RE: `completing-read` without RET

[Drew Adams]

> You're right, it's when require-match is t.
> 
> > Choose: a TAB
> > Nothing happens.
> 
> Don't use TAB, use RET.

Sure, but that's not what the OP requested.

There is a difference between (a) knowing that what you type
is sufficient to choose a single candidate, and so you hit RET,
and (b) not knowing whether there is only one match, and maybe
not even knowing whether completion is lax or REQUIRE-MATCH is
t, so you hit TAB.

If you hit TAB and there is only one matching candidate, the
OP wants that candidate to be chosen (returned).  That's my
understanding of the request.  Vanilla `completing-read' does
not offer that possibility (feature).