[cfe] Wildcard import prefixes are non-binding.

Bug: https://github.com/dart-lang/sdk/issues/55655
Change-Id: I938c254fad2c656c9342e9cea5b146373b9f63e5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/372603
Commit-Queue: Kallen Tu <kallentu@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>

pkg/front_end/lib/src/fasta/builder/prefix_builder.dart

@@ -30,8 +30,11 @@ class PrefixBuilder extends BuilderImpl {
 
   final LoadLibraryBuilder? loadLibraryBuilder;
 
+  final bool isWildcard;
+
   PrefixBuilder(this.name, this.deferred, this.parent, this.loadLibraryBuilder,
-      this.charOffset, this.importIndex) {
+      this.charOffset, this.importIndex)
+      : isWildcard = name == '_' {
     assert(deferred == (loadLibraryBuilder != null),
         "LoadLibraryBuilder must be provided iff prefix is deferred.");
     if (loadLibraryBuilder != null) {

pkg/front_end/lib/src/fasta/kernel/body_builder.dart

@@ -3515,6 +3515,14 @@ class BodyBuilder extends StackListenerImpl
           memberBuilder.parent, memberBuilder.member, null);
     } else if (declaration is PrefixBuilder) {
       assert(prefix == null);
+      // Wildcard import prefixes are non-binding and cannot be used.
+      if (libraryFeatures.wildcardVariables.isEnabled &&
+          declaration.isWildcard) {
+        // TODO(kallentu): Provide a helpful error related to wildcard prefixes.
+        return new UnresolvedNameGenerator(this, nameToken,
+            new Name(declaration.name, libraryBuilder.nameOrigin),
+            unresolvedReadKind: UnresolvedKind.Unknown);
+      }
       return new PrefixUseGenerator(this, nameToken, declaration);
     } else if (declaration is LoadLibraryBuilder) {
       return new LoadLibraryGenerator(this, nameToken, declaration);

pkg/front_end/testcases/wildcard_variables/import_prefix.dart

@@ -0,0 +1,11 @@
+// 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:collection' as _;
+
+test() {
+  _.Queue<int>();
+  Queue<int>();
+  [1].firstOrNull;
+}

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

@@ -0,0 +1,27 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/wildcard_variables/import_prefix.dart:8:3: Error: Undefined name '_'.
+//   _.Queue<int>();
+//   ^
+//
+// pkg/front_end/testcases/wildcard_variables/import_prefix.dart:9:3: Error: Method not found: 'Queue'.
+//   Queue<int>();
+//   ^^^^^
+//
+import self as self;
+import "dart:core" as core;
+import "dart:collection" as col;
+
+import "dart:collection" as _;
+
+static method test() → dynamic {
+  invalid-expression "pkg/front_end/testcases/wildcard_variables/import_prefix.dart:8:3: Error: Undefined name '_'.
+  _.Queue<int>();
+  ^"{dynamic}.Queue<core::int>();
+  invalid-expression "pkg/front_end/testcases/wildcard_variables/import_prefix.dart:9:3: Error: Method not found: 'Queue'.
+  Queue<int>();
+  ^^^^^";
+  col::IterableExtensions|get#firstOrNull<core::int>(<core::int>[1]);
+}

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

@@ -0,0 +1,27 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/wildcard_variables/import_prefix.dart:8:3: Error: Undefined name '_'.
+//   _.Queue<int>();
+//   ^
+//
+// pkg/front_end/testcases/wildcard_variables/import_prefix.dart:9:3: Error: Method not found: 'Queue'.
+//   Queue<int>();
+//   ^^^^^
+//
+import self as self;
+import "dart:core" as core;
+import "dart:collection" as col;
+
+import "dart:collection" as _;
+
+static method test() → dynamic {
+  invalid-expression "pkg/front_end/testcases/wildcard_variables/import_prefix.dart:8:3: Error: Undefined name '_'.
+  _.Queue<int>();
+  ^"{dynamic}.Queue<core::int>();
+  invalid-expression "pkg/front_end/testcases/wildcard_variables/import_prefix.dart:9:3: Error: Method not found: 'Queue'.
+  Queue<int>();
+  ^^^^^";
+  col::IterableExtensions|get#firstOrNull<core::int>(<core::int>[1]);
+}

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

@@ -0,0 +1,7 @@
+library;
+import self as self;
+
+import "dart:collection" as _;
+
+static method test() → dynamic
+  ;

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

@@ -0,0 +1,27 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/wildcard_variables/import_prefix.dart:8:3: Error: Undefined name '_'.
+//   _.Queue<int>();
+//   ^
+//
+// pkg/front_end/testcases/wildcard_variables/import_prefix.dart:9:3: Error: Method not found: 'Queue'.
+//   Queue<int>();
+//   ^^^^^
+//
+import self as self;
+import "dart:core" as core;
+import "dart:collection" as col;
+
+import "dart:collection" as _;
+
+static method test() → dynamic {
+  invalid-expression "pkg/front_end/testcases/wildcard_variables/import_prefix.dart:8:3: Error: Undefined name '_'.
+  _.Queue<int>();
+  ^"{dynamic}.Queue<core::int>();
+  invalid-expression "pkg/front_end/testcases/wildcard_variables/import_prefix.dart:9:3: Error: Method not found: 'Queue'.
+  Queue<int>();
+  ^^^^^";
+  col::IterableExtensions|get#firstOrNull<core::int>(core::_GrowableList::_literal1<core::int>(1));
+}

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

@@ -0,0 +1,3 @@
+import 'dart:collection' as _;
+
+test() {}

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

@@ -0,0 +1,3 @@
+import 'dart:collection' as _;
+
+test() {}

tests/language/wildcard_variables/import/import_error_test.dart

@@ -15,21 +15,21 @@ main() {
   _.topLevel;
 //^
 // [analyzer] unspecified
-// [cfe] unspecified
+// [cfe] Undefined name '_'.
 
   _.C(value);
 //^
 // [analyzer] unspecified
-// [cfe] unspecified
+// [cfe] Undefined name '_'.
 
   // Private extensions can't be used.
   value.bar;
-//^
-// [analyzer] unspecified
-// [cfe] unspecified
+//      ^^^
+// [analyzer] COMPILE_TIME_ERROR.UNDEFINED_GETTER
+// [cfe] The getter 'bar' isn't defined for the class 'String'.
 
   value.fn;
-//^
-// [analyzer] unspecified
-// [cfe] unspecified
+//      ^^
+// [analyzer] COMPILE_TIME_ERROR.UNDEFINED_GETTER
+// [cfe] The getter 'fn' isn't defined for the class 'String'.
 }