PR23833, DR2140: an lvalue-to-rvalue conversion on a glvalue of type
nullptr_t does not access memory.

We now reuse CK_NullToPointer to represent a conversion from a glvalue
of type nullptr_t to a prvalue of nullptr_t where necessary.

llvm-svn: 345562
diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index 29119a7..0bf2578 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -7644,9 +7644,13 @@
 
     case SK_LValueToRValue: {
       assert(CurInit.get()->isGLValue() && "cannot load from a prvalue");
-      CurInit = ImplicitCastExpr::Create(S.Context, Step->Type,
-                                         CK_LValueToRValue, CurInit.get(),
-                                         /*BasePath=*/nullptr, VK_RValue);
+      // C++ [conv.lval]p3:
+      //   If T is cv std::nullptr_t, the result is a null pointer constant.
+      CastKind CK =
+          Step->Type->isNullPtrType() ? CK_NullToPointer : CK_LValueToRValue;
+      CurInit =
+          ImplicitCastExpr::Create(S.Context, Step->Type, CK, CurInit.get(),
+                                   /*BasePath=*/nullptr, VK_RValue);
       break;
     }