Fix the ranking of reference bindings during overload resolution

(C++0x [over.ics.rank]p3) when one binding is an lvalue reference and
the other is an rvalue reference that binds to an rvalue. In
particular, we were using the predict "is an rvalue reference" rather
than "is an rvalue reference that binds to an rvalue", which was
incorrect in the one case where an rvalue reference can bind to an
lvalue: function references.

This particular issue cropped up with std::forward, where Clang was
picking an std::forward overload while forwarding an (lvalue)
reference to a function. However (and unfortunately!), the right
answer for this code is that the call to std::forward is
ambiguous. Clang now gets that right, but we need to revisit the
std::forward implementation in libc++.

llvm-svn: 124216

2 files changed, 27 insertions, 3 deletions

diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index d32ff869534..85ae6f73260 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -2966,7 +2966,7 @@ TryReferenceInit(Sema &S, Expr *&Init, QualType DeclType,
       ICS.Standard.setToType(2, T1);
       ICS.Standard.ReferenceBinding = true;
       ICS.Standard.DirectBinding = true;
-      ICS.Standard.RRefBinding = isRValRef;
+      ICS.Standard.RRefBinding = isRValRef && InitCategory.isRValue();
       ICS.Standard.CopyConstructor = 0;
 
       // Nothing more to do: the inaccessibility/ambiguity check for
@@ -3036,7 +3036,7 @@ TryReferenceInit(Sema &S, Expr *&Init, QualType DeclType,
     ICS.Standard.DirectBinding = 
       S.getLangOptions().CPlusPlus0x || 
       (InitCategory.isPRValue() && !T2->isRecordType());
-    ICS.Standard.RRefBinding = isRValRef;
+    ICS.Standard.RRefBinding = isRValRef && InitCategory.isRValue();
     ICS.Standard.CopyConstructor = 0;
     return ICS; 
   }
diff --git a/clang/test/CXX/over/over.match/over.match.best/over.ics.rank/p3-0x.cpp b/clang/test/CXX/over/over.match/over.match.best/over.ics.rank/p3-0x.cpp
index cff9e9709a1..faff058a505 100644
--- a/clang/test/CXX/over/over.match/over.match.best/over.ics.rank/p3-0x.cpp
+++ b/clang/test/CXX/over/over.match/over.match.best/over.ics.rank/p3-0x.cpp
@@ -15,7 +15,7 @@ namespace std_example {
   float &k2 = g2(f1());
   float &l2 = g2(f2());
 
-  // FIXME: We don't support ref-qualifiers set.
+  // FIXME: We don't support ref-qualifiers yet.
 #if 0
   struct A { 
     A& operator<<(int); 
@@ -33,3 +33,27 @@ namespace std_example {
   a.p();
 #endif
 }
+
+template<typename T>
+struct remove_reference {
+  typedef T type;
+};
+
+template<typename T>
+struct remove_reference<T&> {
+  typedef T type;
+};
+
+template<typename T>
+struct remove_reference<T&&> {
+  typedef T type;
+};
+
+namespace FunctionReferencesOverloading {
+  template<typename T> int &f(typename remove_reference<T>::type&); // expected-note{{candidate function [with T = int (&)(int)]}}
+  template<typename T> float &f(typename remove_reference<T>::type&&); // expected-note{{candidate function [with T = int (&)(int)]}}
+
+  void test_f(int (&func_ref)(int)) {
+    f<int (&)(int)>(func_ref); // expected-error{{call to 'f' is ambiguous}}
+  }
+}