re PR c++/55993 ([C++11] derived-to-base conversion fails in constant expression)

	PR c++/55993
	* semantics.c (cxx_fold_indirect_ref): Handle empty bases at
	non-zero offsets, too.

From-SVN: r196023

gcc/cp/ChangeLog

 2013-02-13  Jason Merrill  <jason@redhat.com>
 
+	PR c++/55993
+	* semantics.c (cxx_fold_indirect_ref): Handle empty bases at
+	non-zero offsets, too.
+
 	PR c++/56155
 	* decl.c (build_enumerator): Always convert the value to a
 	fixed underlying type.

gcc/cp/semantics.c

@@ -7448,6 +7448,15 @@ cxx_fold_indirect_ref (location_t loc, tree type, tree op0, bool *empty_base)
 	      return build4_loc (loc, ARRAY_REF, type, op00, op01,
 				 NULL_TREE, NULL_TREE);
 	    }
+	  /* Also handle conversion to an empty base class, which
+	     is represented with a NOP_EXPR.  */
+	  else if (is_empty_class (type)
+		   && CLASS_TYPE_P (op00type)
+		   && DERIVED_FROM_P (type, op00type))
+	    {
+	      *empty_base = true;
+	      return op00;
+	    }
 	  /* ((foo *)&struct_with_foo_field)[1] => COMPONENT_REF */
 	  else if (RECORD_OR_UNION_TYPE_P (op00type))
 	    {

gcc/testsuite/g++.dg/cpp0x/constexpr-empty6.C

+// PR c++/55993
+// { dg-do compile { target c++11 } }
+
+struct A {};
+struct B:A {};
+struct C:A {};
+struct D:B,C {};
+
+constexpr D d {};
+constexpr const C& e=d; // OK
+constexpr auto f=static_cast<const C&>(d); // FAIL