[Bug c++/99686] ICE when using both concepts and full specialization

["StevenSun2021 at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99686

--- Comment #3 from Steven Sun <StevenSun2021 at hotmail dot com> ---
@W E Brown: I got your idea. So are all uses like this ill-formed? This seems
unexpected for me. I would expect the complete specialization is full
specialization for both primary templates.

I also find facts that support your idea:

If I modify the full specialization to 
--------------------
template <>
void func<int&&> (int&& arg){}
--------------------
It compiles. See https://godbolt.org/z/fh9Mx7Krr


If I modify the full specialization to 
--------------------
template <>
void func<int> (int&& arg){}
--------------------
It produce the same ICE output as comment 1. See
https://godbolt.org/z/eMa5YcrrE

It looks like the compiler doesn't knows which primary template to specialize.

I look up in the C++20 standard, and did not find anything about 2 primary
templates exists.

The code in Comment 1 compiles for g++ 6.1 to 9.3, then breaks since 10.1 (all
under -std=c++17 -fconcepts).

In conclusion, this makes sences but I didn't see that coming. Anyway, I think
a possible improvement is make ICE to an error of "ambigous full
specialization". Or even better, a change in C++23 standard.