[dart2wasm] Use default value sentinel when member body unreachable

This issue was partially fixed by:
https://dart-review.googlesource.com/c/sdk/+/428541

However, this only looked at the case of a member in an abstract class
that itself was made abstract if it was unused. TFA does not make the
member abstract if the enclosing class is not abstract:
pkg/vm/lib/transformations/type_flow/transformer.dart#L2289

Instead we can use the unreachable metadata to identify these
unreachable body members (that have default values cleared):
pkg/vm/lib/transformations/type_flow/transformer.dart#L736

Attached test fails prior to this fix.

Bug: https://github.com/dart-lang/sdk/issues/62273
Change-Id: I1ccb818ec41bbe805ee828be0b96ba23e73beae4
Tested: Introduced unit test.
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/469280
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Nate Biggs <natebiggs@google.com>

pkg/dart2wasm/lib/dispatch_table.dart

@@ -7,6 +7,7 @@ import 'dart:math' show min;
 import 'package:kernel/ast.dart';
 import 'package:vm/metadata/procedure_attributes.dart';
 import 'package:vm/metadata/table_selector.dart';
+import 'package:vm/metadata/unreachable.dart';
 import 'package:wasm_builder/wasm_builder.dart' as w;
 
 import 'class_info.dart';
@@ -131,7 +132,11 @@ class SelectorInfo {
     final references = source.readList(source.readReference);
 
     final paramInfo = _parameterInfoFromReferences(
-        references, useSentinelForOptionalParameters);
+        source.component
+                .metadata[UnreachableNodeMetadataRepository.repositoryTag]
+            as UnreachableNodeMetadataRepository,
+        references,
+        useSentinelForOptionalParameters);
     return SelectorInfo._(dispatchTable, id, name, callCount,
         isSetter: isSetter)
       ..useMultipleEntryPoints = useMultipleEntryPoints
@@ -747,7 +752,9 @@ class DispatchTable {
         selector.useMultipleEntryPoints = false;
         selector._useSentinelForOptionalParameters = true;
         selector.paramInfo = _parameterInfoFromReferences(
-            selector._references, selector._useSentinelForOptionalParameters);
+            translator.unreachableMetadata,
+            selector._references,
+            selector._useSentinelForOptionalParameters);
       } else {
         // Will be initialized in the `selectorTargets.forEach()` below.
       }
@@ -814,7 +821,9 @@ class DispatchTable {
         selector._useSentinelForOptionalParameters = true;
       }
       selector.paramInfo = _parameterInfoFromReferences(
-          selector._references, selector._useSentinelForOptionalParameters);
+          translator.unreachableMetadata,
+          selector._references,
+          selector._useSentinelForOptionalParameters);
 
       // Split up [ranges] into those that are statically dispatched to and
       // those are used via dispatch table.
@@ -1076,15 +1085,26 @@ bool _isUsedViaDispatchTableCall(SelectorInfo selector) {
 }
 
 ParameterInfo _parameterInfoFromReferences(
-    List<Reference> references, bool useDefaultValueSentinel) {
+    UnreachableNodeMetadataRepository unreachableMetadata,
+    List<Reference> references,
+    bool useDefaultValueSentinel) {
+  final unreachableNodeMapping = unreachableMetadata.mapping;
   // We know all target implementations (closed world) if all of them use
   // the same default value for optionals, we can make the caller pass it.
   final first = references.first;
+  final firstMember = first.asMember;
   final paramInfo = ParameterInfo.fromMember(
-      first, useDefaultValueSentinel || first.asMember.isAbstract);
+      first,
+      useDefaultValueSentinel ||
+          firstMember.isAbstract ||
+          unreachableNodeMapping[firstMember] != null);
   for (final target in references.skip(1)) {
+    final targetMember = target.asMember;
     paramInfo.merge(ParameterInfo.fromMember(
-        target, useDefaultValueSentinel || target.asMember.isAbstract));
+        target,
+        useDefaultValueSentinel ||
+            targetMember.isAbstract ||
+            unreachableNodeMapping[targetMember] != null));
   }
   return paramInfo;
 }

pkg/dart2wasm/lib/serialization.dart

@@ -178,11 +178,12 @@ class DataSerializer {
 }
 
 class DataDeserializer {
+  final Component component;
   final _BinaryDataSource _source;
   final _IdToEntityMapper _mapper;
   final List<String> _stringIndexer = [];
 
