[Bug libstdc++/80564] bind on SFINAE unfriendly generic lambda

[columbo@gmx-topmail.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80564

--- Comment #6 from Robert Haberlach <columbo@gmx-topmail.de> ---
(In reply to TC from comment #4)
> (In reply to Eric Fiselier from comment #3)
> > Here is an example of why `_Bind::operator()(...) const` must be considered
> > during overload resolution even if the call wrapper itself is not const.
> > 
> > ------------------------------
> > #include <functional>
> > 
> > struct Func {
> >   template <class ...Args>
> >   void operator()(Args&&...) = delete;
> > 
> >   template <class ...Args>
> >   void operator()(Args&&...) const {}
> > };
> > 
> > int main() {
> >     Func f;
> >     std::bind(f)();
> > }
> > -------------------------
> 
> Interesting, libstdc++ rejects this as an attempt to call a deleted
> function. That seems more correct than libc++'s approach which calls the
> const overload.

See also the discussion on Phabricator: https://reviews.llvm.org/D32824

I agree. AFAICS [func.bind.bind] is clear on this: the type of the Func
object used to call the member operator() is non-const.