Re: [PATCH] c++: coercing variable template from current inst [PR110580]

[Jason Merrill <jason@redhat.com>]

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Ok.

On Tue, Jul 11, 2023, 9:16 AM Patrick Palka <ppalka@redhat.com> wrote:

> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?
>
> -- >8 --
>
> Here during ahead of time coercion of the variable template-id v1<int>,
> since we pass only the innermost arguments to coerce_template_parms (and
> outer arguments are still dependent at this point), substitution of the
> default template argument V=U prematurely lowers U from level 2 to level 1.
> Thus we incorrectly resolve v1<int> to v1<int, T> (effectively) instead
> of to v1<int, int>.
>
> Coercion of a class/alias template-id on the other hand is always done
> using the full set of arguments relative to the most general template,
> so ahead of time coercion there does the right thing.  I suppose we
> should do the same for variable template-ids.
>
>         PR c++/110580
>
> gcc/cp/ChangeLog:
>
>         * pt.cc (lookup_template_variable): Pass all arguments to
>         coerce_template_parms, and use the innermost parameters from
>         the most general template.
>
> gcc/testsuite/ChangeLog:
>
>         * g++.dg/cpp1y/var-templ83.C: New test.
> ---
>  gcc/cp/pt.cc                             |  4 +++-
>  gcc/testsuite/g++.dg/cpp1y/var-templ83.C | 16 ++++++++++++++++
>  2 files changed, 19 insertions(+), 1 deletion(-)
>  create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ83.C
>
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 076f788281e..fa15b75b9c5 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -10345,7 +10345,9 @@ lookup_template_variable (tree templ, tree
> arglist, tsubst_flags_t complain)
>    if (flag_concepts && variable_concept_p (templ))
>      return build_concept_check (templ, arglist, tf_none);
>
> -  tree parms = DECL_INNERMOST_TEMPLATE_PARMS (templ);
> +  tree gen_templ = most_general_template (templ);
> +  tree parms = DECL_INNERMOST_TEMPLATE_PARMS (gen_templ);
> +  arglist = add_outermost_template_args (templ, arglist);
>    arglist = coerce_template_parms (parms, arglist, templ, complain);
>    if (arglist == error_mark_node)
>      return error_mark_node;
> diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ83.C
> b/gcc/testsuite/g++.dg/cpp1y/var-templ83.C
> new file mode 100644
> index 00000000000..f5268f258d7
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/var-templ83.C
> @@ -0,0 +1,16 @@
> +// PR c++/110580
> +// { dg-do compile { target c++14 } }
> +
> +template<class T>
> +struct A {
> +  template<typename U, typename V = U>
> +  static constexpr bool v1 = __is_same(U, V);
> +
> +  template<typename U, typename V = T>
> +  static constexpr bool v2 = !__is_same(U, V);
> +
> +  static_assert(v1<int>, "");
> +  static_assert(v2<int>, "");
> +};
> +
> +template struct A<char>;
> --
> 2.41.0.327.gaa9166bcc0
>
>