[Sema] Fix PR28623.

In atomic builtins, we assumed that the LValue conversion on the first
argument would succeed. So, we would crash given code like:

```
void ovl(char);
void ovl(int);
__atomic_store_n(ovl, 0, 0);
```

This patch makes us not assume that said conversion is successful. :)

llvm-svn: 276232

clang/lib/Sema/SemaChecking.cpp

@@ -2398,7 +2398,11 @@ ExprResult Sema::SemaAtomicOpsOverloaded(ExprResult TheCallResult,
 
   // Inspect the first argument of the atomic operation.
   Expr *Ptr = TheCall->getArg(0);
-  Ptr = DefaultFunctionArrayLvalueConversion(Ptr).get();
+  ExprResult ConvertedPtr = DefaultFunctionArrayLvalueConversion(Ptr);
+  if (ConvertedPtr.isInvalid())
+    return ExprError();
+
+  Ptr = ConvertedPtr.get();
   const PointerType *pointerType = Ptr->getType()->getAs<PointerType>();
   if (!pointerType) {
     Diag(DRE->getLocStart(), diag::err_atomic_builtin_must_be_pointer)

clang/test/SemaCXX/atomic-ops.cpp

@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 %s -verify -fsyntax-only -triple=i686-linux-gnu -std=c++11
+
+// We crashed when we couldn't properly convert the first arg of __atomic_* to
+// an lvalue.
+void PR28623() {
+  void helper(int); // expected-note{{target}}
+  void helper(char); // expected-note{{target}}
+  __atomic_store_n(helper, 0, 0); // expected-error{{reference to overloaded function could not be resolved}}
+}