Original text:
Append the result of running Parsing an Item or Inner List (parse-item-or-list) with input_string to members. <del> Discard any leading SP characters from input_string. </del>  If input_string is empty, return members. Consume the first character of input_string; if it is not ",", fail parsing. <del> Discard any leading SP characters from input_string. </del>  If input_string is empty, there is a trailing comma; fail parsing.

Reviewer comments:
From Magnus's feedback
also NAME PTAL
This does not instill much of a panicked reaction in me, which is probably a good sign. I trust it is intentional to not apply a similar treatment to inner lists (which also has a prose description I had commented on previously) on the grounds of them not resembling existing top-level header field structures? That's certainly defensible, though I have to wonder whether someone will simply assume that it must be the same (not that we have much hope of changing such a person's behavior).
How does this vibe with ? I know we spent a bit of time on it, but it's long enough ago that I can't recall detail.
Ah, right, I think I remember. Doing it this way means inner-list is different: The original motivation to remove tabs was because the above was going to be but there was consternation about RWS. Most of the discussion is in
NAME was where my mind went first too, but that seems to be just about in inner lists (which makes sense). I think this is separate; intermediaries that don't know about the semantics and syntactic limitations of a given field shouldn't be meddling inside it (which inner list delimitation is).
NAME I think so -- that's the only reason to allow this. Existing servers may already legitimately be using TAB when combining multiple field lines. And I suppose it makes sense, given the recent discussion about not breaking up individual list or dictionary members over multiple lines, that we don't need to replace *SP with OWS everywhere. I'm okay with this change -- does it warrant a note in the prose somewhere that this special case only exists to accommodate field concatenation? Let's hope we don't need to add VTAB or FF to this, too :)
NAME I don't know that that's necessary; we already pretty clearly. In SF, I looked at changing "whitespace", but in each case the ABNF and algorithms make it clear what we're talking about, so I don't see a strong need to disambiguate.
I'm ok with the change over here; that said, the Semantics spec could be clearer. Opened URL
NAME and I have been talking about this, and have come to the conclusion that this change went too far. Specifically, steps 1 and 5 in "Parsing Structured Fields" should just be SP, not OWS, because there isn't any risk of an intermediary inserting TAB before the first field line or after the last field line; it's only between List and Dictionary members that we need to be concerned. Reverting that part of the change also makes writing the test cases considerably simpler. See 6842793e.
This is so that field line combination as per the Semantics spec can happen, right? I note that currently says: Note that it says "comma an optional whitespace", not "OWS". Depending on how you read it, it might include other whitespace characters. I believe this needs to be fixed in the Semantics spec.I dont think I ever saw a TAB in a HTTP header in real life, but I am OK with this.

Revised text:
Append the result of running Parsing an Item or Inner List (parse-item-or-list) with input_string to members.  <ins> Discard any leading OWS characters from input_string. </ins> If input_string is empty, return members. Consume the first character of input_string; if it is not ",", fail parsing.  <ins> Discard any leading OWS characters from input_string. </ins> If input_string is empty, there is a trailing comma; fail parsing.