[cfe] Handle extensions in Extension?.member check

Closes https://github.com/dart-lang/sdk/issues/49127

Change-Id: I26ce537dbed908789333f4aa6baa6b8487e5e993
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/246447
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>

pkg/_fe_analyzer_shared/lib/src/messages/codes_generated.dart

@@ -3845,6 +3845,29 @@ const MessageCode messageExtensionDeclaresInstanceField = const MessageCode(
     correctionMessage:
         r"""Try removing the field declaration or making it a static field""");
 
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const Template<Message Function(String name)>
+    templateExtensionInNullAwareReceiver =
+    const Template<Message Function(String name)>(
+        problemMessageTemplate: r"""The extension '#name' cannot be null.""",
+        correctionMessageTemplate: r"""Try replacing '?.' with '.'""",
+        withArguments: _withArgumentsExtensionInNullAwareReceiver);
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const Code<Message Function(String name)> codeExtensionInNullAwareReceiver =
+    const Code<Message Function(String name)>("ExtensionInNullAwareReceiver",
+        severity: Severity.warning);
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+Message _withArgumentsExtensionInNullAwareReceiver(String name) {
+  if (name.isEmpty) throw 'No name provided';
+  name = demangleMixinApplicationName(name);
+  return new Message(codeExtensionInNullAwareReceiver,
+      problemMessage: """The extension '${name}' cannot be null.""",
+      correctionMessage: """Try replacing '?.' with '.'""",
+      arguments: {'name': name});
+}
+
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
 const Template<Message Function(String name)>
     templateExtensionMemberConflictsWithObjectMember =

pkg/front_end/lib/src/fasta/kernel/body_builder.dart

@@ -2947,7 +2947,7 @@ class BodyBuilder extends StackListenerImpl
       assert(declaration.isStatic || declaration.isTopLevel);
       MemberBuilder memberBuilder = declaration as MemberBuilder;
       return new StaticAccessGenerator(
-          this, token, name, memberBuilder.member, null);
+          this, token, name, memberBuilder.parent, memberBuilder.member, null);
     } else if (declaration is PrefixBuilder) {
       assert(prefix == null);
       return new PrefixUseGenerator(this, token, declaration);

pkg/front_end/lib/src/fasta/kernel/expression_generator.dart

@@ -20,6 +20,7 @@ import '../builder/class_builder.dart';
 import '../builder/declaration_builder.dart';
 import '../builder/extension_builder.dart';
 import '../builder/invalid_type_declaration_builder.dart';
+import '../builder/library_builder.dart';
 import '../builder/member_builder.dart';
 import '../builder/named_type_builder.dart';
 import '../builder/nullability_builder.dart';
@@ -1386,12 +1387,18 @@ class StaticAccessGenerator extends Generator {
   final int? typeOffset;
   final bool isNullAware;
 
+  /// The builder for the parent of [readTarget] and [writeTarget]. This is
+  /// either the builder for the enclosing library,  class, or extension.
+  final Builder? parentBuilder;
+
   StaticAccessGenerator(ExpressionGeneratorHelper helper, Token token,
-      this.targetName, this.readTarget, this.writeTarget,
+      this.targetName, this.parentBuilder, this.readTarget, this.writeTarget,
       {this.typeOffset, this.isNullAware: false})
       // ignore: unnecessary_null_comparison
       : assert(targetName != null),
         assert(readTarget != null || writeTarget != null),
+        assert(parentBuilder is DeclarationBuilder ||
+            parentBuilder is LibraryBuilder),
         super(helper, token);
 
   factory StaticAccessGenerator.fromBuilder(
@@ -1402,19 +1409,44 @@ class StaticAccessGenerator extends Generator {
       MemberBuilder? setterBuilder,
       {int? typeOffset,
       bool isNullAware: false}) {
-    return new StaticAccessGenerator(helper, token, targetName,
-        getterBuilder?.readTarget, setterBuilder?.writeTarget,
-        typeOffset: typeOffset, isNullAware: isNullAware);
+    // If both [getterBuilder] and [setterBuilder] exist, they must both be
+    // either top level (potentially from different libraries) or from the same
+    // class/extension.
+    assert(getterBuilder == null ||
+        setterBuilder == null ||
+        (getterBuilder.parent is LibraryBuilder &&
+            setterBuilder.parent is LibraryBuilder) ||
+        getterBuilder.parent == setterBuilder.parent);
+    return new StaticAccessGenerator(
+        helper,
+        token,
+        targetName,
+        getterBuilder?.parent ?? setterBuilder?.parent,
+        getterBuilder?.readTarget,
+        setterBuilder?.writeTarget,
+        typeOffset: typeOffset,
+        isNullAware: isNullAware);
   }
 
