Awk output redirection to expression - defined or not?

[Ed Morton]

I'm certain I remember years ago reading a document that said 
(paraphrasing) "an unparenthesized expression on the right side of input 
or output redirection is undefined behavior" and I thought it was an 
older version of the POSIX spec. I now can't find that (or similar) 
statement in any of these:

 * SUSV2 - https://pubs.opengroup.org/onlinepubs/7990989775/xcu/awk.html
 * SUSV3 -
   https://pubs.opengroup.org/onlinepubs/009695399/utilities/awk.html
 * Current POSIX spec -
   https://pubs.opengroup.org/onlinepubs/9699919799/utilities/awk.html

or by googling.

What I do see in the current POSIX spec is a related statement just 
about input redirection:

> Historical practice has been that:
> getline < "a" "b"
>
> is parsed as:
>
> ( getline < "a" ) "b"
>
> although many would argue that the intent was that the file *ab* 
> should be read. However:
>
> getline < "x" + 1
>
> parses as:
>
> getline < ( "x" + 1 )
>
> ...
>
> Since in most cases such constructs are not (or at least should not) 
> be used (because they have a natural ambiguity for which there is no 
> conventional parsing), the meaning of these constructs has been made 
> explicitly unspecified.

and:

> The *getline* operator can form ambiguous constructs when there are 
> unparenthesized binary operators (including concatenate) to the right 
> of the '<' (up to the end of the expression containing the *getline*). 
> The result of evaluating such a construct is unspecified

but nothing about output redirection. I know gawk doesn't require parens 
around the expression for output redirection but other awks do (e.g. see 
https://stackoverflow.com/q/21093626/1745001) and it's not obvious to me 
why `getline < "a" "b"` should be undefined behavior while `print > "a" 
"b"` wouldn't be so intuitively if one of them is undefined then so 
should the other be.

Does anyone else recall seeing a statement about output redirection to 
an expression requiring parens and, if so, do you recall where it existed?

    Ed.