[Bug libstdc++/125514] [16/17 Regression] libstdc++: reference_wrapper triggers -Wsfinae-incomplete for forward-declared type in C++17

[tkaminsk at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125514

--- Comment #3 from Tomasz Kamiński <tkaminsk at gcc dot gnu.org> ---
Correction, to above. In C++17 the result_type, argument_type,
first_argument_type and second_argument_type are required to be present, so we
couldn't remove them. See section 4.1 of p0357r1.