Don't treat 'T &forward(T&&)' as builtin.

This allows the standard library to diagnose it properly. Suppress
warning in libc++ testsuite for unused result of call to std::forward.

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -621,6 +621,20 @@ static bool isRelevantAttr(Sema &S, const Decl *D, const Attr *A) {
     return true;
   }
 
+  if (const auto *BA = dyn_cast<BuiltinAttr>(A)) {
+    // Do not treat 'std::forward' as a builtin if it takes an rvalue reference
+    // type and returns an lvalue reference type. The library implementation
+    // will produce an error in this case; don't get in its way.
+    if (BA->getID() == Builtin::BIforward) {
+      const FunctionDecl *FD = dyn_cast<FunctionDecl>(D);
+      if (FD && FD->getNumParams() >= 1 &&
+          FD->getParamDecl(0)->getType()->isRValueReferenceType() &&
+          FD->getReturnType()->isLValueReferenceType()) {
+        return false;
+      }
+    }
+  }
+
   return true;
 }
 

clang/test/SemaCXX/builtin-std-move.cpp

@@ -30,11 +30,18 @@ namespace std {
   template<typename T> struct remove_reference<T&> { using type = T; };
   template<typename T> struct remove_reference<T&&> { using type = T; };
 
+  template<typename T> struct is_lvalue_reference { static constexpr bool value = false; };
+  template<typename T> struct is_lvalue_reference<T&> { static constexpr bool value = true; };
+
   template<typename T> CONSTEXPR T &&forward(typename remove_reference<T>::type &x) {
     static_assert(T::moveable, "instantiated forward"); // expected-error {{no member named 'moveable' in 'B'}}
                                                         // expected-error@-1 {{no member named 'moveable' in 'C'}}
     return static_cast<T&&>(x);
   }
+  template<typename T> CONSTEXPR T &&forward(typename remove_reference<T>::type &&x) {
+    static_assert(!is_lvalue_reference<T>::value, "should not forward rval as lval"); // expected-error {{static_assert failed}}
+    return static_cast<T&&>(x);
+  }
 
   template<typename T> CONSTEXPR const T &as_const(T &x) {
     static_assert(T::moveable, "instantiated as_const"); // expected-error {{no member named 'moveable' in 'B'}}
@@ -76,6 +83,11 @@ static_assert(f({}), "should be constexpr");
 // expected-note@#call {{}}
 #endif
 
+A &forward_rval_as_lval() {
+  std::forward<A&&>(A()); // expected-warning {{const attribute}}
+  return std::forward<A&>(A()); // expected-note {{instantiation of}}
+}
+
 struct B {};
 B &&(*pMove)(B&) = std::move; // #1 expected-note {{instantiation of}}
 B &&(*pMoveIfNoexcept)(B&) = &std::move_if_noexcept; // #2 expected-note {{instantiation of}}

libcxx/test/std/utilities/utility/forward/forward.fail.cpp

@@ -22,7 +22,7 @@ const A csource() {return A();}
 int main(int, char**)
 {
     {
-        std::forward<A&>(source());  // expected-note {{requested here}}
+        (void)std::forward<A&>(source());  // expected-note {{requested here}}
         // expected-error-re@*:* 1 {{static_assert failed{{.*}} "cannot forward an rvalue as an lvalue"}}
     }
     {