[Bug c++/100288] [11/12 Regression] g++-11 internal error and fails to precompile a concept

["f.b.brokken at rug dot nl" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100288

--- Comment #4 from Frank B. Brokken <f.b.brokken at rug dot nl> ---
Dear ppalka at gcc dot gnu.org, you wrote:
> 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100288
> 
> Patrick Palka <ppalka at gcc dot gnu.org> changed:
> 
>            What    |Removed                     |Added
> ----------------------------------------------------------------------------
>            Keywords|                            |ice-on-invalid-code
>            See Also|                            |https://gcc.gnu.org/bugzill
>                    |                            |a/show_bug.cgi?id=99599
> 
> --- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
> Agreed, the testcase looks invalid much like PR99599.  One workaround for
> avoiding the constraint recursion here would be to change the signature of
> 
>   template <OstreamInsertable Type>
>   inline void operator<<(CSVTabIns &&tab, Type const &value)
> 
> to something like
> 
>   template <std::same_as<std::remove_cvref_t<CSVTabIns>> U, OstreamInsertable
> Type>
>   inline void operator<<(U &&tab, Type const &value)
> 
> so that the constraint OstreamInsertable<Type> is checked on this overload only
> if the first argument to << has the expected type.
> 
> -- 
> You are receiving this mail because:
> You reported the bug.

Hi Patrick,

Thanks for your e-mail.

Since (AFAICS) you directly and only sent your e-mail to me I'm wondering
whether you want me to comment on your e-mail. It's getting kind of late here,
but I could send a more extensive reply tomorrow. Let me know if that's what
you want.

I tried your suggestion, and it seems to solve the issue. But at the same time
it puzzles me why 

    template <OstreamInsertable Type>
    inline void operator<<(CSVTabIns &&tab, Type const &value)
    {}

won't be instantiated for FMT in

    inline void operator<<(CSVTabIns &tab, FMT::FMTHline hline)
    {
        tab << (*hline)(1);      // insert hline in the next column
    }

when FMT defines FMTHline as  'typedef FMT (*FMTHline)(unsigned)' and 

    std::ostream &operator<<(std::ostream &out, FMT const &fmt)

is declared. I would have expected that 

        tab << (*hline)(1);      // insert hline in the next column

would cause the compiler to instantiate 

    template <OstreamInsertable Type>
    inline void operator<<(CSVTabIns &&tab, Type const &value)
    {}

for Type = FMT.

Maybe I'm missing something here? And it's also not something that caused the
compiler's internal error.