On Sat, 13 Dec 2025, Egas Ribeiro wrote:
> Regtested on x86_64-pc-linux-gnu, OK for trunk?
>
> -- >8 --
>
> When processing inherited CTAD in C++23, type_targs_deducible_from can
> be called with a synthetic alias template whose TREE_VALUE is a type
> alias. Since TYPE_TEMPLATE_INFO_MAYBE_ALIAS can return NULL for type
> aliases, we need to fall back to TYPE_TEMPLATE_INFO to get the template
> info of the underlying type before calling TI_TEMPLATE, and otherwise
> return NULL_TREE when the type lacks template info, indicating it can't
> be deduced.
>
> PR c++/122070
>
> gcc/cp/ChangeLog:
>
> * pt.cc (type_targs_deducible_from): Fall back to
> TYPE_TEMPLATE_INFO when TYPE_TEMPLATE_INFO_MAYBE_ALIAS is NULL.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp23/class-deduction-inherited10.C: New test.
> * g++.dg/cpp23/class-deduction-inherited9.C: New test.
>
> Signed-off-by: Egas Ribeiro <[email protected]>
> ---
> gcc/cp/pt.cc | 12 ++++++-
> .../cpp23/class-deduction-inherited10.C | 10 ++++++
> .../g++.dg/cpp23/class-deduction-inherited9.C | 31 +++++++++++++++++++
> 3 files changed, 52 insertions(+), 1 deletion(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp23/class-deduction-inherited10.C
> create mode 100644 gcc/testsuite/g++.dg/cpp23/class-deduction-inherited9.C
>
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 341e5ab8808..50d1cac5a7a 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -31641,7 +31641,17 @@ type_targs_deducible_from (tree tmpl, tree type)
> per alias_ctad_tweaks. */
> tparms = INNERMOST_TEMPLATE_PARMS (TREE_PURPOSE (tmpl));
> ttype = TREE_VALUE (tmpl);
> - tmpl = TI_TEMPLATE (TYPE_TEMPLATE_INFO_MAYBE_ALIAS (ttype));
> + tree ti = TYPE_TEMPLATE_INFO_MAYBE_ALIAS (ttype);
> +
> + /* Fallback to using TYPE_TEMPLATE_INFO if
> + TYPE_TEMPLATE_INFO_MAYBE_ALIAS is NULL_TREE. */
> + if (!ti)
> + {
> + ti = TYPE_TEMPLATE_INFO (ttype);
> + if (!ti)
> + return NULL_TREE;
Looks good, though I don't think TYPE_TEMPLATE_INFO will ever be null
here, this code path is only taken when called from inherited_ctad_tweaks
which already verifies TYPE_TEMPLATE_INFO is non-NULL.
Patch pushed to trunk as r16-6247-ga0908b67231287 with the second null
check removed, thanks!
-- >8 --
> + }
> + tmpl = TI_TEMPLATE (ti);
> }
>
> int len = TREE_VEC_LENGTH (tparms);
> diff --git a/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited10.C
> b/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited10.C
> new file mode 100644
> index 00000000000..7f9c9f21645
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited10.C
> @@ -0,0 +1,10 @@
> +// { dg-do compile { target c++23 } }
> +// PR c++/122070
> +template<class T> struct A { A(T) {}; };
> +
> +using B = A<int>;
> +
> +template<class T=void>
> +struct C : B { using B::B; };
> +
> +C c = 0;
> diff --git a/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited9.C
> b/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited9.C
> new file mode 100644
> index 00000000000..b13394bf8f9
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited9.C
> @@ -0,0 +1,31 @@
> +// { dg-do compile { target c++23 } }
> +// PR c++/122070
> +
> +namespace std {
> + using size_t = decltype(sizeof(0));
> + template<typename CharT>
> + struct basic_string_view {
> + const CharT* ptr;
> + size_t len;
> + constexpr basic_string_view(const CharT* p) : ptr(p), len(0) { while
> (p && p[len]) ++len; }
> + };
> + using string_view = basic_string_view<char>;
> +}
> +
> +template<std::size_t N>
> +struct sized_string_view: std::string_view {
> + using std::string_view::string_view;
> +};
> +template<std::size_t N>
> +sized_string_view(const char (&str)[N]) -> sized_string_view<N - 1>;
> +
> +constexpr auto string_builder(auto first, auto second, auto... trailing) {
> + constexpr auto is_last = sizeof...(trailing) == 0;
> + auto buffer = 1;
> + if constexpr (is_last) {
> + return buffer;
> + } else
> + return string_builder(buffer, trailing...);
> +}
> +
> +constexpr auto copyright = string_builder(sized_string_view("a"),
> sized_string_view("b"), sized_string_view("c"));
> --
> 2.52.0
>
>
