[analyzer] Handle display for type parameters without name

The ElementDisplayStringBuilder crash when encountering a type parameter
without a name. This chance handles the case by using the empty string as
the name in the computation of unique type parameters.

Fixes b/418755155

Change-Id: I4fd1a6df64a31614e0c4548963d0c5507cc94ca8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/429640
Commit-Queue: Johnni Winther <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

Author: johnniwinther

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

@@ -623,7 +623,8 @@ class ElementDisplayStringBuilder {
 
     var newTypeParameters = <TypeParameterElementImpl2>[];
     for (var typeParameter in type.typeParameters) {
-      var name = typeParameter.name3!;
+      // The type parameter name can be null in erroneous cases.
+      var name = typeParameter.name3 ?? '';
       for (var counter = 0; !namesToAvoid.add(name); counter++) {
         const unicodeSubscriptZero = 0x2080;
         const unicodeZero = 0x30;

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

@@ -863,6 +863,23 @@ AssignmentExpression
 ''');
   }
 
+  test_indexExpression_unresolved_missing_type_parameter_name() async {
+    await assertErrorsInCode(
+      r'''
+abstract class A {
+   void b< extends int>();
+}
+void f(A a) {
+  a.b[0] = 0;
+}
+''',
+      [
+        error(ParserErrorCode.MISSING_IDENTIFIER, 30, 7),
+        error(CompileTimeErrorCode.UNDEFINED_OPERATOR, 67, 3),
+      ],
+    );
+  }
+
   test_indexExpression_unresolvedTarget_compound() async {
     await assertErrorsInCode(
       r'''

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

@@ -38,6 +38,12 @@ NamedType
     alias: <testLibrary>::@typeAlias::G
       typeArguments
         int
+''');
+  }
+
+  test_type_missing_type_parameter_name() async {
+    await resolveTestCode(r'''
+typedef F = void Function< extends int>();
 ''');
   }
 }