[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 #5 from Steven Sun <StevenSun2021 at hotmail dot com> ---
I learn a little about gcc recently. I think I got a vague idea of what's going
on inside.

In c++17 mode with concepts, and with my code in comment 1.

The compiler decides to instantiate from the concept constrained template.

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Usually the compiler should generate a new function on the AST, but this time
there's already one function (fully specialized) on the AST.

So, the function `start_preparsed_function` in the below link returns at the
quoted line. It's thinking "Ah! There's already a function, no need to allocate
a new one."

https://gcc.gnu.org/git?p=gcc.git;a=blob;f=gcc/cp/decl.c;h=316ad4c1426940bd4f51197a6297eefc24064fec;hb=HEAD#l16696

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Then the flow goes back to the function in the link below. It's thinking "Since
I'm gonna add a new function, new function is allocated on AST. Everything
should be prepared for me!"

More specifially, it will assume the `cfun` in the correct state set by the
`start_preparsed_function`.

But `cfun` is null, since the `start_preparsed_function` returns early. This is
the direct cause of segfault, which triggered at the line below.

https://gcc.gnu.org/git?p=gcc.git;a=blob;f=gcc/cp/pt.c;h=36a8cb5df5d36337c18e1547e775b747f59a087f;hb=HEAD#l25932

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In conclusion, The compiler doesn't consider there would be a situation that an
instatiation is still needed when full specialzation is provided. This break
the invariants.