[gcc r15-10711] c++: Fix ICE with type aliases in inherited CTAD [PR122070]

[Patrick Palka]

https://gcc.gnu.org/g:d549413f11d42872ad751c77afaa81b36a67639a

commit r15-10711-gd549413f11d42872ad751c77afaa81b36a67639a
Author: Egas Ribeiro <egas.g.ribeiro@gmail.com>
Date:   Sat Dec 13 13:14:47 2025 +0000

    c++: Fix ICE with type aliases in inherited CTAD [PR122070]
    
    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, which should
    always be non-NULL when called from inherited_ctad_tweaks.
    
            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 <egas.g.ribeiro@gmail.com>
    Co-authored-by: Patrick Palka <ppalka@redhat.com>
    Reviewed-by: Patrick Palka <ppalka@redhat.com>
    (cherry picked from commit a0908b6723128799a68c9fdab65f65761f657488)

Diff:
---
 gcc/cp/pt.cc                                       |  6 ++++-
 .../g++.dg/cpp23/class-deduction-inherited10.C     | 11 ++++++++
 .../g++.dg/cpp23/class-deduction-inherited9.C      | 31 ++++++++++++++++++++++
 3 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index e8755a2637d8..5823e9bf7cd5 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -31296,7 +31296,11 @@ 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)
+	/* TTYPE is a typedef to a template-id.  */
+	ti = TYPE_TEMPLATE_INFO (ttype);
+      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 000000000000..d3b1b18077d9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited10.C
@@ -0,0 +1,11 @@
+// PR c++/122070
+// { dg-do compile { target c++23 } }
+
+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 000000000000..c7d27202ccca
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited9.C
@@ -0,0 +1,31 @@
+// PR c++/122070
+// { dg-do compile { target c++23 } }
+
+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"));