From d04b0c75794545f1f7a942764285e21eaf2915a1 Mon Sep 17 00:00:00 2001
From: Patrick Palka <ppalka@redhat.com>
Date: Fri, 16 Jul 2021 16:21:10 -0400
Subject: [PATCH] c++: covariant reference return types [PR99664]

This implements the wording changes of CWG 960 which clarifies that two
reference types are covariant only if they're both lvalue references
or both rvalue references.

	DR 960
	PR c++/99664

gcc/cp/ChangeLog:

	* search.c (check_final_overrider): Compare TYPE_REF_IS_RVALUE
	when the return types are references.

gcc/testsuite/ChangeLog:

	* g++.dg/inherit/covariant23.C: New test.
---
 gcc/cp/search.c                            |  8 +++++++-
 gcc/testsuite/g++.dg/inherit/covariant23.C | 14 ++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/inherit/covariant23.C

diff --git a/gcc/cp/search.c b/gcc/cp/search.c
index af41bfe5835..943671acff8 100644
--- a/gcc/cp/search.c
+++ b/gcc/cp/search.c
@@ -1948,7 +1948,13 @@ check_final_overrider (tree overrider, tree basefn)
       fail = !INDIRECT_TYPE_P (base_return);
       if (!fail)
 	{
-	  fail = cp_type_quals (base_return) != cp_type_quals (over_return);
+	  if (cp_type_quals (base_return) != cp_type_quals (over_return))
+	    fail = 1;
+
+	  if (TYPE_REF_P (base_return)
+	      && (TYPE_REF_IS_RVALUE (base_return)
+		  != TYPE_REF_IS_RVALUE (over_return)))
+	    fail = 1;
 
 	  base_return = TREE_TYPE (base_return);
 	  over_return = TREE_TYPE (over_return);
diff --git a/gcc/testsuite/g++.dg/inherit/covariant23.C b/gcc/testsuite/g++.dg/inherit/covariant23.C
new file mode 100644
index 00000000000..b27be15ef45
--- /dev/null
+++ b/gcc/testsuite/g++.dg/inherit/covariant23.C
@@ -0,0 +1,14 @@
+// PR c++/99664
+// { dg-do compile { target c++11 } }
+
+struct Res { };
+
+struct A {
+  virtual Res &&f();
+  virtual Res &g();
+};
+
+struct B : A {
+  Res &f() override; // { dg-error "return type" }
+  Res &&g() override; // { dg-error "return type" }
+};