Polymorphic type not found in lambda

Suppose the following CameLIGO contract:

let const = fun (type a b) (a, b : a * b) : a -> a
 
let main (_ : unit * unit) : operation list * unit =
  [], const ((), 0)

Trying to compile it in the LIGO Playground results in "Type "a" not found."

Note that using an ordinary declaration works as intended:

let const (type a b) (a, b : a * b) : a = a
 
let main (_ : unit * unit) : operation list * unit =
  [], const ((), 0)

Another variation that works is this:

let const (type a b) = fun (a, b : a * b) : a -> a
 
let main (_ : unit * unit) : operation list * unit =
  [], const ((), 0)