On 15/08/19 17:04 +0100, Jonathan Wakely wrote:
>In C++17 a function can return a prvalue of a type that cannot be moved
>or copied. The current implementation of std::is_invocable_r uses
>std::is_convertible to test the conversion to R required by INVOKE<R>.
>That fails for non-copyable prvalues, because std::is_convertible is
>defined in terms of std::declval which uses std::add_rvalue_reference.
>In C++17 conversion from R to R involves no copies and so is not the
>same as conversion from R&& to R.
>
>This commit changes std::is_invocable_r to check the conversion without
>using std::is_convertible.
>
>std::function also contains a similar check using std::is_convertible,
>which can be fixed by simply reusing std::is_invocable_r (but because
>std::is_invocable_r is not defined for C++11 it uses the underlying
>std::__is_invocable_impl trait directly).
>
>	PR libstdc++/91456
>	* include/bits/std_function.h (__check_func_return_type): Remove.
>	(function::_Callable): Use std::__is_invocable_impl instead of
>	__check_func_return_type.
>	* include/std/type_traits (__is_invocable_impl): Add another defaulted
>	template parameter. Define a separate partial specialization for
>	INVOKE and INVOKE<void>. For INVOKE<R> replace is_convertible check
>	with a check that models delayed temporary materialization.
>	* testsuite/20_util/function/91456.cc: New test.
>	* testsuite/20_util/is_invocable/91456.cc: New test.

With some minor changes to __is_convertible_helper we could make that
usable by both std::is_convertible and __is_invokable_impl.

I don't plan to commit this now but might do at a later date.