Elements. Unify interface-cycle handling for extension types

Unifies extension type cycle breaking with the existing
`breakInterfaceCycles()` flow used for other elements and removes the
bespoke walker in `summary2/extension_type.dart`. This consolidates the
logic and ensures consistent behavior across element kinds.

Key changes:
- Move `hasImplementsSelfReference` and
  `hasRepresentationSelfReference` from fragments to
  `ExtensionTypeElementImpl`. Flags are now owned and serialized by the
  element.
- Update summary I/O: writer emits and reader consumes the flags at the
  element level; fragment-level reads/writes are removed.
- Adjust `ErrorVerifier` to read flags via `fragment.element`.
- Handle extension types inside `interface_cycles.dart`: on an SCC,
  mark `hasImplementsSelfReference` and replace `interfaces` with
  `Object` or `Object?` depending on the representation’s nullability.
- Bump `DATA_VERSION` to 524 due to summary layout changes.

Why:
- Eliminates duplicate, divergent cycle-breaking logic for extension
  types.
- Clarifies ownership of self-reference state and simplifies
  serialization.
- Produces consistent diagnostics and interface normalization across
  element kinds.

Impact:
- Requires a format bump; older summaries are invalidated as intended.
- Fragment-level self-reference bits are removed in favor of
  element-level flags.

Change-Id: Ifabb0acb2ce60353bbfe10ee5877c0c66cee5202
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/445961
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Paul Berry <[email protected]>

Author: scheglov

pkg/analyzer/lib/src/dart/analysis/driver.dart

@@ -109,7 +109,7 @@ testFineAfterLibraryAnalyzerHook;
 // TODO(scheglov): Clean up the list of implicitly analyzed files.
 class AnalysisDriver {
   /// The version of data format, should be incremented on every format change.
-  static const int DATA_VERSION = 523;
+  static const int DATA_VERSION = 524;
 
   /// The number of exception contexts allowed to write. Once this field is
   /// zero, we stop writing any new exception contexts in this process.

pkg/analyzer/lib/src/dart/element/element.dart

@@ -2421,6 +2421,14 @@ class ExtensionTypeElementImpl extends InterfaceElementImpl
   @override
   final ExtensionTypeFragmentImpl _firstFragment;
 
+  /// Whether the element has direct or indirect reference to itself,
+  /// in representation.
+  bool hasRepresentationSelfReference = false;
+
+  /// Whether the element has direct or indirect reference to itself,
+  /// in implemented superinterfaces.
+  bool hasImplementsSelfReference = false;
+
   late DartType _typeErasure;
 
   ExtensionTypeElementImpl(this.reference, this._firstFragment) {
@@ -2443,30 +2451,6 @@ class ExtensionTypeElementImpl extends InterfaceElementImpl
     ];
   }
 
-  /// Whether the element has direct or indirect reference to itself,
-  /// in implemented superinterfaces.
-  bool get hasImplementsSelfReference {
-    return _firstFragment.hasImplementsSelfReference;
-  }
-
-  /// Whether the element has direct or indirect reference to itself,
-  /// in implemented superinterfaces.
-  set hasImplementsSelfReference(bool value) {
-    _firstFragment.hasImplementsSelfReference = value;
-  }
-
-  /// Whether the element has direct or indirect reference to itself,
-  /// in representation.
-  bool get hasRepresentationSelfReference {
-    return _firstFragment.hasRepresentationSelfReference;
-  }
-
-  /// Whether the element has direct or indirect reference to itself,
-  /// in representation.
-  set hasRepresentationSelfReference(bool value) {
-    _firstFragment.hasRepresentationSelfReference = value;
-  }
-
   @override
   ElementKind get kind => ElementKind.EXTENSION_TYPE;
 
@@ -2535,14 +2519,6 @@ class ExtensionTypeFragmentImpl extends InterfaceFragmentImpl
   @override
   late final ExtensionTypeElementImpl element;
 
