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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/113884] GCC rejects valid program saying ambiguous call
 when using std::vector
Date: Mon, 12 Feb 2024 05:59:32 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113884

--- Comment #8 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Jason Liam from comment #7)
> (In reply to Andrew Pinski from comment #5)
> > std::vector 's constructor which takes std::size_t is marked as explici=
t.
>=20
> But you're missing that the initializer list ctor is preferred/choosen ov=
er
> the size_t arg ctor.=20

That is different from your original example.
This is closer to your original example:
```
#include <initializer_list>

struct B
{
  B(std::initializer_list<double>);
};

struct C
{
  explicit C(int);
};


void func(B);
void func(C);

int main() {
    func({ 4.2 });
}
```

here we have two calls to func, one which takes B and the other which takes=
 C.
In the case case of copy-list-initialization, it is ambigous which is to be
constructed as copy-list-initialization for overload resolution; explicit
constructors are looked at but only an error if it was chosen. Note narrowi=
ng
is not taken into account for overload resolution too; maybe that is what y=
ou
are missing.

Again read https://wg21.link/cwg1228 which talks about this not being a def=
ect
in the C++ standard; this is how the standard is written.

Anyways clang's bug report is https://github.com/llvm/llvm-project/issues/2=
8016
.

>implying the the second overload is not even viable so how can it make the=
 call ambiguous.

I should note that is not have overload resolution works either.=