bug#27158: 25.2; Eliminating old usage of completing-read from built-in files

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

> > Would you make a default arg be mandatory instead of optional?
> > Is that it?  If not, what default default would you propose?
> 
> It's not necessary.

What's not necessary?  The argument is either optional or mandatory.
If optional, what default behavior do you want?

Or are you saying that the default arg should just be removed
from `completing-read', because "it's not necessary"?  Not
necessary for whom?  Why isn't it ever a useful thing to have?

Just because some callers might not need it does not mean that
it is not useful for other callers.

> > What should be returned if no explicit default is provided and
> > the user hits RET with no input?
> 
> Prohibit them from finishing completion, except through entering
> a valid value, or pressing C-g.

(while <no valid value> <get input with completion>)

Is that hard?

> > In any case, nothing stops someone from defining their own
> > `my-completing-read', which does not have this feature, er,
> > quirk.  I don't see the problem.
> 
> That doesn't help if we want to augment completing-read-function
> and all packages using it.

What happens when you set `completing-read-function' to your
`my-completing-read'?