Add Error Message for Bogus Cases Branches

[peblair]

I know that there is static information wiring that is needed to facilitate this (which I am currently working on implementing), but I wanted to make a note of something we should likely fix once we have the ability to do so. Currently, the following runs without issue:

fun foo(x):
  cases(Option) x:
    | none => 0
    | some(n) => n
    | other(f) => f
  end
end

Even worse, so does this (spot the bug!):

fun foo(x):
  cases(Option) x:
    | none => 0
    | som(n) => n
  end
where:
  foo(none) is 0
end

It would be great if we could have some form of error message (even at runtime, if necessary) for things such as these.
(Bonus points if we can have an exhastiveness-checking system like OCaml! 😄)

[schanzer]

Related to #366 ?

[peblair]

Oops. Basically. If we want to mark this as a duplicate, we should add the fact that you can put utter garbage in the comments of that issue (since it would seem to imply that the constructor names need to be bound)

[schanzer]

Why don't we close the other issue, since this seems to be a superset?