Paolo Tranquilli e13a85c8f1 NFC: make headers compatible with C++20 ...

In a downstream project we are using the Swift code as a library and
compiling with C++20. This mostly works except for two backward
incompatibilities introduced by C++20:

* [P1008] disallows aggregate initialization when defaulted or deleted
  constructors are specified. This lead to a compilation error for
  aggregate initialization of `swift::Compilation::Result`. The backward
  and forward compatible fix for that is to provide a constructor
  turning aggregate initializations into a normal constructor call.
* [P1185] introduces more candidates for overload resolution of
  comparison operator calls. As explained in [P1630], this leads in some
  cases to ambiguity compilation errors in C++20, which is exactly the
  case with `swift::ValueOwnershipKind`. The fix in this case is to
  remove some redundant operator calls conditionally on the
  `__cpp_impl_three_way_comparison` feature test macro, which [includes
  the papers mentioned above][feature_test].

[P1008]: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1008r1.pdf
[P1185]: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1185r2.html
[P1630]: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1630r1.html
[feature_test]: https://en.cppreference.com/w/cpp/feature_test

2023-07-31 17:34:05 +02:00

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