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Implement the forwarding performed by std::bind via deducing this when available, instead of needing 4 operator() overloads. Using deducing this here is more complicated than in other standard call wrappers because std::bind is not really "perfect forwarding": it doesn't consider value category, and along with const-ness it also forwards volatile-ness (until C++20). The old implementation suffers from the same problem that other pre-C++23 SFINAE-friendly call wrappers have which is solved by using deducing this (see p5.5 of the deducing this paper P0847R7). PR libstdc++/80564 libstdc++-v3/ChangeLog: * include/std/functional (__cv_like): New. (_Bind::_Res_type): Don't define when not needed. (_Bind::__dependent): Likewise. (_Bind::_Res_type_cv): Likewise. (_Bind::operator()) [_GLIBCXX_EXPLICIT_THIS_PARAMETER]: Define as two instead of four overloads using deducing this. * testsuite/20_util/bind/cv_quals_2.cc: Ignore SFINAE diagnostics inside headers. * testsuite/20_util/bind/ref_neg.cc: Likewise. * testsuite/20_util/bind/80564.cc: New test. Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com> Reviewed-by: Jonathan Wakely <jwakely@redhat.com>