Introducing implicits on the LHS works when an infix operator is matched in prefix form but fails to parse when introduced on a match in infix form.

implicit_break.idr

data MyList : Type -> Type where
  Nil : {a : Type} -> MyList a
  (::) : {a : Type} -> a -> MyList a -> MyList a

f : MyList b -> ()
f [] = ()
f ((::) {a} x xs) = ?normal

g : MyList b -> ()
g [] = ()
g ({a} x :: xs) = ?broken