   void _reportNonNullableInNullAwareWarningIfNeeded() {
     if (isNullAware && _helper.libraryBuilder.isNonNullableByDefault) {
-      String className = (readTarget ?? writeTarget)!.enclosingClass!.name;
-      _helper.libraryBuilder.addProblem(
-          templateClassInNullAwareReceiver.withArguments(className),
-          typeOffset ?? fileOffset,
-          typeOffset != null ? className.length : noLength,
-          _helper.uri);
+      DeclarationBuilder declarationBuilder =
+          parentBuilder as DeclarationBuilder;
+      if (declarationBuilder.isExtension) {
+        String extensionName = declarationBuilder.name;
+        _helper.libraryBuilder.addProblem(
+            templateExtensionInNullAwareReceiver.withArguments(extensionName),
+            typeOffset ?? fileOffset,
+            typeOffset != null ? extensionName.length : noLength,
+            _helper.uri);
+      } else {
+        String className = declarationBuilder.name;
+        _helper.libraryBuilder.addProblem(
+            templateClassInNullAwareReceiver.withArguments(className),
+            typeOffset ?? fileOffset,
+            typeOffset != null ? className.length : noLength,
+            _helper.uri);
+      }
     }
   }
 

pkg/front_end/messages.status

@@ -314,6 +314,7 @@ ExtendsVoid/example: Fail # Feature not yet enabled by default.
 ExtensionDeclaresAbstractMember/example: Fail
 ExtensionDeclaresConstructor/example: Fail
 ExtensionDeclaresInstanceField/example: Fail
+ExtensionInNullAwareReceiver/analyzerCode: Fail
 ExtensionMemberConflictsWithObjectMember/analyzerCode: Fail
 ExternalConstructorWithBody/part_wrapped_script1: Fail
 ExternalConstructorWithBody/script1: Fail

pkg/front_end/messages.yaml

@@ -5223,6 +5223,19 @@ ClassInNullAwareReceiver:
       C?.field;
     }
 
+ExtensionInNullAwareReceiver:
+  problemMessage: "The extension '#name' cannot be null."
+  correctionMessage: "Try replacing '?.' with '.'"
+  severity: WARNING
+  configuration: nnbd-strong
+  script: |
+    extension E on int {
+      static var field;
+    }
+    method() {
+      E?.field;
+    }
+
 NonNullableNotAssignedError:
   problemMessage: "Non-nullable variable '#name' must be assigned before it can be used."
   configuration: nnbd-strong

pkg/front_end/test/fasta/generator_to_string_test.dart

@@ -187,7 +187,8 @@ Future<void> main() async {
         "StaticAccessGenerator(offset: 4, targetName: foo,"
         " readTarget: $uri::myGetter,"
         " writeTarget: $uri::mySetter)",
-        new StaticAccessGenerator(helper, token, 'foo', getter, setter));
+        new StaticAccessGenerator(
+            helper, token, 'foo', libraryBuilder, getter, setter));
     check(
         "LoadLibraryGenerator(offset: 4,"
         " builder: Instance of 'LoadLibraryBuilder')",

pkg/front_end/testcases/general/issue49127.dart

@@ -0,0 +1,11 @@
+// Copyright (c) 2022, 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 E on int {
+  static String s = "Lily was here";
+}
+
+test() {
+  E?.s;
+}

pkg/front_end/testcases/general/issue49127.dart.textual_outline.expect

@@ -0,0 +1,5 @@
+extension E on int {
+  static String s = "Lily was here";
+}
+
+test() {}

pkg/front_end/testcases/general/issue49127.dart.textual_outline_modelled.expect

@@ -0,0 +1,5 @@
+extension E on int {
+  static String s = "Lily was here";
+}
+
+test() {}

pkg/front_end/testcases/general/issue49127.dart.weak.expect

@@ -0,0 +1,19 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/general/issue49127.dart:10:3: Warning: The extension 'E' cannot be null.
+// Try replacing '?.' with '.'
+//   E?.s;
+//   ^
+//
+import self as self;
+import "dart:core" as core;
+
+extension E on core::int {
+  static field s = self::E|s;
+}
+static field core::String E|s = "Lily was here";
+static method test() → dynamic {
+  self::E|s;
+}

pkg/front_end/testcases/general/issue49127.dart.weak.modular.expect

@@ -0,0 +1,19 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/general/issue49127.dart:10:3: Warning: The extension 'E' cannot be null.
+// Try replacing '?.' with '.'
+//   E?.s;
+//   ^
+//
+import self as self;
+import "dart:core" as core;
+
+extension E on core::int {
+  static field s = self::E|s;
+}
+static field core::String E|s = "Lily was here";
+static method test() → dynamic {
+  self::E|s;
+}

pkg/front_end/testcases/general/issue49127.dart.weak.outline.expect

@@ -0,0 +1,10 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+extension E on core::int {
+  static field s = self::E|s;
+}
+static field core::String E|s;
+static method test() → dynamic
+  ;

pkg/front_end/testcases/general/issue49127.dart.weak.transformed.expect

@@ -0,0 +1,19 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/general/issue49127.dart:10:3: Warning: The extension 'E' cannot be null.
+// Try replacing '?.' with '.'
+//   E?.s;
+//   ^
+//
+import self as self;
+import "dart:core" as core;
+
+extension E on core::int {
+  static field s = self::E|s;
+}
+static field core::String E|s = "Lily was here";
+static method test() → dynamic {
+  self::E|s;
+}