From 8161c4adea7f1842f9d28633d82e912ebb7a4cf9 Mon Sep 17 00:00:00 2001
From: Patrick Palka <ppalka@redhat.com>
Date: Tue, 15 Oct 2024 13:23:17 -0400
Subject: [PATCH] c++: unifying lvalue vs rvalue (non-forwarding) ref
 [PR116710]

When unifying two (non-forwarding) reference types, unify immediately
recurses into the referenced type without first comparing rvalueness.
(Note that at this point forwarding references and other reference
parameters have already been stripped to their referenced type by
maybe_adjust_types_for_deduction, so this code path applies only to
nested reference types.)

	PR c++/116710

gcc/cp/ChangeLog:

	* pt.cc (unify) <case REFERENCE_TYPE>: Compare rvalueness.

gcc/testsuite/ChangeLog:

	* g++.dg/template/unify12.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>
---
 gcc/cp/pt.cc                            |  3 ++-
 gcc/testsuite/g++.dg/template/unify12.C | 24 ++++++++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/template/unify12.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index c9219d5b3a5a..0141c53b617c 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -25161,7 +25161,8 @@ unify (tree tparms, tree targs, tree parm, tree arg, int strict,
       }
 
     case REFERENCE_TYPE:
-      if (!TYPE_REF_P (arg))
+      if (!TYPE_REF_P (arg)
+	  || TYPE_REF_IS_RVALUE (parm) != TYPE_REF_IS_RVALUE (arg))
 	return unify_type_mismatch (explain_p, parm, arg);
       return unify (tparms, targs, TREE_TYPE (parm), TREE_TYPE (arg),
 		    strict & UNIFY_ALLOW_MORE_CV_QUAL, explain_p);
diff --git a/gcc/testsuite/g++.dg/template/unify12.C b/gcc/testsuite/g++.dg/template/unify12.C
new file mode 100644
index 000000000000..bed52d0fa36c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/unify12.C
@@ -0,0 +1,24 @@
+// PR c++/116710
+// { dg-do compile { target c++11 } }
+
+template<class T> struct A : T {};
+
+template<class T>
+void f(void (*)(T &), typename A<T>::type * = 0);
+
+void f(...);
+
+void g(int &&);
+
+void q() { f(g); } // OK
+
+template<class T>
+struct B { operator B<T&>(); };
+
+template<class T>
+void h(B<T&>);
+
+int main() {
+  B<int&&> b;
+  h(b); // { dg-error "no match" }
+}
-- 
2.47.2