Make FragmentedElementMixin and TypeParameterElementImpl get baseElement more precise

Change-Id: I4fe7140b72a0576c59087a9148c62ee1b93c1553
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/440540
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Paul Berry <[email protected]>

Author: fshcheglov

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

@@ -2960,8 +2960,8 @@ class FieldFragmentImpl extends PropertyInducingFragmentImpl
 
 class FormalParameterElementImpl extends PromotableElementImpl
     with
-        FragmentedAnnotatableElementMixin<FormalParameterFragment>,
-        FragmentedElementMixin<FormalParameterFragment>,
+        FragmentedAnnotatableElementMixin<FormalParameterFragmentImpl>,
+        FragmentedElementMixin<FormalParameterFragmentImpl>,
         FormalParameterElementMixin,
         _HasSinceSdkVersionMixin,
         _NonTopLevelVariableOrParameter {
@@ -3413,7 +3413,7 @@ class FormalParameterFragmentImpl extends VariableFragmentImpl
   ) => FormalParameterElementImpl(firstFragment as FormalParameterFragmentImpl);
 }
 
-mixin FragmentedAnnotatableElementMixin<E extends Fragment>
+mixin FragmentedAnnotatableElementMixin<E extends FragmentImpl>
     implements FragmentedElementMixin<E> {
   String? get documentationComment {
     var buffer = StringBuffer();
@@ -3457,14 +3457,9 @@ mixin FragmentedAnnotatableElementMixin<E extends Fragment>
   }
 }
 
-mixin FragmentedElementMixin<E extends Fragment> implements _Fragmented<E> {
+mixin FragmentedElementMixin<E extends FragmentImpl> implements _Fragmented<E> {
   bool get isSynthetic {
-    if (firstFragment is FragmentImpl) {
-      return (firstFragment as FragmentImpl).isSynthetic;
-    }
-    // We should never get to this point.
-    assert(false, 'Fragment does not implement ElementImpl');
-    return false;
+    return firstFragment.isSynthetic;
   }
 
   /// A list of all of the fragments from which this element is composed.
@@ -3520,9 +3515,7 @@ mixin FragmentedExecutableElementMixin<E extends ExecutableFragmentImpl>
   bool get isStatic => (firstFragment as ExecutableFragmentImpl).isStatic;
 }
 
-mixin FragmentedTypeParameterizedElementMixin<
-  E extends TypeParameterizedFragment
->
+mixin FragmentedTypeParameterizedElementMixin<E extends FragmentImpl>
     implements FragmentedElementMixin<E> {
   bool get isSimplyBounded {
     var fragment = firstFragment;
@@ -9591,8 +9584,8 @@ class TopLevelVariableFragmentImpl extends PropertyInducingFragmentImpl
 
 class TypeAliasElementImpl extends TypeDefiningElementImpl
     with
-        FragmentedAnnotatableElementMixin<TypeAliasFragment>,
-        FragmentedElementMixin<TypeAliasFragment>,
+        FragmentedAnnotatableElementMixin<TypeAliasFragmentImpl>,
+        FragmentedElementMixin<TypeAliasFragmentImpl>,
         DeferredResolutionReadingMixin,
         _HasSinceSdkVersionMixin
     implements AnnotatableElementImpl, TypeAliasElement {
@@ -9926,8 +9919,8 @@ abstract class TypeDefiningElementImpl extends ElementImpl
 
 class TypeParameterElementImpl extends TypeDefiningElementImpl
     with
-        FragmentedAnnotatableElementMixin<TypeParameterFragment>,
-        FragmentedElementMixin<TypeParameterFragment>,
+        FragmentedAnnotatableElementMixin<TypeParameterFragmentImpl>,
+        FragmentedElementMixin<TypeParameterFragmentImpl>,
         _NonTopLevelVariableOrParameter
     implements TypeParameterElement, SharedTypeParameter {
   @override
@@ -9950,7 +9943,7 @@ class TypeParameterElementImpl extends TypeDefiningElementImpl
   }
 
   @override
-  TypeParameterElement get baseElement => this;
+  TypeParameterElementImpl get baseElement => this;
 
   @override
   TypeImpl? get bound => firstFragment.bound;

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

@@ -1789,7 +1789,7 @@ class TypeSystemImpl implements TypeSystem {
       if (isSubtypeOf(to, from.bound)) {
         var declaration = from.element.baseElement;
         return TypeParameterTypeImpl(
-          element: declaration as TypeParameterElementImpl,
+          element: declaration,
           nullabilitySuffix: _promotedTypeParameterTypeNullability(
             from.nullabilitySuffix,
             to.nullabilitySuffix,