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.
From-SVN: r274542