-  /// Whether the element has direct or indirect reference to itself,
-  /// in representation.
-  bool hasRepresentationSelfReference = false;
-
-  /// Whether the element has direct or indirect reference to itself,
-  /// in implemented superinterfaces.
-  bool hasImplementsSelfReference = false;
-
   ExtensionTypeFragmentImpl({required super.name});
 
   @override

pkg/analyzer/lib/src/generated/error_verifier.dart

@@ -3334,9 +3334,9 @@ class ErrorVerifier extends RecursiveAstVisitor<void>
 
   void _checkForExtensionTypeImplementsItself(
     ExtensionTypeDeclarationImpl node,
-    ExtensionTypeFragmentImpl element,
+    ExtensionTypeFragmentImpl fragment,
   ) {
-    if (element.hasImplementsSelfReference) {
+    if (fragment.element.hasImplementsSelfReference) {
       diagnosticReporter.atToken(
         node.name,
         CompileTimeErrorCode.extensionTypeImplementsItself,
@@ -3388,9 +3388,9 @@ class ErrorVerifier extends RecursiveAstVisitor<void>
 
   void _checkForExtensionTypeRepresentationDependsOnItself(
     ExtensionTypeDeclarationImpl node,
-    ExtensionTypeFragmentImpl element,
+    ExtensionTypeFragmentImpl fragment,
   ) {
-    if (element.hasRepresentationSelfReference) {
+    if (fragment.element.hasRepresentationSelfReference) {
       diagnosticReporter.atToken(
         node.name,
         CompileTimeErrorCode.extensionTypeRepresentationDependsOnItself,

pkg/analyzer/lib/src/summary2/bundle_reader.dart

@@ -617,6 +617,9 @@ class LibraryReader {
       var element = ExtensionTypeElementImpl(reference, fragments.first);
       element.linkFragments(fragments);
 
+      element.hasRepresentationSelfReference = _reader.readBool();
+      element.hasImplementsSelfReference = _reader.readBool();
+
       // TODO(scheglov): consider reading lazily
       for (var fragment in element.fragments) {
         fragment.ensureReadMembers();
@@ -648,8 +651,6 @@ class LibraryReader {
         create: (name) {
           var fragment = ExtensionTypeFragmentImpl(name: name);
           fragment.readModifiers(_reader);
-          fragment.hasRepresentationSelfReference = _reader.readBool();
-          fragment.hasImplementsSelfReference = _reader.readBool();
           fragment.typeParameters = _readTypeParameters();
 
           // TODO(scheglov): consider reading lazily

pkg/analyzer/lib/src/summary2/bundle_writer.dart

@@ -357,6 +357,10 @@ class BundleWriter {
       _writeReference(element.reference);
       _writeFragments(element.fragments);
 
+      // TODO(fshcheglov): Put these separate flags into modifiers
+      _sink.writeBool(element.hasRepresentationSelfReference);
+      _sink.writeBool(element.hasImplementsSelfReference);
+
       // TODO(scheglov): consider reading lazily
       _resolutionSink.withTypeParameters(element.typeParameters, () {
         _writeFieldElements(element.fields);
@@ -379,9 +383,6 @@ class BundleWriter {
 
   void _writeExtensionTypeFragment(ExtensionTypeFragmentImpl fragment) {
     _writeTemplateFragment(fragment, () {
-      // TODO(fshcheglov): Put these separate flags into modifiers
-      _sink.writeBool(fragment.hasRepresentationSelfReference);
-      _sink.writeBool(fragment.hasImplementsSelfReference);
       _writeTypeParameters(fragment.typeParameters, () {
         _resolutionSink._writeMetadata(fragment.metadata);
 

pkg/analyzer/lib/src/summary2/extension_type.dart

@@ -30,17 +30,6 @@ void buildExtensionTypes(Linker linker, List<AstNode> declarations) {
   for (var node in nodes) {
     walker.walk(node);
   }
-
-  _breakImplementsCycles(elements);
-}
-
-/// Clears interfaces for extension types that have cycles.
-void _breakImplementsCycles(List<ExtensionTypeElementImpl> elements) {
-  var walker = _ImplementsWalker();
-  for (var element in elements) {
-    var node = walker.getNode(element);
-    walker.walk(node);
-  }
 }
 
