[dart2js] Fix scope visitor missing not visiting structural parameter default type.

By not visiting the default type of structural type parameters, the scope visitor was not registering the T type parameter as being potentially needed for RTI. So then the SSA RTI builder did not have the necessary scope info to create the RTI objects for the closures' signatures.

Fixed: 54451
Change-Id: I96f2af985bd176e71a6779fd27b09efdd76fe6cf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/343660
Commit-Queue: Nate Biggs <natebiggs@google.com>
Reviewed-by: Stephen Adams <sra@google.com>

pkg/compiler/lib/src/ir/scope_visitor.dart

@@ -403,6 +403,9 @@ class ScopeModelBuilder extends ir.VisitorDefault<EvaluationComplexity>
   @override
   EvaluationComplexity visitStructuralParameter(
       ir.StructuralParameter typeParameter) {
+    // Visit the default type to register any necessary type parameters that RTI
+    // might need if the associated function is used as a generic tear off.
+    visitNode(typeParameter.defaultType);
     return const EvaluationComplexity.constant();
   }
 

tests/web/regress/issue/54451_test.dart

@@ -0,0 +1,25 @@
+// Copyright (c) 2023, 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.
+
+// Tests that the bounds for structural parameters of a Function type are
+// visited and any type variables RTI might need are registered.
+
+class A<T> {
+  void foo1() {
+    void bar1<Z extends void Function<Y extends T>()>() {}
+    print(bar1.runtimeType); // Crashes compiler if A.T is not accessible.
+  }
+}
+
+extension<T> on T {
+  void foo2() {
+    void bar2<Z extends void Function<Y extends T>()>() {}
+    print(bar2.runtimeType); // Crashes compiler if T is not accessible.
+  }
+}
+
+void main() {
+  A<int>().foo1();
+  1.foo2();
+}