[dart2wasm] Fix compiler bug regarding void values that can be observed

Issue #54800

Change-Id: I26b8c35f60955ee46eeae19797aab7e6c84bf354
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/350580
Reviewed-by: Slava Egorov <[email protected]>
Commit-Queue: Martin Kustermann <[email protected]>

Author: mkustermann

pkg/dart2wasm/lib/code_generator.dart

@@ -1031,12 +1031,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
 
   @override
   void visitVariableDeclaration(VariableDeclaration node) {
-    if (node.type is VoidType) {
-      if (node.initializer != null) {
-        wrap(node.initializer!, voidMarker);
-      }
-      return;
-    }
     w.ValueType type = translateType(node.type);
     w.Local? local;
     Capture? capture = closures.captures[node];
@@ -2166,10 +2160,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
 
   @override
   w.ValueType visitVariableGet(VariableGet node, w.ValueType expectedType) {
-    // Return `void` for a void [VariableGet].
-    if (node.variable.type is VoidType) {
-      return voidMarker;
-    }
     w.Local? local = locals[node.variable];
     Capture? capture = closures.captures[node.variable];
     if (capture != null) {
@@ -2192,10 +2182,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
 
   @override
   w.ValueType visitVariableSet(VariableSet node, w.ValueType expectedType) {
-    // Return `void` for a void [VariableSet].
-    if (node.variable.type is VoidType) {
-      return wrap(node.value, voidMarker);
-    }
     w.Local? local = locals[node.variable];
     Capture? capture = closures.captures[node.variable];
     bool preserved = expectedType != voidMarker;

tests/language/void/regress_54800_test.dart

@@ -0,0 +1,26 @@
+// 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 'dart:async';
+
+import 'package:async_helper/async_helper.dart';
+import 'package:expect/expect.dart';
+
+void main() async {
+  asyncStart();
+  testVoid();
+  await testFutureVoid();
+  asyncEnd();
+}
+
+void testVoid() {
+  void x = int.parse('42');
+  Expect.equals(42, x as dynamic);
+}
+
+Future testFutureVoid() async {
+  final controller = StreamController(onCancel: () async => int.parse('42'));
+  final subscription = controller.stream.listen(null);
+  Expect.equals(42, (await subscription.cancel()) as dynamic);
+}