Elements. Add NeverElementImpl2, make NeverElementImpl its fragment.

Change-Id: I3ef636dec6a678e30c44f0551276a8b3557a7641
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/394240
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Phil Quitslund <[email protected]>

Author: scheglov

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

@@ -39,6 +39,10 @@ class ElementDisplayStringBuilder {
     _write(element.name ?? '<unnamed $runtimeType>');
   }
 
+  void writeAbstractElement2(ElementImpl2 element) {
+    _write(element.name3 ?? '<unnamed $runtimeType>');
+  }
+
   void writeClassElement(ClassElementImpl element) {
     if (element.isAugmentation) {
       _write('augment ');

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

@@ -3201,11 +3201,23 @@ abstract class ElementImpl2 implements Element2 {
     return enclosingElement2?.session;
   }
 
+  /// Append a textual representation of this element to the given [builder].
+  void appendTo(ElementDisplayStringBuilder builder) {
+    builder.writeAbstractElement2(this);
+  }
+
   @override
   String displayString2({
     bool multiline = false,
     bool preferTypeAlias = false,
-  });
+  }) {
+    var builder = ElementDisplayStringBuilder(
+      multiline: multiline,
+      preferTypeAlias: preferTypeAlias,
+    );
+    appendTo(builder);
+    return builder.toString();
+  }
 
   @override
   bool isAccessibleIn2(LibraryElement2 library) {
@@ -3240,6 +3252,15 @@ abstract class ElementImpl2 implements Element2 {
   String toString() {
     return displayString2();
   }
+
+  /// Use the given [visitor] to visit all of the children of this element.
+  /// There is no guarantee of the order in which the children will be visited.
+  @override
+  void visitChildren2<T>(ElementVisitor2<T> visitor) {
+    for (var child in children2) {
+      child.accept2(visitor);
+    }
+  }
 }
 
 /// A concrete implementation of an [ElementLocation].
@@ -8198,10 +8219,8 @@ class MultiplyDefinedElementImpl implements MultiplyDefinedElement {
   }
 }
 
-class MultiplyDefinedElementImpl2 implements MultiplyDefinedElement2 {
-  @override
-  final int id = ElementImpl._NEXT_ID++;
-
+class MultiplyDefinedElementImpl2 extends ElementImpl2
+    implements MultiplyDefinedElement2 {
   final CompilationUnitElementImpl libraryFragment;
 
   @override
@@ -8365,7 +8384,8 @@ class MultiplyDefinedFragmentImpl implements MultiplyDefinedFragment {
 }
 
 /// The synthetic element representing the declaration of the type `Never`.
-class NeverElementImpl extends ElementImpl implements TypeDefiningElement {
+class NeverElementImpl extends ElementImpl
+    implements TypeDefiningElement, TypeDefiningFragment {
   /// The unique instance of this class.
   static final instance = NeverElementImpl._();
 
@@ -8380,9 +8400,33 @@ class NeverElementImpl extends ElementImpl implements TypeDefiningElement {
   @override
   List<Element2> get children2 => const [];
 
+  @override
+  List<Fragment> get children3 => const [];
+
+  @override
+  NeverElementImpl2 get element => NeverElementImpl2.instance;
+
+  @override
+  Null get enclosingFragment => null;
+
   @override
   ElementKind get kind => ElementKind.NEVER;
 
+  @override
+  Null get libraryFragment => null;
+
+  @override
+  String get name2 => 'Never';
+
+  @override
+  Null get nameOffset2 => null;
+
+  @override
+  Null get nextFragment => null;
+
+  @override
+  Null get previousFragment => null;
+
   @override
   T? accept<T>(ElementVisitor<T> visitor) => null;
 
@@ -8401,6 +8445,59 @@ class NeverElementImpl extends ElementImpl implements TypeDefiningElement {
   }
 }
 
