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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Date: Fri, 24 Mar 2023 20:47:01 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109277

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #2)
> The difference is is_convertible definition.
> 12:
>   template<typename _From, typename _To>
>     struct is_convertible
>     : public __is_convertible_helper<_From, _To>::type
>     { };
>=20
>=20
> 13:
>   template<typename _From, typename _To>
>     struct is_convertible
>     : public __bool_constant<__is_convertible(_From, _To)>
>     { };
>=20
> So it might be a front-end issue after all.

and yes that is definitely the difference here. If I add back the
__is_convertible_helper and use that for is_convertible to the GCC 13
preprocessed source, the code then compiles.=