[Bug c/65892] gcc fails to implement N685 aliasing of union members

[jameskuyper at verizon dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892

James Kuyper Jr. <jameskuyper at verizon dot net> changed:

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                 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.