Can you pattern match constructors on a type class constrained parameter?

See code example below. It won't compile. I had thought that maybe it's because it has to have a single type for the first parameter in the test function. But that doesn't make sense because if I don't pattern match on it so it will compile, I can call it with both MyObj11 5 and MyObj21 5 which are two different types.

So what is it that restricts so you can't pattern match on constructors with a type class constrained parameter? Or is there some mechanism by which you can?

class SomeClass a where toString :: a -> String

instance SomeClass MyType1 where toString v = "MyType1"
instance SomeClass MyType2 where toString v = "MyType2"

data MyType1 = MyObj11 Int | MyObj12 Int Int 
data MyType2 = MyObj21 Int | MyObj22 Int Int 

test :: SomeClass a => a -> String
test (MyObj11 x) = "11"
test (MyObj12 x y) = "12" -- Error here if remove 3rd line: rigid type bound error
test (MyObj22 x y) = "22" -- Error here about not match MyType1.