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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/96330] Constexpr variables cannot be used in the template
 context.
Date: Fri, 05 Mar 2021 16:43:45 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96330

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #1)
> Tried
> --- gcc/cp/pt.c.jj	2020-07-27 10:38:19.000000000 +0200
> +++ gcc/cp/pt.c	2020-07-27 17:25:09.748240198 +0200
> @@ -16838,14 +16838,17 @@ tsubst_copy (tree t, tree args, tsubst_f
>      case TEMPLATE_ID_EXPR:
>        {
>  	/* Substituted template arguments */
> -	tree fn =3D TREE_OPERAND (t, 0);
> +	tree expr =3D TREE_OPERAND (t, 0);
>  	tree targs =3D TREE_OPERAND (t, 1);
>=20=20
> -	fn =3D tsubst_copy (fn, args, complain, in_decl);
> +	expr =3D tsubst_copy (expr, args, complain, in_decl);
>  	if (targs)
>  	  targs =3D tsubst_template_args (targs, args, complain, in_decl);
>=20=20
> -	return lookup_template_function (fn, targs);
> +        if (variable_template_p (expr))
> +	  return lookup_and_finish_template_variable (expr, targs, complain);
> +	else
> +	  return lookup_template_function (expr, targs);
>        }
>=20=20
>      case TREE_LIST:
> but that just means an ICE elsewhere.

Hmm, seems using lookup_template_variable instead of
lookup_and_finish_template_variable works.=