A read of the C11 standard draft, which would have this amendment if it were accepted by the C standards committee, shows that this has not been added:
Hum, from your wg14 link: 6.5.2.3 comma 3 and note 95. I thought that was the note that was added on TC3.
Also the note is non-normative, so it is only clarifying preexisting behaviour.
But I'm far from an expert on the C standard. Also that was the C11 draft, maybe the note was removed before the final standard.
Edit: I believe the alias rules are in 6.5 comma 7; specifically:
> An object shall have its stored value accessed only by an lvalue expression that has one of
the following types:
[...]
>an aggregate or union type that includes one of the aforementioned types among its
members (including, recursively, a member of a subaggregate or contained union),
Edit2: neither commas nor the note have changed in the 202y Draft.
You need more language in order to say that type punning is allowed. Implicitly, only the type of the last write is permitted reads, and anything else is undefined behavior. At least, this is my understanding based on my own read and the guidance from the GCC developers.
From a cursory search I can't find any languages in the C standard that disallow reading not from the last written member.
I'm familiar with such language in the C++ standard.
Edit: On the contrary, this note 92:
> 92)If the member used to read the contents of a union object is not the same as the member last used to store a value in the
object the appropriate part of the object representation of the value is reinterpreted as an object representation in the new
type as described in 6.2.6 (a process sometimes called type punning). This can possibly be a non-value representation.
Edit2: and that's specifically the text that was added by dr283. I think you might be confusing with a different DR (don't remember the number) that specifically asked if generalized type punning was possible as long as an union containing the aliased types was visible in the translation unit. I think that's still open although GCC definitely forbids it.
Also the note is non-normative, so it is only clarifying preexisting behaviour.
But I'm far from an expert on the C standard. Also that was the C11 draft, maybe the note was removed before the final standard.
Edit: I believe the alias rules are in 6.5 comma 7; specifically:
> An object shall have its stored value accessed only by an lvalue expression that has one of the following types:
[...]
>an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union),
Edit2: neither commas nor the note have changed in the 202y Draft.