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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/102257] call of overloaded 'tuple' is ambiguous
Date: Tue, 14 Sep 2021 15:31:23 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102257

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to =E5=BA=B7=E6=A1=93=E7=91=8B from comment #3)
> (In reply to Andrew Pinski from comment #2)
> > See https://wg21.link/cwg1228 this might be invalid code and GCC is cor=
rect
> > in rejecting it.
>=20
> Maybe. But why does GCC accept the following?
>=20
> #include <tuple>
> #include <string>
>=20
> int main() {
>   std::tuple<std::string, int> t{{}, {}};
> }
>=20
> https://godbolt.org/z/ePovjh3fa

Looks like we accept this testcase because the template parameter _Alloc for
the template candidate

  template<typename _Alloc,
           _ImplicitDefaultCtor<is_object<_Alloc>::value, _T1, _T2> =3D tru=
e>
  tuple(std::allocator_arg_t, const _Alloc&)

is no longer deducible (whereas in the original testcase _Alloc is deduced =
to
int here), so this candidate is discarded and the result of overload resolu=
tion
is no longer ambiguous.=20

(In reply to Andrew Pinski from comment #2)
> See https://wg21.link/cwg1228 this might be invalid code and GCC is corre=
ct
> in rejecting it.

Agreed FWIW=