[Bug libstdc++/126949] New: [15/16/17 Regression] Can't construct an unordered set with a hasher whose ctor is explicit

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126949

            Bug ID: 126949
           Summary: [15/16/17 Regression] Can't construct an unordered set
                    with a hasher whose ctor is explicit
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gnu.ojxq8 at dralias dot com
  Target Milestone: ---

libstdc++15 and 16 do not allow to construct an unordered set with a
hasher whose ctor is explicit.

This is an issue with Clang, or with GCC -Wsystem-headers. See
https://godbolt.org/z/EGzKbjh84

Code:

```
// { dg-do compile { target c++11 } }
#include <unordered_set>
struct Hash {
    explicit Hash(bool = false) {}
    std::size_t operator()(int value) const { return value; }
};
int main() { std::unordered_set<int, Hash> set; }
```

error:

/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/bits/hashtable_policy.h:1056:70:
error: chosen constructor is explicit in copy-initialization