GADT, pattern matching anon fun inference regression

[scabug]

sealed trait Op[A]
case class IntOp(i: Int) extends Op[Int]

object Test {
  def run[A](op: Op[A]): Int => A = op match {
    case IntOp(i) => 
      {case x => i} // broken
  }
  // def runOkay[A](op: Op[A]): Int => A = op match {
  //   case IntOp(i) =>
  //     x => i // okay
  // }
}

This no longer typechecks ("found A required Int"). I bisected the regression to http://github.com/scala/scala/pull/4971, support for SAM types.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-10085?orig=1
Reporter: @retronym
Affected Versions: 2.12.0-M4, 2.12.0