Active pattern '{0}' has a result type containing type variables that are not determined by the input. The common cause is ...

Active pattern '{0}' has a result type containing type variables that are not determined by the input. The common cause is a when a result case is not mentioned, e.g. 'let (|A|B|) (x:int) = A x'. This can be fixed with a type constraint, e.g. 'let (|A|B|) (x:int) : Choice = A x'