bug#65459: completing-read INITIAL-VALUE unaware of COLLECTION and REQUIRE-MATCH

[Stefan Monnier]

>> Whether the default only shows up when instructed is a UI issue, which
>> can/should be under control of a UI customization. - Stefan
> I would need an example.  Suppose one changes the UI customization, would that
> apply to all calls of that function ?  I would want prefilling occasionally.
> Would that be possible ?

There are some cases, such as `read-file-name` where we do want some
prefilling, so of course it's possible, but it has to be linked to some
specific property of the thing we're requesting from the user and how it
relates to the string with which we want to prefill the buffer.

So it goes back to the question of why you want this specific call to
`completing-read` to be prefilled.  The answer shouldn't be "because
I prefer it this way" (that answer should be satisfied instead by
a customization setting since it may not apply to other users of that
code).

E.g. in the case of `read-file-name` it's because in 99% of the cases
the end result wants to include all or most of that prefilled string.

> All these UI accept any entries by the user.

I don't know what you mean by that.

> I am of the school of thought that doing so automatically is also
> a valid operation.

And I didn't understand this part either.


        Stefan