https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91456
--- Comment #4 from Jonathan Wakely <redi at gcc dot 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 <jwak...@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
