Incorrect infering "Nothing" as a type argument of `fun` in 'Unapply'

[DarkDimius]

object test {
  class Foo[a](val arg : a)

  object Foo  {
    def unapply [a](m : Foo[a]) = Some (m.arg)
  }
  def matchAndGetArgFromFoo[a]( e:Foo[a]):a = {e match { case Foo(x) => x }}
// Unapply node here will have type argument [a] instantiated to scala.Nothing:
// UnApply(TypeApply(Select(Ident(Foo),unapply),List(TypeTree[TypeVar(PolyParam(a) -> TypeRef(ThisType(TypeRef(NoPrefix,scala)),Nothing))])),List(),List(Bind(x,Ident(_))))
// but the type of the UnApply node itself is correct: RefinedType(TypeRef(ThisType(TypeRef(ThisType(TypeRef(NoPrefix,<empty>)),test$)),Foo), test$$Foo$$a, TypeAlias(TypeRef(NoPrefix,a)))
}

Given such types the rewriting from UnApply(fun,Nil,_) to
val result = fun(matchSelector) cannot be applied as sel has type not matching fun(because fun was instantiated with wrong type arguments)

[DarkDimius]

@odersky that is a minification of example that I've showed you yesterday. Could you please have a look?

[DarkDimius]

Fixed