bug#31350: 27.0; `pcase' message: "Redundant pcase pattern"

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

> > I'll repeat the request, which is what I think a user
> > would like to know:
> >
> >   Can the messaging at least tell you:
> >   (1) all of a set of clauses that are mutually redundant and
> >   (2) which one of them will actually be used by the compiled
> >       code, the others presumably having been pruned?
> >
> > IOW, what's the actual effect, for users?  How does pcase
> > deal with the redundancy?  Can that at least be documented
> > somewhere (maybe it is already)?
> 
> I think you make an error in reasoning: we don't have mutual redundancy
> here.  A case can be redundant when it will never match because whenever
> would match, a previous case in the cases list always matches, so it is
> effectively shadowed.  But this is not symmetric (cases are always tried
> from first to last (!), and e.g. a `_' catchall pattern in one case will
> make all following cases redundant, but not the other way round).
> 
> I think the message is more like a warning that a case can never match,
> and in all cases where this happened to me, as also in your case, the
> reason was a very obvious editing mistake.  I don't think there is much
> to say here, Emacs just tells you: this case here will never be used,
> look what you have done wrong.  There is a problem with your code
> whenever you see that message.  And there is nothing to say about the
> semantics as well.
> 
> BTW, I think the implementation only covers the most obvious and
> simplistic cases, like those involving catchall patterns or duplicated
> patterns.

If what you say is the case then the help should tell
users that.  To me it's not obvious.

And even if a user (somehow) understands that later
cases are made redundant by earlier ones, how to tell
which earlier ones are implicated?