Re: [PATCH 2/2] c++: maybe_dependent_member_ref and typenames [PR118626]

[Jason Merrill]

On 3/31/25 7:51 PM, Patrick Palka wrote:

Here during maybe_dependent_member_ref for accepted_type<_Up>, we
correctly don't strip the typedef because it's a complex one (its
defaulted template parameter isn't used in its definition) and so
we recurse to consider its corresponding TYPE_DECL.

We then incorrectly decide to not rewrite this use because of the
TYPENAME_TYPE shortcut.  But I don't think this shortcut should apply to
a typedef TYPE_DECL.

        PR c++/118626

gcc/cp/ChangeLog:

        * pt.cc (maybe_dependent_member_ref): Restrict TYPENAME_TYPE
        shortcut to non-typedef TYPE_DECL.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/class-deduction-alias25a.C: New test.
---
  gcc/cp/pt.cc                                  |  3 ++-
  .../g++.dg/cpp2a/class-deduction-alias25a.C   | 19 +++++++++++++++++++
  2 files changed, 21 insertions(+), 1 deletion(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp2a/class-deduction-alias25a.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 6509089efdc..a1b0df428dc 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -17786,7 +17786,8 @@ maybe_dependent_member_ref (tree t, tree args, 
tsubst_flags_t complain,

if (TREE_CODE (t) == TYPE_DECL)

      {
-      if (TREE_CODE (TREE_TYPE (t)) == TYPENAME_TYPE
+      if (!DECL_ORIGINAL_TYPE (t)

I think let's check !is_typedef_decl instead.  OK with that tweak.

+         && TREE_CODE (TREE_TYPE (t)) == TYPENAME_TYPE
          && TYPE_NAME (TREE_TYPE (t)) == t)
        /* The TYPE_DECL for a typename has DECL_CONTEXT of the typename
           scope, but it doesn't need to be rewritten again.  */
diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias25a.C 
b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias25a.C
new file mode 100644
index 00000000000..74ef1e4d890
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias25a.C
@@ -0,0 +1,19 @@
+// PR c++/118626
+// { dg-do compile { target c++20 } }
+
+template<long> struct _Nth_type { using type = _Nth_type; };
+
+template<class _Up>
+struct variant {
+  template<class _Tp> static constexpr long __accepted_index = 0;
+  template<long _Np> using __to_type = typename _Nth_type<_Np>::type;
+  template<class _Tp, int = sizeof(_Tp)> using __accepted_type = 
__to_type<__accepted_index<_Tp>>;
+  template<class = __accepted_type<_Up>> variant(_Up);
+};
+
+template<class _Tp>
+struct Node { Node(_Tp); };
+
+template<class R> using Tree = variant<Node<R>>;
+using type = decltype(Tree{Node{42}});
+using type = Tree<int>;