error: definition with same mangled name when specializing a concepted template

[Alcaro]

#include <cstdio>
#include <concepts>

template <typename T>
void fun(T);

template <>
void fun(char c)
{
    std::puts("foo<char>()");
}

template <std::integral I>
void fun(I i)
{
    std::puts("foo<std::integral>()");
}

int main()
{
    fun<char>(' ');
}

Compile with -std=c++20.

Result:

<source>:14:6: error: definition with same mangled name '_Z3funIcEvT_' as another definition
<source>:8:6: note: previous definition is here

Expected: Prints foo<char>(), or at least a less cryptic error

https://godbolt.org/z/axvdbK1Eh

GCC and MSVC have similar bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107151
https://developercommunity.visualstudio.com/t/Template-explicit-specializationconcept/10012835

[llvmbot]

@llvm/issue-subscribers-clang-codegen

[usx95]

Very similar to #48216

[erichkeane]

While a similar look, this isn't the same issue as #48216 has a fix for the friends instantiation, this one is unofortunately different.

[zygoloid]

Fixed in 4b163e3. Contrary to the suggestion at the start of this issue, the program prints

foo<std::integral>()

because the fun(char) explicit specialization specializes the primary template, not the constrained template<integral> overload, because the latter is not yet declared. And overload resolution picks the template<integral> version because it's more constrained.