Fixes #2902. Add tests for an extension applicability

LanguageFeatures/Extension-methods/applicability_A01_t01.dart

@@ -0,0 +1,51 @@
+// 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.
+
+/// @assertion An extension `E` is applicable to a simple or composite member
+/// invocation with corresponding member basename m and target expression `e`,
+/// where `e` has static type `S`, if
+/// ...
+/// - The type S does not have a member with the basename m. For this, the type
+///   `dynamic` is considered as having all member names, and an expression of
+///   type `Never` or `void` cannot occur as the target of a member invocation,
+///   so none of these can ever have applicable extensions. Function types and
+///   the type `Function` are considered as having a `call` member. This ensure
+///   that if there is an applicable extension, the existing invocation would
+///   otherwise be a compile-time error. Members of `Object` exists on all
+///   types, so they can never be the target of implicit member invocations
+///   (they can also not be declared as extension members).
+///
+/// @description Check that an extension member with the basename `m` is not
+/// applicable if on type has a static member with the same basename.
+/// @author [email protected]
+/// @issue 56818
+
+class C {
+  static String get m1 => "m1";
+  static String m2() => "m2";
+  static void set m3(String _) {}
+}
+
+extension Ext on C {
+  String get m1 => "Ext.m1";
+  String m2() => "Ext.m2";
+  void set m3(String _) {}
+}
+
+main() {
+  print(C().m1);
+//          ^^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+  print(C().m2());
+//          ^^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+  C().m3 = "";
+//    ^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}

LanguageFeatures/Extension-methods/applicability_A01_t02.dart

@@ -0,0 +1,50 @@
+// 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.
+
+/// @assertion An extension `E` is applicable to a simple or composite member
+/// invocation with corresponding member basename m and target expression `e`,
+/// where `e` has static type `S`, if
+/// ...
+/// - The type S does not have a member with the basename m. For this, the type
+///   `dynamic` is considered as having all member names, and an expression of
+///   type `Never` or `void` cannot occur as the target of a member invocation,
+///   so none of these can ever have applicable extensions. Function types and
+///   the type `Function` are considered as having a `call` member. This ensure
+///   that if there is an applicable extension, the existing invocation would
+///   otherwise be a compile-time error. Members of `Object` exists on all
+///   types, so they can never be the target of implicit member invocations
+///   (they can also not be declared as extension members).
+///
+/// @description Check that an extension member with the basename `m` is not
+/// applicable if on type has a static member with the same basename.
+/// @author [email protected]
+
+class C {
+  static String get m1 => "m1";
+  static String m2() => "m2";
+  static void set m3(String _) {}
+}
+
+extension Ext on C {
+  void set m1(String _) {}
+  void set m2(String _) {}
+  String m3() => "Ext.m2";
+}
+
+main() {
+  print(C().m1);
+//          ^^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+  print(C().m2());
+//          ^^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+  C().m3 = "";
+//    ^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}