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.

This reinstates r363337, reverted in r363352.

llvm-svn: 363429
diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
index 77d4441..72e1119 100644
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -1885,6 +1885,11 @@
                                            ExprValueKind VK) {
   unsigned PathSize = (BasePath ? BasePath->size() : 0);
   void *Buffer = C.Allocate(totalSizeToAlloc<CXXBaseSpecifier *>(PathSize));
+  // Per C++ [conv.lval]p3, lvalue-to-rvalue conversions on class and
+  // std::nullptr_t have special semantics not captured by CK_LValueToRValue.
+  assert((Kind != CK_LValueToRValue ||
+          !(T->isNullPtrType() || T->getAsCXXRecordDecl())) &&
+         "invalid type for lvalue-to-rvalue conversion");
   ImplicitCastExpr *E =
     new (Buffer) ImplicitCastExpr(T, Kind, Operand, PathSize, VK);
   if (PathSize)