[cfe] Allow unnamed optional wildcard parameters to have no default value.

Wildcard optional parameters aren't used anyways, so it doesn't matter if they have a default value.
This CL removes the error that's typically there.

Bug: https://github.com/dart-lang/sdk/issues/55655
Change-Id: Ie7286e1c3650fa347b1c28f1cc4ebd657cca33de
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/371560
Commit-Queue: Kallen Tu <kallentu@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>

pkg/front_end/lib/src/fasta/source/source_library_builder.dart

@@ -2109,7 +2109,8 @@ class SourceCompilationUnitImpl implements SourceCompilationUnit {
     FormalParameterBuilder formal = new FormalParameterBuilder(
         kind, modifiers, type, name, _sourceLibraryBuilder, charOffset,
         fileUri: fileUri,
-        hasImmediatelyDeclaredInitializer: initializerToken != null)
+        hasImmediatelyDeclaredInitializer: initializerToken != null,
+        isWildcard: libraryFeatures.wildcardVariables.isEnabled && name == '_')
       ..initializerToken = initializerToken;
     return formal;
   }
@@ -5242,6 +5243,14 @@ class SourceLibraryBuilder extends LibraryBuilderImpl {
       if (isOptional &&
           formal.variable!.type.isPotentiallyNonNullable &&
           !formal.hasDeclaredInitializer) {
+        // Wildcard optional parameters can't be used so we allow having no
+        // initializer.
+        if (libraryFeatures.wildcardVariables.isEnabled &&
+            formal.isWildcard &&
+            !formal.isSuperInitializingFormal &&
+            !formal.isInitializingFormal) {
+          continue;
+        }
         addProblem(
             templateOptionalNonNullableWithoutInitializerError.withArguments(
                 formal.name, formal.variable!.type),

pkg/front_end/lib/src/fasta/type_inference/inference_visitor_base.dart

@@ -2120,6 +2120,13 @@ abstract class InferenceVisitorBase implements InferenceVisitor {
       if ((isOptionalPositional || isOptionalNamed) &&
           formal.type.isPotentiallyNonNullable &&
           !formal.hasDeclaredInitializer) {
+        // Wildcard optional parameters can't be used so we allow having no
+        // initializer.
+        if (libraryFeatures.wildcardVariables.isEnabled &&
+            formal.isWildcard &&
+            !formal.isInitializingFormal) {
+          continue;
+        }
         libraryBuilder.addProblem(
             templateOptionalNonNullableWithoutInitializerError.withArguments(
                 formal.name!, formal.type),

pkg/front_end/testcases/wildcard_variables/unnamed_optional_parameter_no_default.dart

@@ -0,0 +1,13 @@
+// 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.
+
+void foo([int _]) {}
+
+void main() {
+  void bar([int _]) {}
+  void bar1([int _, int _]) {}
+  void bar2([int _, int _ = 2]) {}
+  void bar3([int _ = 2, int _]) {}
+  void bar4([int x = 2, int? _, int _]) {}
+}

pkg/front_end/testcases/wildcard_variables/unnamed_optional_parameter_no_default.dart.strong.expect

@@ -0,0 +1,17 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+static method foo([wildcard core::int _ = #C1]) → void {}
+static method main() → void {
+  function bar([wildcard core::int _ = #C1]) → void {}
+  function bar1([wildcard core::int _ = #C1, wildcard core::int _ = #C1]) → void {}
+  function bar2([wildcard core::int _ = #C1, wildcard core::int _ = #C2]) → void {}
+  function bar3([wildcard core::int _ = #C2, wildcard core::int _ = #C1]) → void {}
+  function bar4([core::int x = #C2, wildcard core::int? _ = #C1, wildcard core::int _ = #C1]) → void {}
+}
+
+constants  {
+  #C1 = null
+  #C2 = 2
+}

pkg/front_end/testcases/wildcard_variables/unnamed_optional_parameter_no_default.dart.strong.modular.expect

@@ -0,0 +1,17 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+static method foo([wildcard core::int _ = #C1]) → void {}
+static method main() → void {
+  function bar([wildcard core::int _ = #C1]) → void {}
+  function bar1([wildcard core::int _ = #C1, wildcard core::int _ = #C1]) → void {}
+  function bar2([wildcard core::int _ = #C1, wildcard core::int _ = #C2]) → void {}
+  function bar3([wildcard core::int _ = #C2, wildcard core::int _ = #C1]) → void {}
+  function bar4([core::int x = #C2, wildcard core::int? _ = #C1, wildcard core::int _ = #C1]) → void {}
+}
+
+constants  {
+  #C1 = null
+  #C2 = 2
+}

pkg/front_end/testcases/wildcard_variables/unnamed_optional_parameter_no_default.dart.strong.outline.expect

@@ -0,0 +1,8 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+static method foo([wildcard core::int _]) → void
+  ;
+static method main() → void
+  ;

pkg/front_end/testcases/wildcard_variables/unnamed_optional_parameter_no_default.dart.strong.transformed.expect

@@ -0,0 +1,17 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+static method foo([wildcard core::int _ = #C1]) → void {}
+static method main() → void {
+  function bar([wildcard core::int _ = #C1]) → void {}
+  function bar1([wildcard core::int _ = #C1, wildcard core::int _ = #C1]) → void {}
+  function bar2([wildcard core::int _ = #C1, wildcard core::int _ = #C2]) → void {}
+  function bar3([wildcard core::int _ = #C2, wildcard core::int _ = #C1]) → void {}
+  function bar4([core::int x = #C2, wildcard core::int? _ = #C1, wildcard core::int _ = #C1]) → void {}
+}
+
+constants  {
+  #C1 = null
+  #C2 = 2
+}

pkg/front_end/testcases/wildcard_variables/unnamed_optional_parameter_no_default.dart.textual_outline.expect

@@ -0,0 +1,3 @@
+void foo([int _]) {}
+
+void main() {}

pkg/front_end/testcases/wildcard_variables/unnamed_optional_parameter_no_default.dart.textual_outline_modelled.expect

@@ -0,0 +1,3 @@
+void foo([int _]) {}
+
+void main() {}

tests/language/wildcard_variables/unnamed_optional/unnamed_optional_error_test.dart

@@ -10,11 +10,9 @@
 
 class SuperClass {
   SuperClass([int _]);
-  //              ^
-  // [analyzer] COMPILE_TIME_ERROR.MISSING_DEFAULT_VALUE_FOR_PARAMETER
-  // [cfe] The parameter '_' can't have a value of 'null' because of its type 'int', but the implicit default value is 'null'.
   SuperClass.nullable([int? _]);
 }
+
 class SubClass extends SuperClass {
   final int _;
   SubClass([
@@ -31,6 +29,7 @@ class SubClass extends SuperClass {
   // [cfe] Type 'int' of the optional super-initializer parameter '_' doesn't allow 'null', but the parameter doesn't have a default value, and the default value can't be copied from the corresponding parameter of the super constructor.
   ]);
 }
+
 class TypedSubClass extends SuperClass {
   final int? _;
   TypedSubClass([