PR c++/11072

	* ginclude/stddef.h (offsetof): Remove cast to 'char &'. Explain why.
testsuite:
	PR c++/11072
	* g++.dg/other/offsetof2.C: XFAIL.
	* g++.dg/other/offsetof5.C: New.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@68831 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 16 insertions, 6 deletions

diff --git a/gcc/ginclude/stddef.h b/gcc/ginclude/stddef.h
index ad091ea1383..d19d78a3140 100644
--- a/gcc/ginclude/stddef.h
+++ b/gcc/ginclude/stddef.h
@@ -409,16 +409,26 @@ typedef __WINT_TYPE__ wint_t;
 
 #ifdef _STDDEF_H
 
-/* Offset of member MEMBER in a struct of type TYPE.  */
+/* Offset of member MEMBER in a struct of type TYPE. */
 #ifndef __cplusplus
 #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
-#else /* C++ */
-/* The reference cast is necessary to thwart an operator& that might
-   be applicable to MEMBER's type.  See DR 273 for details.  */
+#else
+/* In C++ a POD type can have a user defined address-of operator, and
+   that will break offsetof. C++ core defect 273 addresses this and
+   claims that reinterpret_casts to char & type are sufficient to
+   overcome this problem.
+
+   (reinterpret_cast <size_t>
+     (&reinterpret_cast <char &>(static_cast <TYPE *> (0)->MEMBER)))
+
+   But, such casts are not permitted in integral constant expressions,
+   which offsetof is supposed to be.
+
+   It appears that offsetof is unimplementable in C++ without a
+   compiler extension.  */
 #define offsetof(TYPE, MEMBER) (reinterpret_cast <size_t> \
-    (&reinterpret_cast <char &>(static_cast <TYPE *> (0)->MEMBER)))
+	(&static_cast<TYPE *> (0)->MEMBER))
 #endif /* C++ */
-
 #endif /* _STDDEF_H was defined this time */
 
 #endif /* !_STDDEF_H && !_STDDEF_H_ && !_ANSI_STDDEF_H && !__STDDEF_H__