r277380 - in /branches/gcc-9-branch/libstdc++-v...

[redi@gcc.gnu.org]

Author: redi
Date: Thu Oct 24 12:54:13 2019
New Revision: 277380

URL: https://gcc.gnu.org/viewcvs?rev=277380&root=gcc&view=rev
Log:
PR libstdc++/91456 make INVOKE<R> work with uncopyable prvalues

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).

Backport from mainline
2019-08-15  Jonathan Wakely  <jwakely@redhat.com>

	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.

Added:
    branches/gcc-9-branch/libstdc++-v3/testsuite/20_util/function/91456.cc
    branches/gcc-9-branch/libstdc++-v3/testsuite/20_util/is_invocable/91456.cc
Modified:
    branches/gcc-9-branch/libstdc++-v3/ChangeLog
    branches/gcc-9-branch/libstdc++-v3/include/bits/std_function.h
    branches/gcc-9-branch/libstdc++-v3/include/std/type_traits