LanguageFeatures/Constructor-tear-offs/ambiguities_A11_t03.dart

// Copyright (c) 2021, 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 This new syntax also introduces new ambiguities in the grammar, 
/// similar to the one we introduced with generic functions. Examples include:
/// 
/// f(a<b,c>(d)); // Existing ambiguity, resolved to a generic method call.
/// f(x.a<b,c>[d]); // f((x.a<b, c>)[d]) or f((x.a < b), (c > [d]))
/// f(x.a<b,c>-d);  // f((x.a<b, c>)-d) or f((x.a < b), (c > -d]))
/// The x.a<b,c> can be an explicitly instantiated generic function tear-off or 
/// an explicitly instantiated type literal named using a prefix, which is new. 
/// While neither type objects nor functions declare operator- or operator[], 
/// such could be added using extension methods.
/// 
/// We will disambiguate such situations heuristically based on the token 
/// following the > that matches the < we are ambiguous about. In the existing 
/// ambiguity we treat ( as a sign that the < starts a generic invocation. We 
/// extend the number of tokens which, when following a potential type argument 
/// list, makes us choose to parse the previous tokens as that type argument 
/// list.
/// 
/// There is a number of tokens which very consistently end an expression, and 
/// we include all those:
/// 
/// ), }, ], ;, :, ,
/// 
/// Then we include tokens which we predict will continue a generic instantiation:
/// 
/// ( . == !=
/// 
/// The first six are tokens which cannot possibly start an expression, and 
/// therefore cannot occur after a greater-than infix operator. The last four 
/// tokens can continue an expression, and of those only ( can also start an 
/// expression, and we already decided how to disambiguate that).
/// 
/// There are many other tokens which currently cannot continue an expression 
/// (and therefore cannot validly follow a type argument list) or which cannot 
/// start an expression (and therefore cannot validly follow a greater-than 
/// operator), but in the service of keeping our future options open, we choose 
/// a design that does not rely on those restrictions. For example we omit most 
/// infix operators from being "continuation tokens", even though they currently 
/// cannot start a new expression, and therefore cannot follow a > infix 
/// operator. This leaves us open to allowing some of those operators as prefix 
/// operators in the future, like we currently allow the - operator.
///
/// @description Checks disambiguate by '!=' token. Test that a<b, c>!= is
/// parsed as (a<b, c>!=. Test constructor tear-off
/// @author sgrekhov@unipro.ru

// SharedOptions=--enable-experiment=constructor-tearoffs

import "../../Utils/expect.dart";

String f(a, [b]) => "$a, $b";

class a<T1, T2> {
  int x;
  a(this.x);

  @override
  String toString() => "a<$T1, $T2>($x)";
}

typedef b = int;
typedef c = String;

main() {
  var x = a<b, c>;
  Expect.equals("false, null", f(a<b, c> != x));
}