-  DataDeserializer(Uint8List bytes, Component component)
+  DataDeserializer(Uint8List bytes, this.component)
       : _source = _BinaryDataSource(bytes),
         _mapper = _IdToEntityMapper(component);
 

pkg/dart2wasm/lib/translator.dart

@@ -15,6 +15,7 @@ import 'package:vm/metadata/direct_call.dart';
 import 'package:vm/metadata/inferred_type.dart';
 import 'package:vm/metadata/procedure_attributes.dart';
 import 'package:vm/metadata/unboxing_info.dart';
+import 'package:vm/metadata/unreachable.dart';
 import 'package:wasm_builder/wasm_builder.dart' as w;
 
 import 'class_info.dart';
@@ -184,6 +185,9 @@ class Translator with KernelNodes {
       (component.metadata[ProcedureAttributesMetadataRepository.repositoryTag]
               as ProcedureAttributesMetadataRepository)
           .mapping;
+  late final UnreachableNodeMetadataRepository unreachableMetadata =
+      component.metadata[UnreachableNodeMetadataRepository.repositoryTag]
+          as UnreachableNodeMetadataRepository;
 
   // Other parts of the global compiler state.
   @override

pkg/dart2wasm/lib/util.dart

@@ -16,6 +16,7 @@ import 'package:vm/metadata/procedure_attributes.dart'
     show ProcedureAttributesMetadataRepository;
 import 'package:vm/metadata/table_selector.dart'
     show TableSelectorMetadataRepository;
+import 'package:vm/metadata/unreachable.dart';
 
 final bool compilerAssertsEnabled = (() {
   bool compilerAsserts = false;
@@ -104,6 +105,7 @@ String intToMinString(int i) {
 
 Component createEmptyComponent() {
   return Component()
+    ..addMetadataRepository(UnreachableNodeMetadataRepository())
     ..addMetadataRepository(ProcedureAttributesMetadataRepository())
     ..addMetadataRepository(TableSelectorMetadataRepository())
     ..addMetadataRepository(DirectCallMetadataRepository())

pkg/vm/lib/metadata/unreachable.dart

@@ -19,8 +19,10 @@ class UnreachableNode {
 /// Repository for [UnreachableNode].
 class UnreachableNodeMetadataRepository
     extends MetadataRepository<UnreachableNode> {
+  static const repositoryTag = 'vm.unreachable.metadata';
+
   @override
-  final String tag = 'vm.unreachable.metadata';
+  final String tag = repositoryTag;
 
   @override
   final Map<TreeNode, UnreachableNode> mapping = <TreeNode, UnreachableNode>{};

tests/web/wasm/regress_62273_test.dart

@@ -0,0 +1,33 @@
+// Copyright (c) 2026, 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.
+
+class A {
+  void foo({String s = 'hi'}) {
+    print(s);
+  }
+}
+
+class B extends A {
+  @override
+  void foo({String s = 'there'}) {
+    print(s);
+  }
+}
+
+A getA(int x) {
+  if (x < 10) {
+    return B();
+  } else {
+    // A is instantiated but the type cannot flow into a 'foo' call so A.foo's
+    // body is not reachable.
+    throw 'here';
+    return A();
+  }
+}
+
+void main() {
+  getA(9).foo();
+  // Ensure the default value is not inlined into the function.
+  getA(8).foo(s: 'wow');
+}