When an overloaded comparison operator returns a reference, do not consider

it for -Wunused-comparion warnings.  This fixes PR19724.

llvm-svn: 208824

clang/lib/AST/Expr.cpp

@@ -2094,6 +2094,8 @@ bool Expr::isUnusedResultAWarning(const Expr *&WarnE, SourceLocation &Loc,
     case OO_Greater:
     case OO_GreaterEqual:
     case OO_LessEqual:
+      if (Op->getCallReturnType()->isReferenceType())
+        break;
       WarnE = this;
       Loc = Op->getOperatorLoc();
       R1 = Op->getSourceRange();

clang/test/SemaCXX/warn-unused-comparison.cpp

@@ -106,3 +106,16 @@ namespace PR10291 {
 
   X<int> x;
 }
+
+namespace PR19724 {
+class stream {
+} cout, cin;
+
+stream &operator<(stream &s, int);
+bool operator<(stream &s, stream &s2);
+
+void test() {
+  cout < 5;    // no waring, operator returns a reference
+  cout < cin;  // expected-warning {{relational comparison result unused}}
+}
+}