Weird error message for incorrect default arg type

[cknitt]

let f = (~test: option<int>=42) => ()

gives

We've found a bug for you!
  /.../fixtures/optional_default_arg_type_mismatch.res:1:29-30

  1 │ let f = (~test: option<int>=42) => ()
  2 │

  This has type: int
  But this switch is expected to return: option<int>

  All branches in a switch must return the same type.
  To fix this, change your branch to return the expected type.

However, there is no switch visible to the user here.