 /// Collector of referenced extension types in a type.
@@ -60,63 +49,6 @@ class _DependenciesCollector extends RecursiveTypeVisitor {
   }
 }
 
-class _ImplementsNode extends graph.Node<_ImplementsNode> {
-  final _ImplementsWalker walker;
-  final ExtensionTypeElementImpl element;
-
-  @override
-  bool isEvaluated = false;
-
-  _ImplementsNode(this.walker, this.element);
-
-  @override
-  List<_ImplementsNode> computeDependencies() {
-    return element.interfaces
-        .map((interface) => interface.element)
-        .whereType<ExtensionTypeElementImpl>()
-        .map(walker.getNode)
-        .toList();
-  }
-
-  void _evaluate() {
-    isEvaluated = true;
-  }
-
-  void _markCircular() {
-    isEvaluated = true;
-    element.hasImplementsSelfReference = true;
-
-    var representationType = element.representation.type;
-    var typeSystem = element.library.typeSystem;
-
-    var superInterface =
-        typeSystem.isNonNullable(representationType)
-            ? typeSystem.objectNone
-            : typeSystem.objectQuestion;
-    element.interfaces = [superInterface];
-  }
-}
-
-class _ImplementsWalker extends graph.DependencyWalker<_ImplementsNode> {
-  final Map<ExtensionTypeElementImpl, _ImplementsNode> nodeMap = Map.identity();
-
-  @override
-  void evaluate(_ImplementsNode v) {
-    v._evaluate();
-  }
-
-  @override
-  void evaluateScc(List<_ImplementsNode> scc) {
-    for (var node in scc) {
-      node._markCircular();
-    }
-  }
-
-  _ImplementsNode getNode(ExtensionTypeElementImpl element) {
-    return nodeMap[element] ??= _ImplementsNode(this, element);
-  }
-}
-
 class _Node extends graph.Node<_Node> {
   final _Walker walker;
   final ExtensionTypeDeclarationImpl node;

pkg/analyzer/lib/src/summary2/interface_cycles.dart

@@ -14,12 +14,6 @@ void breakInterfaceCycles(Linker linker, List<AstNode> declarations) {
   for (var declaration in declarations) {
     if (declaration is DeclarationImpl) {
       var element = declaration.declaredFragment!.element;
-
-      // Handled elsewhere.
-      if (element is ExtensionTypeElementImpl) {
-        continue;
-      }
-
       if (element is InterfaceElementImpl) {
         elements.add(element);
       }
@@ -64,19 +58,26 @@ class _ImplementsNode extends graph.Node<_ImplementsNode> {
 
     element.interfaceCycle = elements;
 
+    var typeProvider = element.library.typeProvider;
+    var typeSystem = element.library.typeSystem;
+
     switch (element) {
       case ClassElementImpl element:
-        var typeProvider = element.library.typeProvider;
         element.supertype = typeProvider.objectType;
         element.mixins = [];
         element.interfaces = [];
       case EnumElementImpl element:
         element.mixins = [];
         element.interfaces = [];
       case ExtensionTypeElementImpl element:
-        element.interfaces = [];
+        element.hasImplementsSelfReference = true;
+        var representationType = element.representation.type;
+        var superInterface =
+            typeSystem.isNonNullable(representationType)
+                ? typeSystem.objectNone
+                : typeSystem.objectQuestion;
+        element.interfaces = [superInterface];
       case MixinElementImpl element:
-        var typeProvider = element.library.typeProvider;
         element.superclassConstraints = [typeProvider.objectType];
         element.interfaces = [];
       default: