https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892
--- Comment #52 from Davin McCall <davmac at davmac dot org> --- (In reply to James Kuyper Jr. from comment #48) > > The "one special guarantee" clause appears in the section describing union > > member access via the "." or "->" operators, implying that it only applies > > to the access of union members via the union. ... > > I find nothing objectionable about that statement - it is indeed impossible > to create code which relies upon the special guarantee in 6.5.2.3p6 without > accessing the union members via the '.' or '->' operators. However, I > believe that you mean something more restricted than what you're actually > saying, because the code given in the original bug report does in fact > access the union members via '.' operator, in the expressions &u.s1 and > &u.s2, to create a situation where, as I understand it, that special > guarantee is fully applicable. > Could you expand on your description of what you think is required, to make > it clear why it doesn't apply in this case? It isn't clear that "&u.s1" for example actually accesses either "u" or its member "s1", and I would argue that it doesn't for either. I read it how (if I understand correctly) GCC has up until now interpreted it: the "special guarantee" is for expressions directly involving member access via the union. Once you take the address of the member, and later dereference it via "*", you are dealing with a different operator and the guarantee doesn't apply. I'll admit that this is still making some assumptions, but it's an interpretation that is far more at peace with the rest of the standard. (In reply to Andrew Haley from comment #45) > (In reply to Davin McCall from comment #44) > > The "one special guarantee" clause appears in the section describing union > > member access via the "." or "->" operators, implying that it only applies > > to the access of union members via the union. > > I don't believe that's what is intended, or that you can make such a > conclusion based on the section in which the rule appears. It applies > to other accesses too, as is (somewhat) made clear by the rationale in > http://www.open-std.org/jtc1/sc22/wg14/www/docs/n685.htm: It certainly may not be what is intended by N685, but I think it's normally reasonable to conclude that a statement in a particular section of a document applies to that section and not more universally than that; in this case, the "universal" interpretation flatly contradicts the strict aliasing rule and any other rule which would otherwise disallow access, which seems extremely problematic to me. In general it appears the committee have asserted that the "universal" interpretation (which since N685 requires visibility of the union declaration to be effective) is the correct one, but my argument is that the actual text of the standard strongly implies something different, and that the interpretation being pushed instead turns another portion of the standard text into nonsense. It's extremely problematic in my view that a more reasonable reading is considered incorrect and that this can only be known with external knowledge outside the text of the specification itself. Never the less, I take your point.
