Fix referencing an aliased type parameter. (dart-lang#3784)

What is an aliased type parameter? Good question! `typedef TD<T> = T;` is such
an alias. The fix is pretty simple, we just weren't previously handling this
case, or being safe.

lib/src/element_type.dart

@@ -272,20 +272,28 @@ abstract class DefinedElementType extends ElementType {
 
   factory DefinedElementType._from(DartType type, ModelElement modelElement,
       Library library, PackageGraph packageGraph) {
-    // `TypeAliasElement.alias.element` has different implications.
-    // In that case it is an actual type alias of some kind (generic or
-    // otherwise). Here however `alias.element` signals that this is a type
-    // referring to an alias.
     if (type is! TypeAliasElement && type.alias != null) {
-      return AliasedElementType._(
-          type as ParameterizedType, library, packageGraph, modelElement);
+      // Here, `alias.element` signals that this is a type referring to an
+      // alias. (`TypeAliasElement.alias.element` has different implications.
+      // In that case it is an actual type alias of some kind (generic or
+      // otherwise).)
+      return switch (type) {
+        TypeParameterType() =>
+          TypeParameterElementType._(type, library, packageGraph, modelElement),
+        ParameterizedType() =>
+          AliasedElementType._(type, library, packageGraph, modelElement),
+        _ => throw UnimplementedError(
+            'No ElementType implemented for aliased ${type.runtimeType}'),
+      };
     }
-    if (type is TypeParameterType) {
-      return TypeParameterElementType._(
-          type, library, packageGraph, modelElement);
-    }
-    return ParameterizedElementType._(
-        type as ParameterizedType, library, packageGraph, modelElement);
+    return switch (type) {
+      TypeParameterType() =>
+        TypeParameterElementType._(type, library, packageGraph, modelElement),
+      ParameterizedType() =>
+        ParameterizedElementType._(type, library, packageGraph, modelElement),
+      _ => throw UnimplementedError(
+          'No ElementType implemented for ${type.runtimeType}'),
+    };
   }
 
   @override

test/typedef_test.dart

@@ -40,6 +40,21 @@ typedef T = C;
     expect(tTypedef.aliasedType, isA<InterfaceType>());
   }
 
+  void test_extensionType_generic_referenceToTypeParameter() async {
+    var library = await bootPackageWithLibrary('''
+typedef TD<T> = T;
+
+/// Text [T].
+extension type ET<T>(TD<T> _) {}
+''');
+
+    expect(
+      library.extensionTypes.named('ET').documentationAsHtml,
+      // There is no way to link to a type parameter.
+      contains('<p>Text <code>T</code>.</p>'),
+    );
+  }
+
   void test_extensionType_basic() async {
     var library = await bootPackageWithLibrary('''
 extension type E(int i) {}