LanguageFeatures/Extension-types/syntax_A08_t02.dart

// Copyright (c) 2023, 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 A rule for <extensionTypeDeclaration> is added to the grammar,
/// along with some rules for elements used in extension type declarations:
///
/// <extensionTypeDeclaration> ::=
///   'extension' 'type' 'const'? <typeIdentifier> <typeParameters>?
///   <representationDeclaration> <interfaces>?
///   '{'
///     (<metadata> <extensionTypeMemberDeclaration>)*
///   '}'
///
/// <representationDeclaration> ::=
///   ('.' <identifierOrNew>)? '(' <metadata> <type> <identifier> ')'
///
/// <identifierOrNew> ::= <identifier> | 'new'
///
/// <extensionTypeMemberDeclaration> ::= <classMemberDefinition>
/// ...
/// Some errors can be detected immediately from the syntax:
///
/// A compile-time error occurs if the extension type declaration declares any
/// instance variables, unless they are external
///
/// @description Checks that it is not an error if an extension type declaration
/// declares an external variables
/// @author sgrekhov22@gmail.com

extension type ET1(int id) {
  external int x;
}

extension type ET2<T>(T id) {
  external final T x;
}

main() {
  print(ET1);
  print(ET2);
}