`Const` type inference glitch

Const's phantom type can cause some type inference difficulty in scalac. Here is a minimized example:

   def trav[F[_], A, B](f: A => F[B]): F[B] = ???
    case class Const[A, B](a: A)
    val c: Const[Int, Int] = trav(Const(_:Int))   //compiles
    val a: Int  = trav(Const(_:Int)).a    //doesn’t compile

[error] found : Int => Const[Int,Nothing]
[error] required: Int => Const[Int,B]
[error] val a: Int = trav(Const(_:Int)).a

Making the phantom type covariant will helped the type inference but it's not a very obviously sound solution. See the discussion here by @ceedubs , here by @djspiewak and here by @johnynek.

Shall we live with the type inference inconvenience (not encountered very often though) or the possible ramifications of unnecessarily making the phantom type covariant?

It seems to me that neither incurs significant cost to pay but the former solution is easier to understand.

Start with the minimized Scala example involving Const, trav, and scalac, then read the three linked comments in the discussion. Compare the inference behavior with and without phantom-type covariance. Done requires an agreed resolution for whether the inconvenience or covariance ramifications should be accepted; no implementation file or test is named.