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From: "StevenSun2021 at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/99686] ICE when using both concepts and full specialization
Date: Fri, 07 May 2021 13:15:16 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99686

--- Comment #7 from Steven Sun <StevenSun2021 at hotmail dot com> ---
After digging for two days, I think I may know what's happening inside.=20

Under std=3Dc++20 and my code in comment 1, the flow when parsing a full fu=
nction
specialization would be

1. Found a full specialization of function.
2. Instantiate all possible primary function templates.=20
3. Find the most suitable instantiation (here, the concept constrained is
chosen)
4. Then replace it with full specialization. (this is done in
`reregister_specialization` in `gcc/cp/pt.c`)

So when performing overloading in the main function, there would be one
instantiation and one full specialization to choose. (To be specific, those=
 two
are in the hash table `decl_specialization` defined in `gcc/cp/pt.c`)


While in the c++17 mode, step 4 is broken. So there will be 2 implicit
instantiation to choose when overloading.

Step 4 is performed in the function `duplicate_decls` defined in
`gcc/cp/decl.c`.

Certain conditions must be satisfied to replace the instantiation.

The one failed here is:
"the compiler thinks the full specialization has a different concepts
constraint with primary template (because the primary template has contrain=
ts
but the full specialization doesn't)"=20

So the reregistration is never triggered. This is the source of the bug.

After getting the whole picture, I simplify the test case as
https://godbolt.org/z/9MM6rEf77

----------------------- std=3Dc++17 -fconcepts

template <typename T>
requires requires(T t) { ++t; }
void func(T &&arg)
{
}

template <>
void func(int&& arg)
{
}

int main()
{
    func(1);
}

-----------------------

I'll give more details in the next comment in case any developer would like=
 to
fix it.=