[dart2js] Add implicit null to type for optional non-nullable parameters.

This ensures any code downstream of this knows that it can receive a null and null checks are not removed during SSA.

Change-Id: I8c3d706875a9b70ed8407d00053fb381df168a49
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/406480
Reviewed-by: Mayank Patke <[email protected]>

Author: natebiggs

pkg/compiler/lib/src/inferrer/builder.dart

@@ -287,16 +287,25 @@ class KernelTypeGraphBuilder extends ir.VisitorDefault<TypeInformation?>
     required bool isOptional,
   }) {
     Local local = _localsMap.getLocalVariable(node);
-    _state.setLocal(
-      _inferrer,
-      _capturedAndBoxed,
-      local,
-      _inferrer.typeOfParameter(local),
-    );
+    final parameterType = _inferrer.typeOfParameter(local);
+    _state.setLocal(_inferrer, _capturedAndBoxed, local, parameterType);
+
     if (isOptional) {
       TypeInformation type;
       if (node.initializer != null) {
         type = visit(node.initializer)!;
+        if (_inferrer.abstractValueDomain.isNull(type.type).isDefinitelyTrue &&
+            !node.type.isPotentiallyNullable) {
+          // TODO(52582): Make optional nonnullable parameters nullable until
+          // such a case from redirecting factories is a compile-time error.
+          //
+          // In rare cases (e.g. optional parameters on redirecting factories)
+          // the CFE can give us a non-nullable parameter with a null
+          // initializer. Ideally we would take the type of the initializer on
+          // the target parameter but instead we're conservative and treat the
+          // parameter as nullable.
+          parameterType.isOptionalNoDefault = true;
+        }
       } else {
         type = _types.nullType;
       }

pkg/compiler/lib/src/inferrer/engine.dart

@@ -1262,7 +1262,7 @@ class InferrerEngine {
   }
 
   /// Returns the type of [element].
-  TypeInformation typeOfParameter(Local element) {
+  ParameterTypeInformation typeOfParameter(Local element) {
     return types.getInferredTypeOfParameter(element);
   }
 

pkg/compiler/lib/src/inferrer/type_graph_nodes.dart

@@ -48,27 +48,28 @@ enum _Flag {
   isClosureParameter, // 6
   isInitializingFormal, // 7
   isVirtual, // 8
+  isOptionalNoDefault, // 9
   // ---Flags for [CallSiteTypeInformation]---
-  inLoop, // 9
+  inLoop, // 10
   // ---Flags for [DynamicCallSiteTypeInformation]---
-  isConditional, // 10
-  hasClosureCallTargets, // 11
-  targetsIncludeComplexNoSuchMethod, // 12
-  hasTargetsIncludeComplexNoSuchMethod, // 13
+  isConditional, // 11
+  hasClosureCallTargets, // 12
+  targetsIncludeComplexNoSuchMethod, // 13
+  hasTargetsIncludeComplexNoSuchMethod, // 14
   // ---Flags for [PhiElementTypeInformation]---
-  isTry, // 14
+  isTry, // 15
   // ---Flags for [ValueInMapTypeInformation]---
-  valueInMapNonNull, // 15
+  valueInMapNonNull, // 16
   // ---Flags for [MemberTypeInformation]---
-  isCalled, // 16
-  isCalledMoreThanOnce, // 17
+  isCalled, // 17
+  isCalledMoreThanOnce, // 18
   // ---Flags for [ApplyableTypeInformation]---
-  mightBePassedToFunctionApply, // 18
+  mightBePassedToFunctionApply, // 19
   // ---Flags for [InferredTypeInformation]---
-  inferred, // 19
+  inferred, // 20
   // ---Flags for [TracedTypeInformation]---
-  notBailedOut, // 20
-  analyzed, // 21
+  notBailedOut, // 21
+  analyzed, // 22
 }
 
 /// Common class for all nodes in the graph. The current nodes are:
@@ -831,6 +832,9 @@ class ParameterTypeInformation extends ElementTypeInformation {
   bool get _isInitializingFormal => _flags.contains(_Flag.isInitializingFormal);
   bool _isTearOffClosureParameter = false;
   bool get _isVirtual => _flags.contains(_Flag.isVirtual);
+  bool get isOptionalNoDefault => _flags.contains(_Flag.isOptionalNoDefault);
+  set isOptionalNoDefault(bool value) =>
+      _flags = _flags.update(_Flag.isOptionalNoDefault, value);
 
   ParameterTypeInformation.localFunction(
     super.abstractValueDomain,
@@ -968,7 +972,11 @@ class ParameterTypeInformation extends ElementTypeInformation {
     AbstractValue mask,
     InferrerEngine inferrer,
   ) {
-    return _narrowType(inferrer.abstractValueDomain, mask, _type);
+    final staticType =
+        isOptionalNoDefault
+            ? inferrer.abstractValueDomain.includeNull(_type)
+            : _type;
+    return _narrowType(inferrer.abstractValueDomain, mask, staticType);
   }
 
   AbstractValue checkedType(InferrerEngine inferrer) {

pkg/compiler/test/inference/data/redirecting_factory.dart

@@ -0,0 +1,19 @@
+// Copyright (c) 2025, 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 B implements A {
+  /*member: B.foo:[exact=B]*/
+  B.foo([int? /*[null]*/ x]);
+}
+
+class A {
+  /*member: A._#foo#tearOff:[exact=B]*/
+  factory A.foo([int /*[null]*/ x]) = B.foo;
+}
+
+/*member: main:[null]*/
+void main() {
+  final f = A.foo;
+  f();
+}