Type class instance selector doesn't check the apartness of parameters #3941

class TwoParams a b where
  func :: a -> b

instance equals :: TwoParams a a where
  func = identity
else
instance any :: TwoParams a b where
  func = unsafeCoerce

-- can compile
testEquals :: forall a. a -> a
testEquals = func -- with instance `equals`
testAny :: Unit -> Boolean
testAny = func -- with instance `any`

-- expects compile error, cannot choose instance because `b` can be `Unit`
test :: forall b. Unit -> b
test = func
--     ^^^^
--
--   No type class instance was found for
--
--     Main.TwoParams Unit
--                    b0
--
--
-- while checking that type forall (a :: Type) (b :: Type). TwoParams a b => a -> b
--   is at least as general as type Unit -> b0
-- while checking that expression func
--   has type Unit -> b0
-- in value declaration test

-- `a` and `m a` are never unifiable unless we have infinite types (and of course not)
-- so expected that the instance `any` is chosen.
thisShouldBeCompiled :: forall m a. a -> m a
thisShouldBeCompiled = func
--                     ^^^^
--
--   No type class instance was found for
--
--     Main.TwoParams a0
--                    (m1 a0)
--
--
-- while checking that type forall (a :: Type) (b :: Type). TwoParams a b => a -> b
--   is at least as general as type a0 -> m1 a0
-- while checking that expression func
--   has type a0 -> m1 a0
-- in value declaration thisShouldBeCompiled