https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892
--- Comment #29 from rguenther at suse dot de <rguenther at suse dot de> --- On Thu, 19 Apr 2018, jameskuyper at verizon dot net wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 > > James Kuyper Jr. <jameskuyper at verizon dot net> changed: > > What |Removed |Added > ---------------------------------------------------------------------------- > CC| |jameskuyper at verizon dot > net > > --- Comment #28 from James Kuyper Jr. <jameskuyper at verizon dot net> --- > (In reply to Martin Sebor from comment #17) > > The C Union Visibility rule was intended to cover that case. The trouble is > > that the rule tends to be interpreted differently by different people, users > > and implementers alike: Is it the union object that must be visible at the > > point of the access, or just the union type? > > The relevant wording is "anywhere that a declaration of the completed type of > the union is visible.", so it's unambiguously the type, not the object, which > must be visible. A declaration of the completed type can be visible without > any > objects of that type being visible, and that's sufficient for this rule to > apply. > > > ... Must the access be performed > > using the union object, or just the union type, or neither? > > It says only that "it is permitted to inspect the common initial part"; it > imposes no restrictions on how the inspection may be performed. Clearly, > inspection through an lvalue of the union's type must be permitted, but it is > also permitted to use the more indirect methods which are the subject of this > bug report, simply because the standard says nothing to restrict the > permission > it grants to the more direct cases. Note I repeatedly said this part of the standard is just stupid. It makes most if not all type-based alias analysis useless. Which means I'll refuse any patches implementing it in a way that affects default behavior. A clean patch (I really can't think of any clean approach besides forcing -fno-strict-aliasing!) with some extra flag (well, just use -fno-strict-aliasing ...) would be fine with me.