+/// The synthetic element representing the declaration of the type `Never`.
+class NeverElementImpl2 extends TypeDefiningElementImpl2 {
+  /// The unique instance of this class.
+  static final instance = NeverElementImpl2._();
+
+  NeverElementImpl2._();
+
+  @override
+  List<Element2> get children2 => const [];
+
+  @override
+  Null get documentationComment => null;
+
+  @override
+  NeverElementImpl get firstFragment => NeverElementImpl.instance;
+
+  @override
+  bool get isSynthetic => true;
+
+  @override
+  ElementKind get kind => ElementKind.NEVER;
+
+  @override
+  Null get library2 => null;
+
+  @override
+  Metadata get metadata2 {
+    return MetadataImpl(0, const [], () => null);
+  }
+
+  @override
+  String get name3 => 'Never';
+
+  @override
+  T? accept2<T>(ElementVisitor2<T> visitor) {
+    return null;
+  }
+
+  DartType instantiate({
+    required NullabilitySuffix nullabilitySuffix,
+  }) {
+    switch (nullabilitySuffix) {
+      case NullabilitySuffix.question:
+        return NeverTypeImpl.instanceNullable;
+      case NullabilitySuffix.star:
+        // TODO(scheglov): remove together with `star`
+        return NeverTypeImpl.instanceNullable;
+      case NullabilitySuffix.none:
+        return NeverTypeImpl.instance;
+    }
+  }
+}
+
 /// A [VariableElementImpl], which is not a parameter.
 abstract class NonParameterVariableElementImpl extends VariableElementImpl
     with _HasLibraryMixin {
@@ -10191,13 +10288,6 @@ class TypeAliasElementImpl2 extends TypeDefiningElementImpl2
           required NullabilitySuffix nullabilitySuffix}) =>
       firstFragment.instantiate(
           typeArguments: typeArguments, nullabilitySuffix: nullabilitySuffix);
-
-  @override
-  void visitChildren2<T>(ElementVisitor2<T> visitor) {
-    for (var child in children2) {
-      child.accept2(visitor);
-    }
-  }
 }
 
 abstract class TypeDefiningElementImpl2 extends ElementImpl2

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

@@ -1073,16 +1073,15 @@ class NeverTypeImpl extends TypeImpl implements NeverType {
   @override
   final NeverElementImpl element = NeverElementImpl.instance;
 
+  @override
+  final NeverElementImpl2 element3 = NeverElementImpl2.instance;
+
   @override
   final NullabilitySuffix nullabilitySuffix;
 
   /// Prevent the creation of instances of this class.
   NeverTypeImpl._(this.nullabilitySuffix);
 
-  @override
-  // TODO(augmentations): Implement this.
-  Element2? get element3 => throw UnimplementedError();
-
   @override
   int get hashCode => 0;
 

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

@@ -153,7 +153,7 @@ extension ElementOrNullExtension on Element? {
         self.conflictingElements.map((e) => e.asElement2).nonNulls.toList(),
       );
     } else if (self is NeverElementImpl) {
-      return self;
+      return NeverElementImpl2.instance;
     } else if (self is ParameterMember) {
       // TODO(scheglov): we lose types here
       return self.declaration.asElement2;

pkg/analyzer/test/src/dart/resolution/named_type_test.dart

@@ -803,7 +803,7 @@ f(Never a) {}
 NamedType
   name: Never
   element: Never@-1
-  element2: Never@-1
+  element2: Never
   type: Never
 ''');
   }

pkg/analyzer/test/src/dart/resolution/simple_identifier_test.dart

@@ -739,7 +739,7 @@ main() {
 SimpleIdentifier
   token: Never
   staticElement: Never@-1
-  element: Never@-1
+  element: Never
   staticType: Type
 ''');
   }

pkg/analyzer/test/src/diagnostics/extends_non_class_test.dart

@@ -157,7 +157,7 @@ ExtendsClause
   superclass: NamedType
     name: Never
     element: Never@-1
-    element2: Never@-1
+    element2: Never
     type: Never
 ''');
   }

pkg/analyzer/test/src/diagnostics/mixin_super_class_constraint_non_interface_test.dart

@@ -98,7 +98,7 @@ MixinOnClause
     NamedType
       name: Never
       element: Never@-1
-      element2: Never@-1
+      element2: Never
       type: Never
 ''');
   }

pkg/analyzer/test/util/element_printer.dart

@@ -125,6 +125,8 @@ class ElementPrinter {
       case LocalVariableElementImpl2():
         // TODO(scheglov): nameOffset2 can be `null`
         _sink.write('${element.name3}@${element.firstFragment.nameOffset2}');
+      case NeverElementImpl2():
+        _sink.write('Never');
       case ClassElementImpl2 element:
         writeReference(element.reference);
       case EnumElementImpl2 element: