From 04e8808841c33d94529b144b15af5d11671dde1b Mon Sep 17 00:00:00 2001
From: Patrick Palka <ppalka@redhat.com>
Date: Sat, 15 Jul 2023 09:47:36 -0400
Subject: c++: mangling template-id of unknown template [PR110524]

This fixes a crash when mangling an ADL-enabled call to a template-id
naming an unknown template (as per P0846R0).

	PR c++/110524

gcc/cp/ChangeLog:

	* mangle.cc (write_expression): Handle TEMPLATE_ID_EXPR
	whose template is already an IDENTIFIER_NODE.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/fn-template26.C: New test.

(cherry picked from commit 97ceaa110e1607ec8f4f1223200868e1642f3cc7)
---
 gcc/cp/mangle.cc                           |  3 ++-
 gcc/testsuite/g++.dg/cpp2a/fn-template26.C | 16 ++++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/fn-template26.C

diff --git a/gcc/cp/mangle.cc b/gcc/cp/mangle.cc
index a235f23..7076608 100644
--- a/gcc/cp/mangle.cc
+++ b/gcc/cp/mangle.cc
@@ -3312,7 +3312,8 @@ write_expression (tree expr)
   else if (TREE_CODE (expr) == TEMPLATE_ID_EXPR)
     {
       tree fn = TREE_OPERAND (expr, 0);
-      fn = OVL_NAME (fn);
+      if (!identifier_p (fn))
+	fn = OVL_NAME (fn);
       if (IDENTIFIER_ANY_OP_P (fn))
 	write_string ("on");
       write_unqualified_id (fn);
diff --git a/gcc/testsuite/g++.dg/cpp2a/fn-template26.C b/gcc/testsuite/g++.dg/cpp2a/fn-template26.C
new file mode 100644
index 0000000..d4a17eb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/fn-template26.C
@@ -0,0 +1,16 @@
+// PR c++/110524
+// { dg-do compile { target c++20 } }
+
+template<class T>
+auto f(T t) -> decltype(g<T>(t));
+
+namespace N {
+  struct A { };
+  template<class T> void g(T);
+};
+
+int main() {
+  f(N::A{});
+}
+
+// { dg-final { scan-assembler "_Z1fIN1N1AEEDTcl1gIT_Efp_EES2_" } }
-- 
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