From 37e6c9bd99575752b7122c5d76aa2cf021deb93c Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Fri, 17 Nov 2023 21:57:52 -0500
Subject: [PATCH] c++: mangling for CTAD placeholder

Per https://github.com/itanium-cxx-abi/cxx-abi/issues/109 mangle a C++17
CTAD placeholder as its template.

gcc/cp/ChangeLog:

	* mangle.cc (write_type): Mangle placeholder as its template.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/nontype-class4.C: Specify ABI v18.
	* g++.dg/cpp2a/nontype-class4a.C: New test.
---
 gcc/cp/mangle.cc                             | 10 ++++++++++
 gcc/testsuite/g++.dg/cpp2a/nontype-class4.C  |  3 ++-
 gcc/testsuite/g++.dg/cpp2a/nontype-class4a.C | 18 ++++++++++++++++++
 3 files changed, 30 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class4a.C

diff --git a/gcc/cp/mangle.cc b/gcc/cp/mangle.cc
index 8a44b11bab69..0684f0e60380 100644
--- a/gcc/cp/mangle.cc
+++ b/gcc/cp/mangle.cc
@@ -2539,6 +2539,16 @@ write_type (tree type)
 	    case TEMPLATE_TYPE_PARM:
 	      if (is_auto (type))
 		{
+		  if (template_placeholder_p (type)
+		      && abi_check (19))
+		    {
+		      /* ABI #109: placeholder is mangled as its template.  */
+		      type = CLASS_PLACEHOLDER_TEMPLATE (type);
+		      if (find_substitution (type))
+			return;
+		      write_name (type, 0);
+		      break;
+		    }
 		  if (AUTO_IS_DECLTYPE (type))
 		    write_identifier ("Dc");
 		  else
diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class4.C b/gcc/testsuite/g++.dg/cpp2a/nontype-class4.C
index 6235fc8f3f52..5dd4b03e0ed5 100644
--- a/gcc/testsuite/g++.dg/cpp2a/nontype-class4.C
+++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class4.C
@@ -1,11 +1,12 @@
 // { dg-do compile { target c++20 } }
+// { dg-additional-options "-fabi-version=18 -fabi-compat-version=18 -Wabi=0" }
 
 template <class T>
 struct A {
   constexpr A(T) {}
   // auto operator<=> (const A&) = default;
 };
-template <A a> void f();
+template <A a> void f();	// { dg-warning "mangled name" }
 
 int main()
 {
diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class4a.C b/gcc/testsuite/g++.dg/cpp2a/nontype-class4a.C
new file mode 100644
index 000000000000..717876d07ed6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class4a.C
@@ -0,0 +1,18 @@
+// { dg-do compile { target c++20 } }
+// { dg-additional-options "-fabi-version=0 -fabi-compat-version=0 -Wabi=18" }
+
+template <class T>
+struct A {
+  constexpr A(T) {}
+  // auto operator<=> (const A&) = default;
+};
+template <A a> void f();	// { dg-warning "mangled name" }
+
+int main()
+{
+  constexpr A a = 1;
+  f<a>();
+  f<1>();
+}
+
+// { dg-final { scan-assembler "_Z1fITn1AXtlS0_IiEEEEvv" } }
-- 
2.47.2