During the 2019 Belfast meeting LWG motion 9 was accepted to apply P1878R1. Among the changes [iterator.requirements.general] p1 was changed by the project editors to

An output iterator i has a non-empty set of types that are indirectly_writable to the iterator; [...].

Note that the paper instructions said (P1878R1 p.7):

[ Editor’s note: Change stable name [iterator.concept.writable] to [iterator.concept.indirectly.writable] and globally
replace all occurrences of “writable” (the concept) with “indirectly_writable”. ]

Problem is that the pre-Belfast working draft N4835 p.911 said:

An output iterator i has a non-empty set of types that are writable to the iterator; [...].

At this wording part writable does not denote the concept writable, but is a definition on its own, so this change should not have been applied here. And the application had lead to a problematic situation described in the recently opened issue LWG 4178.

According to the LWG reflector discussion (and confirmed by @jwakely) we would like to ask to solve that LWG issue editorially by correcting the wording state to that one described in the accepted paper. I will create a PULL request with these effects.