wrong copy method with covariant type parameter

[scabug]

  case class U[+T](a: Int)(b: T => Int)

Note that no getter is generated for b, so there's no problem with b's type. The compiler will generate

  def copy[T1](a1: Int)(b1: T1 => Int) = {...}
  def copy$$default$$2: T => Int = b  // T in contravariant position

covariant type T occurs in contravariant position in type [T](a: Int)(T) => Int of method copy$$default$$2
       case class U[+T](a: Int)(b: T => Int)
                                ^

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-2382?orig=1
Reporter: @lrytz

[scabug]

@lrytz said:
another example without case classes:

class U[+T](x: T => Int) { def foo(a: T => Int = x) = 0 }

fixed in scala/scala@fb38e47