Re: emacs-26 8f18d12: Improve documentation of decoding into a unibyte buffer

[Stefan Monnier]

>> I think this is an error.  The patch below seems in order.
>
> I'm not sure.  Be sure to read the comments about the tricky business
> of this function, and the method it employs to solve it, and be sure
> you understand all of the subtleties there.

This only applies to the case where `char` is not ASCII.
I installed a slightly more conservative patch which should make sure
the returned string is always unibyte and that also fixes the ebcdic
case at the same occasion.

>> AFAIK it only happens for pure-ASCII strings.
> What is your reasoning?

For one, the docstring says that, pretty much.  But also the fact that
`undecided` implies that any coding system should be applicable, IOW
`char` is in the intersection of all the coding systems we have, so it
can only happen if the string is pure ASCII (since one of the coding
systems is `us-ascii`, the insection cannot be larger than that.
That doesn't preclude a non-undefined return value for some pure ASCII
strings, admittedly, tho the docstring suggests that any ASCII string
just returns `undecided`).


        Stefan