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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/102283] Inconsistent/wrong overload resolution when using
 an initializer list and a defaulted template parameter
Date: Thu, 16 Sep 2021 13:40:57 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102283

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to Giuseppe D'Angelo from comment #2)
> Hi,
>=20
> Do you think that in my original testcase the call should be rejected as
> ambiguous as well? (It seems "reasonable" to me, but maybe I'm missing so=
me
> niche detail about overload resolution when combined with template
> deduction.)
>=20
>=20
> This small variation over the testcase:
>=20
>=20
> struct A { };
> struct B { };
>=20
> template <typename T =3D A>
> void f(T &&); // #1
> void f(const B&) =3D delete; // #2
>=20
> int main() {
>   f({});
> }
>=20
> This now makes GCC select #2, and fail to compile because it's deleted;

I think this is because GCC considers the two ICSes to be incomparable, but
then #2 wins over #1 anyway because non-templates are preferred over templa=
tes
(as per [over.match.best]).

Whereas with Clang/MSVC, I presume they consider the ICS for #1 to be better
than that for #2 (though I haven't found the wording in the standard that
supports this), so #1 wins during the ICS comparison stage of overload
resolution.

> Clang and MSVC still select #1. But a further, minor change:
>=20
>=20
> struct A { };
> struct B { };
>=20
> template <typename T =3D int> // <-- changed this
> void f(T &&); // #1
> void f(const B&) =3D delete; // #2
>=20
> int main() {
>   f({});
> }
>=20
> makes GCC select #1...

This example is a different because the ICS for #1 is a standard conversion
sequence (since the {} initializes a non-class) whereas for #2 it's a
user-defined conversion sequence (since the {} initializes a class), and the
former kind is always better than the latter.  So #1 wins due to having the
better ICS.=