Antiquotations don't work for generic types (even if they implement ToExpr)

[alexkeizer]

IIUC, when writing q($n) for, say n : Nat, it automatically translates $n into ToExpr.toExpr n.
This mechanism doesn't seem to work when we have an antiquotation of generic type \alpha, even if there is an instance of ToExpr \alpha in scope.

import Qq

open Lean
open Qq

-- Works
example (n : Nat) (m : Q(Nat)) : Q(Nat) :=
  q($n)

-- Doesn't work
example {α : Type} [ToExpr α]  (a : α) : Expr :=
  q($a)
--^^^^^ unknown free variable '_uniq.54'
--^^^^^ level mvars not supported ?u.69

-- Doesn't work either, but different error
example {α : Type} [ToExpr α]  (a : α) : QQ (toTypeExpr α) :=
  q($a)
--^^^^^ unquoteExpr: q.2 : Expr


-- Again, doesn't work, but now the error is even on the type declaration, too
example {α : Type} [ToExpr α]  (a : α) : Q($(toTypeExpr α)) :=
--                                       ^^^^^^^^^^^^^^^^^^ unquoteExpr: ?m.189 : Expr
  q($a)
--^^^^^ unquoteExpr: q.2 : Expr

When hovering my cursor over $a (in the Expr-returning example), I see the following context:

«$a»: _uniq.54
⊢ ?m.70

Which seems to suggest that the \alpha type (and the corresponding ToExpr) instance are not part of this local context, explaining why the antiquotation fails.