/- In the following example, type of `x` and `y` could be any type `α` s.t. `[OfNat α]`.
   It relies on `SyntheticMVarKind.withDefault` to set `α := Nat`.
   Moreover, we must commit to `α := Nat` before we try to build de `matcher` since
   `mkMatcher` assumes `matchType` does not contain metavariables.
   We accomplish that by invoking `synthesizeSyntheticMVarsNoPostponing` at `elabMatch`. -/
def foo : IO Unit := do
let (x, y) ← pure (0, 0);
IO.println x

private def f (x : Nat) : Option Nat :=
none

private def g (xs : List (Nat × Nat)) : IO Unit :=
xs.forM fun x =>
  match f x.fst with
  | _ => pure ()