RE: Null filename ("") is considered to correspond to an existing, readable, and writable file?

["Drew Adams" <drew.adams@oracle.com>]

    In "Relative File Names" we read:
       <Elisp text defining relative and absolute file names>
    In "File Name Expansion" we read:
       <Elisp text on converting relative to absolute names>
    -- Variable: default-directory
       <definition of `default-directory'>

    Could you please state what is unclear in these fragments?

They are clear, as far as they go. As I said, the definition of relative is
essentially "every name that is not absolute". That is fine as a definition,
but it would be clearer to also explicitly point out that a null name ("")
is relative, because it does not start with... It's not needed for the
quasi-formal definition, but it is helpful as documentation. That is all.

    So what _is_ the question at hand, wrt the manual(s)?

1) See above.

2) The doc (both doc strings and manual blurb) for the functions mentioned
should explicitly describe the file-name argument, saying that a) it can be
relative or absolute, and reminding readers that b) "" is a relative name,
so that the function returns non-nil, because it tests the directory. Such a
reminder of the definition of relative might not be needed for each function
in the manual that takes a file-name arg, but these functions, because of
their names, encourage misinterpretation wrt "".

    >     > Sorry, I still don't get it. Why is the design like this?
    >
    >     This was discussed here some months ago, although I
    >     couldn't find that thread in the few minutes I had to
    >     look for it.
    >
    > I already said that I assumed this was "by design". I asked
    > what the design _advantage_ is. No answer, so far.

    I tried to provide a pointer to the answer: if you find and read those
    discussions, you might find it.  I didn't necessarily need a
    thank-you, but something less unkind would be nice.

I don't see anything unkind in my reply (certainly nothing unkind was meant)
or particularly helpful in your pointer. Like you, I don't have the time to
scour the posts of the past "some months" (looking for what keywords?). I
don't expect anyone to do that. If the advantage of this design was
discussed (I must have missed it), someone must be able to summarize the
rationale.