From ea381d87ecc711d0003ae6841477ae8000894844 Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Thu, 25 Jul 2024 23:09:31 -0400
Subject: [PATCH] c++: trait as typename scope [PR116052]

The stdexec library currently wrongly ends up using __decay as the scope of
a typename, which leads to a crash.  Let's give an error instead.

	PR c++/116052

gcc/cp/ChangeLog:

	* mangle.cc (write_prefix): Handle TRAIT_EXPR.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/decay1.C: New test.
---
 gcc/cp/mangle.cc                  |  3 ++-
 gcc/testsuite/g++.dg/ext/decay1.C | 16 ++++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/ext/decay1.C

diff --git a/gcc/cp/mangle.cc b/gcc/cp/mangle.cc
index a04bc584586..e7391220234 100644
--- a/gcc/cp/mangle.cc
+++ b/gcc/cp/mangle.cc
@@ -1300,7 +1300,8 @@ write_prefix (const tree node)
 
   MANGLE_TRACE_TREE ("prefix", node);
 
-  if (TREE_CODE (node) == DECLTYPE_TYPE)
+  if (TREE_CODE (node) == DECLTYPE_TYPE
+      || TREE_CODE (node) == TRAIT_TYPE)
     {
       write_type (node);
       return;
diff --git a/gcc/testsuite/g++.dg/ext/decay1.C b/gcc/testsuite/g++.dg/ext/decay1.C
new file mode 100644
index 00000000000..dd457ba307c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/decay1.C
@@ -0,0 +1,16 @@
+// PR c++/116052
+// { dg-do compile { target c++11 } }
+
+template <class T> using decay_t = __decay(T);
+template <class T> void g(typename decay_t<T>::foo); // { dg-error "built-in" }
+template <class T> void f()
+{
+  g<T>(0);
+}
+
+struct A { using foo = int; };
+
+int main()
+{
+  f<A>();
+}