I’ve found my way through the labyrinth to some great sources on this, and I think I’ve got a pretty comprehensive summary of it. I’m posting this as an answer because it seems to explain both the (IMO very misguided) intention of the C clause and the fact that C++ does not inherit it. This will evolve over time if I discover further supporting material or the situation changes.

This is my first time trying to sum up a very complex situation, which seems ill-defined even to many language architects, so I’ll welcome clarifications/suggestions on how to improve this answer – or simply a better answer if anyone has one.

Finally, some concrete commentary

Through vaguely related threads, I found the following answer by @tab – and much appreciated the contained links to (illuminating, if not conclusive) GCC and Working Group defect reports: answer by tab on StackOverflow

The GCC link contains some interesting discussion and reveals a sizeable amount of confusion and conflicting interpretations on part of the Committee and compiler vendors – surrounding the subject of union member structs, punning, and aliasing in both C and C++.

At the end of that, we’re linked to the main event – another BugZilla thread, Bug 65892, containing an extremely useful discussion. In particular, we find our way to the first of two pivotal documents:

Origin of the added line in C99

C proposal N685 is the origin of the added clause regarding visibility of a union type declaration. Through what some claim (see GCC thread #2) is a total misinterpretation of the “common initial sequence” allowance, N685 was indeed intended to allow relaxation of aliasing rules for “common initial sequence” structs within a TU aware of some union containing instances of said struct types, as we can see from this quote:

The proposed solution is to require that a union declaration be visible
if aliases through a common initial sequence (like the above) are possible.
Therefore the following TU provides this kind of aliasing if desired:

union utag {
    struct tag1 { int m1; double d2; } st1;
    struct tag2 { int m1; char c2; } st2;
};

int similar_func(struct tag1 *pst2, struct tag2 *pst3) {
     pst2->m1 = 2;
     pst3->m1 = 0;   /* might be an alias for pst2->m1 */
     return pst2->m1;
}

Judging by the GCC discussion and comments below such as @ecatmur’s, this proposal – which seems to mandate speculatively allowing aliasing for any struct type that has some instance within some union visible to this TU – seems to have received great derision and rarely been implemented.

It’s obvious how difficult it would be to satisfy this interpretation of the added clause without totally crippling many optimisations – for little benefit, as few coders would want this guarantee, and those who do can just turn on fno-strict-aliasing (which IMO indicates larger problems). If implemented, this allowance is more likely to catch people out and spuriously interact with other declarations of unions, than to be useful.

Omission of the line from C++

Following on from this and a comment I made elsewhere, @Potatoswatter in this answer here on SO states that:

The visibility part was purposely omitted from C++ because it’s widely considered to be ludicrous and unimplementable.

In other words, it looks like C++ deliberately avoided adopting this added clause, likely due to its widely pereceived absurdity. On asking for an “on the record” citation of this, Potatoswatter provided the following key info about the thread’s participants:

The folks in that discussion are essentially “on the record” there. Andrew Pinski is a hardcore GCC backend guy. Martin Sebor is an active C committee member. Jonathan Wakely is an active C++ committee member and language/library implementer. That page is more authoritative, clear, and complete than anything I could write.

Potatoswatter, in the same SO thread linked above, concludes that C++ deliberately excluded this line, leaving no special treatment (or, at best, implementation-defined treatment) for pointers into the common initial sequence. Whether their treatment will in future be specifically defined, versus any other pointers, remains to be seen; compare to my final section below about C. At present, though, it is not (and again, IMO, this is good).

What does this mean for C++ and practical C implementations?

So, with the nefarious line from N685… ‘cast aside’… we’re back to assuming pointers into the common initial sequence are not special in terms of aliasing. Still. it’s worth confirming what this paragraph in C++ means without it. Well, the 2nd GCC thread above links to another gem:

C++ defect 1719. This proposal has reached DRWP status: “A DR issue whose resolution is reflected in the current Working Paper. The Working Paper is a draft for a future version of the Standard” – cite. This is either post C++14 or at least after the final draft I have here (N3797) – and puts forward a significant, and in my opinion illuminating, rewrite of this paragraph’s wording, as follows. I’m bolding what I consider to be the important changes, and {these comments} are mine:

In a standard-layout union with an active member {“active” indicates a union instance, not just type} (9.5 [class.union])
of struct type T1, it is permitted to read {formerly “inspect”} a non-static data member m
of another union member of struct type T2 provided m is part of the
common initial sequence of T1 and T2. [Note: Reading a volatile object
through a non-volatile glvalue has undefined behavior (7.1.6.1
[dcl.type.cv]). —end note]

This seems to clarify the meaning of the old wording: to me, it says that any specifically allowed ‘punning’ among union member structs with common initial sequences must be done via an instance of the parent union – rather than being based on the type of the structs (e.g. pointers to them passed to some function). This wording seems to rule out any other interpretation, a la N685. C would do well to adopt this, I’d say. Hey, speaking of which, see below!

The upshot is that – as nicely demonstrated by @ecatmur and in the GCC tickets – this leaves such union member structs by definition in C++, and practically in C, subject to the same strict aliasing rules as any other 2 officially unrelated pointers. The explicit guarantee of being able to read the common initial sequence of inactive union member structs is now more clearly defined, not including vague and unimaginably tedious-to-enforce “visibility” as attempted by N685 for C. By this definition, the main compilers have been behaving as intended for C++. As for C?

Possible reversal of this line in C / clarification in C++

It’s also very worth noting that C committee member Martin Sebor is looking to get this fixed in that fine language, too:

Martin Sebor 2015-04-27 14:57:16 UTC If one of you can explain the problem with it I’m willing to write up a paper and submit it to WG14 and request to have the standard changed.

Martin Sebor 2015-05-13 16:02:41 UTC I had a chance to discuss this issue with Clark Nelson last week. Clark has worked on improving the aliasing parts of the C specification in the past, for example in N1520 (http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1520.htm). He agreed that like the issues pointed out in N1520, this is also an outstanding problem that would be worth for WG14 to revisit and fix.”

Potatoswatter inspiringly concludes:

The C and C++ committees (via Martin and Clark) will try to find a consensus and hammer out wording so the standard can finally say what it means.

We can only hope!

Again, all further thoughts are welcome.