[dart2wasm] Only share type parameter fields if nullability allows

Closes #55741

Change-Id: I1e542041496d07714431ed40871031a117030736
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/366940
Reviewed-by: Ömer Ağacan <omersa@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>

pkg/dart2wasm/lib/class_info.dart

@@ -318,8 +318,10 @@ class ClassInfoCollector {
             : cls.implementedTypes.single;
         for (TypeParameter parameter in cls.typeParameters) {
           for (int i = 0; i < supertype.typeArguments.length; i++) {
-            DartType arg = supertype.typeArguments[i];
-            if (arg is TypeParameterType && arg.parameter == parameter) {
+            DartType superTypeArg = supertype.typeArguments[i];
+            if (superTypeArg is TypeParameterType &&
+                superTypeArg.parameter == parameter &&
+                superTypeArg.nullability != Nullability.nullable) {
               typeParameterMatch[parameter] = superInfo.cls!.typeParameters[i];
               break;
             }

pkg/dart2wasm/lib/transformers.dart

@@ -149,8 +149,10 @@ class _WasmTransformer extends Transformer {
     }
     for (int i = 0; i < cls.typeParameters.length; i++) {
       TypeParameter parameter = cls.typeParameters[i];
-      DartType arg = supertype.typeArguments[i];
-      if (arg is! TypeParameterType || arg.parameter != parameter) {
+      DartType superTypeArg = supertype.typeArguments[i];
+      if (superTypeArg is! TypeParameterType ||
+          superTypeArg.parameter != parameter ||
+          superTypeArg.nullability == Nullability.nullable) {
         return false;
       }
     }

tests/language/regression_55741_test.dart

@@ -0,0 +1,36 @@
+// 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:expect/expect.dart';
+
+final kTrue = int.parse('1') == 1;
+
+void main() {
+  final base = kTrue ? Base<String>('a') : 1;
+  final baseNullable = kTrue ? Base<String?>('a') : 1;
+  final sub = kTrue ? Sub<String>('a') : 1;
+  final subNullable = kTrue ? Sub<String?>('a') : 1;
+
+  Expect.isTrue(base is Base<String>, 'is Base<String>');
+  Expect.isTrue(base is Base<String?>, 'is Base<String?>');
+  Expect.isTrue(baseNullable is! Base<String>, 'is! Base<String>');
+  Expect.isTrue(baseNullable is Base<String?>, 'is Base<String?>');
+  Expect.isTrue(sub is Sub<String>, 'is Sub<String>');
+  Expect.isTrue(sub is Sub<String?>, 'is Sub<String?>');
+  Expect.isTrue(subNullable is! Sub<String>, 'is! Sub<String>');
+  Expect.isTrue(subNullable is Sub<String?>, 'is Sub<String?>');
+  Expect.isTrue(sub is! Base<String>, 'is! Base<String>');
+  Expect.isTrue(sub is Base<String?>, 'is Base<String?>');
+  Expect.isTrue(subNullable is! Base<String>, 'is! Base<String>');
+  Expect.isTrue(subNullable is Base<String?>, 'is Base<String?>');
+}
+
+class Base<T> {
+  Base(this.data);
+  final T data;
+}
+
+class Sub<T> extends Base<T?> {
+  Sub(super.data);
+}