bug#51368: 29.0.50; `cl-case' should error on incorrect use

[Stefan Monnier]

> Ok, I've attached two patches that implement these suggestions.

BTW, there's a third issue which is that

    (cl-case EXP
      ((a b) FOO)
      ((a c) BAR))

won't warn about the duplicate `a`.

After fixing this, many/most uses of `quote` will already emit
a warning, so maybe it would make the quote/function part of your second
patch unnecessary.

Personally, I just recommend the use of `pcase` over `cl-case` ;-)

BTW, I just noticed the use of the `suspicious` category of
warning messages.  Doesn't this category apply to all warnings?

Oh, and for the `nil` case and the presence of branches after
`otherwise`, the warnings could use the same text saying the branch
is unreachable.


        Stefan