[CodeGen] Fix handling of nullptr in initializers (llvm#137364)

Fixes llvm#137276.

Author: alexfh

clang/include/clang/AST/OperationKinds.def

@@ -119,7 +119,8 @@ CAST_OPERATION(ArrayToPointerDecay)
 CAST_OPERATION(FunctionToPointerDecay)
 
 /// CK_NullToPointer - Null pointer constant to pointer, ObjC
-/// pointer, or block pointer.
+/// pointer, or block pointer. The result of this conversion can
+/// still be a null pointer constant if it has type std::nullptr_t.
 ///   (void*) 0
 ///   void (^block)() = 0;
 CAST_OPERATION(NullToPointer)

clang/lib/CodeGen/CodeGenTypes.cpp

@@ -850,12 +850,14 @@ CodeGenTypes::getCGRecordLayout(const RecordDecl *RD) {
 }
 
 bool CodeGenTypes::isPointerZeroInitializable(QualType T) {
-  assert((T->isAnyPointerType() || T->isBlockPointerType()) && "Invalid type");
+  assert((T->isAnyPointerType() || T->isBlockPointerType() ||
+          T->isNullPtrType()) &&
+         "Invalid type");
   return isZeroInitializable(T);
 }
 
 bool CodeGenTypes::isZeroInitializable(QualType T) {
-  if (T->getAs<PointerType>())
+  if (T->getAs<PointerType>() || T->isNullPtrType())
     return Context.getTargetNullPointerValue(T) == 0;
 
   if (const auto *AT = Context.getAsArrayType(T)) {

clang/test/CodeGenCXX/nullptr.cpp

@@ -70,3 +70,10 @@ namespace PR39528 {
   void f(nullptr_t);
   void g() { f(null); }
 }
+
+// CHECK-LABEL: define {{.*}}pr137276
+// CHECK: {{^}}  store i64 0, ptr %arr, align 8{{$}}
+void pr137276(nullptr_t np, int i) {
+  long arr[] = { long(np), i, 0 };
+  (void)arr;
+}