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3680. Constructor ofmove_only_function with empty ref-qualifier is over-constrained
Section: 22.10.17.4.3 [func.wrap.move.ctor] Status: New Submitter: Zhihao Yuan Opened: 2022-02-27 Last modified: 2022-03-04
Priority: 2
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Discussion:
The following test compiles and holds:
struct X
{
int operator()() & { return 1; } // #1
int operator()() && { return 2; } // #2
};
using T = move_only_function<int()>;
assert(T(X{})() == 1);
In other words, #2 is never used. But if you really remove #2, the code doesn't compile.
move_only_function<R(Args...) cv> erasing callable objects with operator() without ref-qualifiers. But the actual outcome outlawed programs that were valid when using std::function. It also outlaws future programs such as
T x = [captures](this auto&) { return ...; }
where declaring this auto&& without forwarding only to satisfy move_only_function's constraints would be strange.
[2022-03-04; Reflector poll]
Set priority to 2 after reflector poll. Probably needs a paper for LEWG.
Proposed resolution:
This wording is relative to N4901.
Modify 22.10.17.4.3 [func.wrap.move.ctor] as indicated:
template<class VT> static constexpr bool is-callable-from = see below;-1- If
noexistrue,is-callable-from<VT>is equal to:is_nothrow_invocable_r_v<R, VT cv ref, ArgTypes...> && is_nothrow_invocable_r_v<R, VT inv-quals, ArgTypes...>Otherwise,
is-callable-from<VT>is equal to:is_invocable_r_v<R, VT cv ref, ArgTypes...> && is_invocable_r_v<R, VT inv-quals, ArgTypes...>
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