<functional>: Fixes around not_fn#4057
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StephanTLavavej merged 2 commits intomicrosoft:mainfrom Oct 4, 2023
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1. Make the return type (`_Not_fn`) perfect forwarding call wrapper since C++20, which is a change in WG21-P0356R5. 2. Make `not_fn` reject non-move-constructible types.
Contributor
|
You should probably also add a struct OnlyMovableFun {
OnlyMovableFun() = default;
OnlyMovableFun(const OnlyMovableFun&) = delete;
OnlyMovableFun(OnlyMovableFun&&) = default;
bool operator()(auto) const;
};
int main() {
OnlyMovableFun f;
auto nf = std::not_fn(f); // should be static_assert?
} |
Contributor
That wouldn't "avoid hard errors", it would transform a hard error into a different hard error. |
StephanTLavavej
approved these changes
Oct 3, 2023
Member
|
Thanks - this compile-time change is low-risk enough that I think we need only one maintainer approval. |
Member
|
I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed. |
Member
|
Thanks for noticing and fixing this issue in |
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Make the return type (
_Not_fn) perfect forwarding call wrapper since C++20, which is a change in WG21-P0356R5.Currently this PR leaves
_Not_fnas-is in C++17 mode, because the return type was nearly fully specified in C++17 (N4659 [func.not_fn]/1), which means thatoperator()overloads, an invocation to a non-const value of the return type might select a const overload, anddecltype(!declval<invoke_result_t<...>>()), if theoperator!takes a non-movable return type ofstd::invoke(args...)by value, the return type ofoperator()would be ill-formed, while!std::invoke(args...)being well-formed.In C++20 the first case should be ill-formed (in an SFINAE-friendly way), and the second case should be well-formed.
Also make
not_fnreject non-move-constructible types. Fixes #4048.I don't know why call wrappers (except for
std::function) are still required to be Cpp17MoveConstructible in C++17 and later. It seems that the restrictions can be lifted.