Stefan Monnier <monnier@iro.umontreal.ca> writes:
>>   (completion-pcm--merge-completions '("ab" "ab") '(prefix "b"))
>>   -> ("b")
>
> Right this is a bug.
>
>     (completion-pcm--merge-completions '("ab" "sab") '(prefix "b"))
>
> returns (correctly)
>
>     ("b" "a" prefix)
>
> so
>
>     (completion-pcm--merge-completions '("ab" "ab") '(prefix "b"))
>
> should return either ("b" "a") or ("b" "a" prefix).
>
> Could you accompany your patch of a regression test using
> `completion-pcm--merge-completions` as above?

Patch with test: