On Sun, 7 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 like std::string_view. Since type aliases don't have their own
> TEMPLATE_INFO, calling TI_TEMPLATE on a NULL
> TYPE_TEMPLATE_INFO_MAYBE_ALIAS result caused a segfault.
>
> This fix adds a NULL check and returns NULL_TREE when the type lacks
> template info, indicating it can't be deduced.
Does this mean CTAD will fail for
template<class T> struct A { A(T); };
using B = A<int>;
template<class T=void>
struct C : B { using B::B; };
C c = 0;
It's a bit of a contrived testcase but I think this should be valid and
behave as if we used A<int> instead of B in the definition of C.
To achieve that we could fall back to using TYPE_TEMPLATE_INFO if
TYPE_TEMPLATE_INFO_MAYBE_ALIAS is NULL.
>
> PR c++/122070
>
> gcc/cp/ChangeLog:
>
> * pt.cc (type_targs_deducible_from): Add NULL check for
> TYPE_TEMPLATE_INFO_MAYBE_ALIAS to prevent ICE with type aliases.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp23/class-deduction-inherited-ice.C: New test.
>
> Signed-off-by: Egas Ribeiro <[email protected]>
> ---
> gcc/cp/pt.cc | 5 ++-
> .../cpp23/class-deduction-inherited-ice.C | 34 +++++++++++++++++++
> 2 files changed, 38 insertions(+), 1 deletion(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp23/class-deduction-inherited-ice.C
>
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 8498730b6e4..c99df255dd7 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -31630,7 +31630,10 @@ 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);
> + if (!ti)
> + return NULL_TREE;
> + tmpl = TI_TEMPLATE (ti);
> }
>
> int len = TREE_VEC_LENGTH (tparms);
> diff --git a/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited-ice.C
> b/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited-ice.C
> new file mode 100644
> index 00000000000..6724ce303d8
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited-ice.C
> @@ -0,0 +1,34 @@
> +// { dg-do compile { target c++23 } }
> +// PR c++/122070
> +
> +namespace std {
> + template<typename T, T v> struct integral_constant { static constexpr T
> value = v; };
> + 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
>
>
