Index: lib/Sema/SemaChecking.cpp
===================================================================
--- lib/Sema/SemaChecking.cpp
+++ lib/Sema/SemaChecking.cpp
@@ -960,6 +960,9 @@
   case Builtin::BI__builtin_isinf_sign:
   case Builtin::BI__builtin_isnan:
   case Builtin::BI__builtin_isnormal:
+  case Builtin::BI__builtin_signbit:
+  case Builtin::BI__builtin_signbitf:
+  case Builtin::BI__builtin_signbitl:
     if (SemaBuiltinFPClassification(TheCall, 1))
       return ExprError();
     break;
@@ -4574,15 +4577,19 @@
                 diag::err_typecheck_call_invalid_unary_fp)
       << OrigArg->getType() << OrigArg->getSourceRange();
 
-  // If this is an implicit conversion from float -> float or double, remove it.
+  // If this is an implicit conversion from float -> float, double, or
+  // long double, remove it.
   if (ImplicitCastExpr *Cast = dyn_cast<ImplicitCastExpr>(OrigArg)) {
     // Only remove standard FloatCasts, leaving other casts inplace
     if (Cast->getCastKind() == CK_FloatingCast) {
       Expr *CastArg = Cast->getSubExpr();
       if (CastArg->getType()->isSpecificBuiltinType(BuiltinType::Float)) {
-          assert((Cast->getType()->isSpecificBuiltinType(BuiltinType::Double) ||
-                  Cast->getType()->isSpecificBuiltinType(BuiltinType::Float)) &&
-               "promotion from float to either float or double is the only expected cast here");
+        assert(
+            (Cast->getType()->isSpecificBuiltinType(BuiltinType::Double) ||
+             Cast->getType()->isSpecificBuiltinType(BuiltinType::Float) ||
+             Cast->getType()->isSpecificBuiltinType(BuiltinType::LongDouble)) &&
+            "promotion from float to either float, double, or long double is "
+            "the only expected cast here");
         Cast->setSubExpr(nullptr);
         TheCall->setArg(NumArgs-1, CastArg);
       }
Index: test/Sema/builtins.c
===================================================================
--- test/Sema/builtins.c
+++ test/Sema/builtins.c
@@ -253,3 +253,26 @@
   __sync_fetch_and_add(ptr, 1); // expected-error{{address argument to atomic builtin cannot be const-qualified ('const int *' invalid)}}
   __atomic_fetch_add(ptr, 1, 0);  // expected-error {{address argument to atomic operation must be a pointer to non-const type ('const int *' invalid)}}
 }
+
+void test22(void) {
+  (void)__builtin_signbit(); // expected-error{{too few arguments to function call, expected 1, have 0}}
+  (void)__builtin_signbit(1.0, 2.0, 3.0); // expected-error{{too many arguments to function call, expected 1, have 3}}
+  (void)__builtin_signbit(1); // expected-error {{floating point classification requires argument of floating point type (passed in 'int')}}
+  (void)__builtin_signbit(1.0);
+  (void)__builtin_signbit(1.0f);
+  (void)__builtin_signbit(1.0L);
+
+  (void)__builtin_signbitf(); // expected-error{{too few arguments to function call, expected 1, have 0}}
+  (void)__builtin_signbitf(1.0, 2.0, 3.0); // expected-error{{too many arguments to function call, expected 1, have 3}}
+  (void)__builtin_signbitf(1);
+  (void)__builtin_signbitf(1.0);
+  (void)__builtin_signbitf(1.0f);
+  (void)__builtin_signbitf(1.0L);
+
+  (void)__builtin_signbitl(); // expected-error{{too few arguments to function call, expected 1, have 0}}
+  (void)__builtin_signbitl(1.0, 2.0, 3.0); // expected-error{{too many arguments to function call, expected 1, have 3}}
+  (void)__builtin_signbitl(1);
+  (void)__builtin_signbitl(1.0);
+  (void)__builtin_signbitl(1.0f);
+  (void)__builtin_signbitl(1.0L);
+}