[CFE] Know about '#this' if finding scopes *on* an extension type

member.

See
#56911 (comment)
for context.

Bug: #56911
Change-Id: If990f41c58268b592d575e1280831b1ed120d54d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/392341
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Jens Johansen <[email protected]>

Author: jensjoha

pkg/front_end/lib/src/base/incremental_compiler.dart

@@ -66,7 +66,8 @@ import '../api_prototype/incremental_kernel_generator.dart'
         IncrementalCompilerResult,
         IncrementalKernelGenerator,
         isLegalIdentifier;
-import '../api_prototype/lowering_predicates.dart' show isExtensionThisName;
+import '../api_prototype/lowering_predicates.dart'
+    show isExtensionThisName, syntheticThisName;
 import '../api_prototype/memory_file_system.dart' show MemoryFileSystem;
 import '../base/nnbd_mode.dart';
 import '../base/processed_options.dart' show ProcessedOptions;
@@ -1917,6 +1918,17 @@ class IncrementalCompiler implements IncrementalKernelGenerator {
                 builder.lookupLocalMember(afterDot, setter: false);
             if (subBuilder is MemberBuilder) {
               if (subBuilder.isExtensionTypeInstanceMember) {
+                List<VariableDeclaration>? positionals =
+                    subBuilder.invokeTarget?.function?.positionalParameters;
+                if (positionals != null &&
+                    positionals.isNotEmpty &&
+                    isExtensionThisName(positionals.first.name) &&
+                    usedDefinitions.containsKey(syntheticThisName)) {
+                  // If we setup the extensionType (and later the
+                  // `extensionThis`) we should also set the type correctly
+                  // (at least in a non-static setting).
+                  usedDefinitions[syntheticThisName] = positionals.first.type;
+                }
                 isStatic = false;
               }
             }

pkg/front_end/testcases/expression/issue_56911_01.expression.yaml

@@ -0,0 +1,41 @@
+# Copyright (c) 2024, 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.
+
+# https://github.com/dart-lang/sdk/issues/56911
+
+# Definition, offset, method etc extracted by starting the VM with
+# `-DDFE_VERBOSE=false`, e.g.
+# ```
+# out/ReleaseX64/dart -DDFE_VERBOSE=false --enable-vm-service \
+# --serve-observatory --disable-service-auth-codes --pause_isolates_on_start \
+# inputFile.dart
+# ```
+# and then issuing the expression compilation. It will then print stuff like
+# `DFE: request[6]: [dart:core, _OneByteString, 1, 0]` in the terminal.
+
+sources: |
+  void main() {
+    List<ExtensionType> list = [new ExtensionType(0)];
+    list.forEach((ExtensionType input) {
+      print(input.value);
+    });
+  }
+
+  extension type ExtensionType._(String s) {
+    ExtensionType(int i) : this._("$i");
+    int get value => s.codeUnitAt(0);
+  }
+
+definitions: ["input"]
+# String
+definition_types: ["dart:core", "_OneByteString", "1", "0"]
+type_definitions: []
+type_bounds: []
+type_defaults: []
+method: "<anonymous closure>"
+static: true
+offset: 122 # at the 'value' of 'print(input.value)' line.
+scriptUri: main.dart
+expression: |
+  input.value

pkg/front_end/testcases/expression/issue_56911_01.expression.yaml.expect

@@ -0,0 +1,4 @@
+Errors: {
+}
+static method /* from org-dartlang-debug:synthetic_debug_expression */ debugExpr(#lib1::ExtensionType% /* erasure=dart.core::String, declared=! */ input, dart.core::List<#lib1::ExtensionType% /* erasure=dart.core::String, declared=! */> list) → dynamic
+  return #lib1::ExtensionType|get#value(input);

pkg/front_end/testcases/expression/issue_56911_02.expression.yaml

@@ -0,0 +1,31 @@
+# Copyright (c) 2024, 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.
+
+# https://github.com/dart-lang/sdk/issues/56911
+
+sources: |
+  void main() {
+    List<ExtensionType> list = [new ExtensionType(0)];
+    list.forEach((ExtensionType input) {
+      print(input.value);
+    });
+  }
+
+  extension type ExtensionType._(String s) {
+    ExtensionType(int i) : this._("$i");
+    int get value => s.codeUnitAt(0);
+  }
+
+definitions: ["#this"]
+# String
+definition_types: ["dart:core", "_OneByteString", "1", "0"]
+type_definitions: []
+type_bounds: []
+type_defaults: []
+method: "ExtensionType.value"
+static: true
+offset: 231 # at the 'value' of 'int get value => s.codeUnitAt(0);' line.
+scriptUri: main.dart
+expression: |
+  s

pkg/front_end/testcases/expression/issue_56911_02.expression.yaml.expect

@@ -0,0 +1,4 @@
+Errors: {
+}
+method /* from org-dartlang-debug:synthetic_debug_expression */ debugExpr(lowered #lib1::ExtensionType% /* erasure=dart.core::String, declared=! */ #this) → dynamic
+  return #this as{Unchecked} dart.core::String;