Michael Albinus writes:
It's about backward compatibility. If id_format is NIL one uses
call2, in order to run also file name handlers which are not aware
of the new parameter.
Unfortunately, it can't provide backward compatibility. The API is
inherently incompatible, and the authors of file handlers have no
control over the compatibility break. (Especially considering that
the new argument is probably used in many places in Emacs's core
An API that is much more likely to be backward-compatible would be to
add the string views as new list elements in the return value.
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