[Bug c/112972] ambiguity in specification for cast to union types

["stephan.stiller at outlook dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112972

Stephan Stiller <stephan.stiller at outlook dot com> changed:

           What    |Removed                     |Added
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             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |---

--- Comment #2 from Stephan Stiller <stephan.stiller at outlook dot com> ---
I disagree; the documentation overall is contradictory with respect to whether
reading from a different member with the same type is also considered "type
punning".

Given the example of union datum p, it wouldn't make sense for any compiler to
not produce code with the intended effect for something like the following:
    p.height = 3.4;
    double d = p.weight;
However, the point is that this is not explicitly documented for the case of
different members having identical types.

For the example from the GCC webpage, if union foo were replaced by union
datum, it would be unclear what the "equivalence" would be. (The choice of
latitute/longitude/height/weight as the designated member of type double
wouldn't matter in practice, but again, this needs to be said explicitly.)