def Set (α : Type) := α → Prop

def Set.union (s₁ s₂ : Set α) : Set α :=
  fun a => s₁ a ∨ s₂ a

def FinSet (n : Nat) := Set (Fin n)

/-!
The type of `x` is unfolded to find `Set.union`
-/
example (x y : FinSet 10) : FinSet 10 :=
  x.union y

namespace FinSet
export Set (union)
end FinSet

/-!
Since the types are defeq, this alias works:
-/
example (x y : FinSet 10) : FinSet 10 :=
  FinSet.union x y

/-!
However, this dot notation fails since there is no `FinSet` argument.
However, unfolding is the preferred method.
-/
/--
error: invalid field notation, function 'Set.union' does not have argument with type (FinSet ...) that can be used, it must be explicit or implicit with a unique name
-/
#guard_msgs in
example (x y : FinSet 10) : FinSet 10 :=
  x.union y