RE: :alnum: broken?

[Drew Adams]

> > signal an error for patterns like [:alnum:].
> 
> AFAIU, Richard suggested to emit a warning from the byte compiler.
> That might be a good-enough solution, better than signaling an error
> at run time, IMO.

+1.  Not only better, but appropriate.

It should be a warning, because it is not an
invalid regexp.

It might generally be a useless regexp, and
it's most likely not what the user intended,
but it's not invalid.

Raising an error would, well, be an error, IMHO.
Displaying a warning during byte-compilation
would be very helpful.