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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/102184] Explicit template instantiation thinks there is an
 ambiguous template specialization when there is none for concepts
Date: Fri, 03 Sep 2021 07:07:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102184

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-09-03
           Keywords|                            |rejects-valid
            Summary|Explicit template           |Explicit template
                   |instantiation is wrongly    |instantiation thinks there
                   |considered as               |is an ambiguous template
                   |specialization              |specialization when there
                   |                            |is none for concepts

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> GCC is saying there is an ambigous specialization.
>=20
> If I do this, GCC accepts the code:
> template<int N> requires(N=3D=3D1) int fib() { return 1; }
> template<int N> requires(N=3D=3D2) int fib() { return 1; }
> template<int N> int fib() requires(N!=3D1 && N!=3D2)  { return fib<N-1>()=
 +
> fib<N-2>(); }
> template int fib<1>();

But I think GCC does not know which specialization to instantiate here.
Because the following works just fine:
int t =3D fib<1>();=