Re: [PATCH] c++: CTAD for class tmpl defined inside partial spec [PR104294]

[Patrick Palka <ppalka@redhat.com>]

On Mon, Jan 31, 2022 at 11:07 AM Patrick Palka <ppalka@redhat.com> wrote:
>
> Here during deduction guide generation for the nested class template
> B<char(int)>::C, the computation of outer_targs yields the template
> arguments relative to the primary template for B (i.e. {char(int)})
> but what we really what is those relative to the enclosing scope, the
> partial specialization of B (i.e. {char, int}).
>
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?
>
>         PR c++/104294
>
> gcc/cp/ChangeLog:
>
>         * pt.cc (ctor_deduction_guides_for): Correct computation of
>         outer_targs.
>
> gcc/testsuite/ChangeLog:
>
>         * g++.dg/cpp1z/class-deduction106.C: New test.
> ---
>  gcc/cp/pt.cc                                  |  4 +++-
>  .../g++.dg/cpp1z/class-deduction106.C         | 19 +++++++++++++++++++
>  2 files changed, 22 insertions(+), 1 deletion(-)
>  create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction106.C
>
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index f46a7ad6655..0fe0ad4e44e 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -29568,7 +29568,9 @@ ctor_deduction_guides_for (tree tmpl, tsubst_flags_t complain)
>    if (DECL_CLASS_SCOPE_P (tmpl)
>        && CLASSTYPE_TEMPLATE_INSTANTIATION (DECL_CONTEXT (tmpl)))
>      {
> -      outer_args = CLASSTYPE_TI_ARGS (DECL_CONTEXT (tmpl));
> +      outer_args = copy_node (CLASSTYPE_TI_ARGS (type));
> +      gcc_assert (TMPL_ARGS_HAVE_MULTIPLE_LEVELS (outer_args));

Whoops, this assert should be slightly stronger: TMPL_ARGS_DEPTH
(outer_args) > 1. Consider that fixed (pending another
bootstrap/regtest cycle).

> +      --TREE_VEC_LENGTH (outer_args);
>        type = TREE_TYPE (most_general_template (tmpl));
>      }
>
> diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction106.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction106.C
> new file mode 100644
> index 00000000000..382f6b70ef5
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction106.C
> @@ -0,0 +1,19 @@
> +// PR c++/104294
> +// { dg-do compile { target c++17 } }
> +
> +template<class>
> +struct B;
> +
> +template <class R, class... Args>
> +struct B<R(Args...)> {
> +  template<class T>
> +  struct C { C(T); };
> +
> +  C(decltype(nullptr)) -> C<void*>;
> +};
> +
> +using ty1 = decltype(B<char(int)>::C{0});
> +using ty1 = B<char(int)>::C<int>;
> +
> +using ty2 = decltype(B<char(int)>::C{nullptr});
> +using ty2 = B<char(int)>::C<void*>;
> --
> 2.35.0
>