Remove type cast in completion to prevent exception

Change-Id: If97adc54523894a3b9df241ffb0308ee1299d03d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/372020
Reviewed-by: Keerti Parthasarathy <keertip@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

pkg/analysis_server/lib/src/services/completion/dart/declaration_helper.dart

@@ -1144,11 +1144,14 @@ class DeclarationHelper {
         );
       }
     }
-    _addExtensionMembers(
-        type: element.thisType as InterfaceType,
-        excludedGetters: {},
-        includeMethods: true,
-        includeSetters: true);
+    var thisType = element.thisType;
+    if (thisType is InterfaceType) {
+      _addExtensionMembers(
+          type: thisType,
+          excludedGetters: {},
+          includeMethods: true,
+          includeSetters: true);
+    }
   }
 
   /// Completion is inside [declaration].

pkg/analysis_server/test/services/completion/dart/location/function_invocation_test.dart

@@ -0,0 +1,49 @@
+// Copyright (c) 2024, the Dart project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:test_reflective_loader/test_reflective_loader.dart';
+
+import '../../../../client/completion_driver_test.dart';
+
+void main() {
+  defineReflectiveSuite(() {
+    defineReflectiveTests(FunctionInvocationTest);
+  });
+}
+
+@reflectiveTest
+class FunctionInvocationTest extends AbstractCompletionDriverTest
+    with FunctionInvocationTestCases {}
+
+mixin FunctionInvocationTestCases on AbstractCompletionDriverTest {
+  Future<void> test_implicitCall() async {
+    await computeSuggestions('''
+extension E<T> on Comparator<T> {
+  Comparator<T> get inverse => (T a0, T b0) => this(^);
+}
+''');
+    // TODO(brianwilkerson): `super` should not be suggested here.
+    assertResponse(r'''
+suggestions
+  a0
+    kind: parameter
+  b0
+    kind: parameter
+  true
+    kind: keyword
+  false
+    kind: keyword
+  null
+    kind: keyword
+  this
+    kind: keyword
+  const
+    kind: keyword
+  super
+    kind: keyword
+  switch
+    kind: keyword
+''');
+  }
+}

pkg/analysis_server/test/services/completion/dart/location/test_all.dart

@@ -33,6 +33,7 @@ import 'for_element_test.dart' as for_element;
 import 'for_statement_test.dart' as for_statement;
 import 'function_declaration_test.dart' as function_declaration;
 import 'function_expression_test.dart' as function_expression;
+import 'function_invocation_test.dart' as function_invocation;
 import 'if_element_test.dart' as if_element;
 import 'if_statement_test.dart' as if_statement;
 import 'implements_clause_test.dart' as implements_clause;
@@ -112,6 +113,7 @@ void main() {
     for_statement.main();
     function_declaration.main();
     function_expression.main();
+    function_invocation.main();
     if_element.main();
     if_statement.main();
     implements_clause.main();