Mysterious error message

[Sixte]

Well, the concatenator '&' works well with two distinct strings. So, their 
types match. The `else` branch of the `when``should not be seen - it is 
statically clear that the first branch gets used. Since the ``else` branch is 
not used, it should not create a new distinct string. And it is a simple cast 
from (distinct string) to (distinct string).