x;
+          ^" && invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:12:24: Error: Variable 'c' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  (a, int b, final int c) = x;
+                       ^"))
+      throw new _in::ReachabilityError::•();
+  } =>#0#0;
+  block {
+    final dynamic #1#0 = y;
+    if(!(#1#0 is{ForNonNullableByDefault} core::List<dynamic> && #1#0{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t2 = a = #1#0{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} in true) && (let final dynamic #t3 = #1#0{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} in invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:13:13: Error: Variable 'd' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  [a, final d] = y;
+            ^")))
+      throw new _in::ReachabilityError::•();
+  } =>#1#0;
+  block {
+    final dynamic #2#0 = z;
+    function ##2#3#initializer() → dynamic
+      return #2#0{self::Class}.{self::Class::field2}{dynamic};
+    late final dynamic #2#3 = ##2#3#initializer(){() → dynamic};
+    function ##2#9#initializer() → dynamic
+      return #2#3{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic};
+    late final dynamic #2#9 = ##2#9#initializer(){() → dynamic};
+    function ##2#16#initializer() → dynamic
+      return #2#3{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic};
+    late final dynamic #2#16 = ##2#16#initializer(){() → dynamic};
+    if(!(#2#0 is{ForNonNullableByDefault} self::Class && (let final dynamic #t4 = a = #2#0{self::Class}.{self::Class::field1}{dynamic} in true) && (#2#3 is{ForNonNullableByDefault} core::List<dynamic> && #2#3{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (#2#9 is{ForNonNullableByDefault} core::List<dynamic> && #2#9{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && (let final dynamic #t5 = #2#9{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} in invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:34: Error: Variable 'e' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+                                 ^") && (let final dynamic #t6 = #2#9{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} in true)) && (#2#16 is{ForNonNullableByDefault} core::List<dynamic> && #2#16{core::List<dynamic>}.{core::List::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C1 && #C2 =={core::num::==}{(core::Object) → core::bool} #2#16{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic} && (let final dynamic #t7 = #2#16{core::List<dynamic>}.{core::List::[]}(1){(core::int) → dynamic} in invalid-expression "pkg/front_end/testcases/patterns/pattern_assignment_declares.dart:14:49: Error: Variable 'f' can't be declared in a pattern assignment.
+Try using a preexisting variable or changing the assignment to a pattern variable declaration.
+  Class(field1: a, field2: [[var e, _], [1, var f]]) = z;
+                                                ^")))))
+      throw new _in::ReachabilityError::•();
+  } =>#2#0;
+}
+
+constants  {
+  #C1 = 2
+  #C2 = 1
+}