Doesn't warn about variable not in scope

Running this works fine

{-# Language GADTs, TypeFamilies, InstanceSigs #-}

data FunC a where
  (:$) :: FunC (a -> b) -> FunC a -> FunC b
  Lam  :: (FunC a -> FunC b) -> FunC (a -> b)
  Add  :: Num a => FunC (a -> a -> a)

class Syntactic a where
  type Internal a
  toFunC   :: a -> FunC (Internal a)
  fromFunC :: FunC (Internal a) -> a

instance Syntactic (FunC a) where
  type Internal (FunC a) = a
  toFunC, fromFunC :: FunC a -> FunC a
  toFunC   ast = ast
  fromFunC ast = ast

instance (Syntactic a, Syntactic b) => Syntactic (a -> b) where
  type Internal (a -> b) = Internal a -> Internal b

  toFunC :: (a -> b) -> FunC (Internal a -> Internal b)
  toFunC f = Lam (toFunC . f . fromFunC)

  fromFunC :: FunC (Internal a -> Internal b) -> (a -> b)
  fromFunC f = fromFunC . (f :$) . toFunC

add :: Num a => FunC a -> FunC a -> FunC a 
add = fromFunC Add

A folklore for infix expressions is writing expr a b as a &expr$ b, if I try that without importing (Data.Function.&) I get

-- tghl.hs:29:23-25: error: …
--     • Couldn't match type ‘Internal t0’ with ‘a0 -> a0 -> a0’
--       Expected type: FunC (Internal t0)
--         Actual type: FunC (a0 -> a0 -> a0)
--       The type variables ‘t0’, ‘a0’ are ambiguous
--     • In the first argument of ‘fromFunC’, namely ‘Add’
--       In the second argument of ‘(&)’, namely ‘fromFunC Add’
--       In the expression: (&) a fromFunC Add
add :: Num a => FunC a -> FunC a -> FunC a 
add a b = a &fromFunC Add$ b

I would expect a Variable not in scope: (&) error.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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