T17563.hs:8:9: error: [GHC-80003]
    • Non type-variable argument in the constraint: Num (T a x)
    • In the quantified constraint ‘forall x. Num (T a x)’
      In the type signature: blah :: (forall x. Num (T a x)) => T a b
    Suggested fix: Perhaps you intended to use FlexibleContexts