I just realized that a function I was compiling had a type error inside a match case. Instead of giving a compile error I was getting a redundant clause warning for the second case. What is the reason for this?

Question

I just realized that a function I was compiling had a type-error inside a match case. Instead of giving a compile error I was getting a redundant clause warning for the second case. What is the reason for this?

Answer

Here is a situation that can cause this error:

datatype AB = A | B
method test(q: AB) {
  var c := match q { case a => true case b => false };
}

The example has an error on the second case saying this branch is redundant.

The problem here is that both a and b are undeclared variables. Consequently they both match in match expression q no matter what q’s value is. Because a matches no matter what, the second case is redundant.

What is usually intended by such code is something like this:

datatype AB = A | B
method test(q: AB) {
  var c := match q { case A => true case B => false };
}

which causes no type errors.

The observation here is that misspelled type names in case patterns can cause silent unexpected behavior. It is likely that a lint warning will be produced for the above anti-pattern in some future version of Dafny.