failure of liberal coverage condition

tf.hs

{-# LANGUAGE DataKinds, PolyKinds, ScopedTypeVariables,
  FlexibleInstances, FlexibleContexts, GADTs, TypeFamilies,
  RankNTypes, LambdaCase, TypeOperators, TemplateHaskell,
  ConstraintKinds, PolyKinds, NoImplicitPrelude,
  UndecidableInstances, MultiParamTypeClasses , 
  GADTSyntax, 
  AllowAmbiguousTypes, 
  InstanceSigs, 
  DeriveFunctor, 
  FunctionalDependencies#-}


class (ValidBroadCast s t ~ True, u ~ EvalBroadcast s t) =>
      BroadCastAlgebra s t u  | s t -> u where
    (+) :: (Tensor s fv a) -> (Tensor t fv a) -> (Tensor u fv a)

instance BroadCastAlgebra s t u where
    (+) :: (Tensor s fv a) -> (Tensor t fv a) -> (Tensor u fv a)
    (+) (Tensor a) (Tensor b) = Tensor $ TF.add a b

class ValidBroadCast s t ~ True => BroadCastAlgebra s t where
        (+) :: (Tensor s fv a) -> (Tensor t fv a) -> (Tensor (EvalBroadcast s t) fv a)