confusing error message on functor

Stream: new members

[Alok Singh]

instance : Functor (Vector · n) where
  map f v := v.map f

/-
Expected type
⊢ ?m.96173 ?m.96191

typeclass instance problem is stuck, it is often due to metavariables
  Functor ?m.96173

-/
#eval Functor.map id #v[1, 2, 3]
#eval #v[1, 2, 3].map id -- works

Why does the second form work when it's supposed to just be .map? Shouldn't it infer the output type?

[Aaron Liu]

The second one uses Vector.map, while the first one uses Functor.map, which generates a second-order unification problem that can't be solved.