No warning for overlapping instances #2957

The compiler only warns for overlapping instances when solving. This is actually a bug, as illustrated by the following example, which prints false, but gives no overlap warning:

class IsInt a where
  isInt :: a -> Boolean
  
instance intIsInt :: IsInt Int where
  isInt _ = true
  
instance isn'tInt :: IsInt a where
  isInt _ = false
  
class F a b | a -> b where
  f :: a -> b
  
instance fInt :: F Int Int where
  f = id
  
main = logShow (isInt (f 1))

The problem is that the type of the call to isInt is not determined, since the F constraint gets solved later. Therefore we pick the isn'tInt instance, which is wrong.

In order to commit to isn'tInt, we must know that a can never become Int (apartness). Alternatively, we could try to warn at the definition site.

cc @LiamGoodacre You will probably find this interesting.