您编写的代码应该按原样正常工作，生成您期望的精确存在类型.如果您针对 shapeless 打开一个错误，以便在下一个版本中不会忘记对此的修复，那将会非常有帮助.

与此同时，如果您真正想要的类型是 List[R[_]](这似乎对我更有用)，那么您可以在比您的解决方法更简单的方法，

scala>进口无形._进口无形._标度>类 R[T] ;A级；乙级;C级定义类 R定义类 A定义的 B 类定义类 C标度>val s2 = new R[A] :: new R[B] :: new R[C] :: HNils2: R[A] :: R[B] :: R[C] :: HNil = R@7a26bc5e :: R@518fdf9 :: R@2bc9e90c :: HNil标度>s2.toList[R[_]]res0: List[R[_]] = List(R@7a26bc5e, R@518fdf9, R@2bc9e90c)

鉴于这种不太精确的类型可能在上下文中更有用，即使在修复了错误之后，您仍希望继续使用 toList 的显式元素类型参数，所以我认为这一点成为正确答案而不是解决方法.

Given the following definitions:

class R[T]

class A
class B
class C

This works:

val s1 = new R[A] :: new R[B] :: HNil
val r1 = s1.toList
// r1 of type: List[R[_ >: A with B]]

While this does not:

val s2 = new R[A] :: new R[B] :: new R[C] :: HNil
val r2 = s2.toList
// could not find implicit value for parameter toList:
// shapeless.ToList[shapeless.::[R[A],
// shapeless.::[R[B],shapeless.::[R[C],shapeless.HNil]]],Lub]

Where I expect:

// r2 of type: List[R[_ >: A with B with C]]

Pseudo solution:

Supply implicit yourself:

val r3 = s2.toList(ToList.hlistToList[R[A], R[B], ::[R[C], HNil],
                                      R[_ >: A with B with C]])

This is of course not a solution, as it eliminates the whole benefit of HLists (the HList is supplied by a caller together with all necessary implicits).

Clarification

I am happy if I get a List[R[_]] at the end without the type bounds.

The code you've written ought to Just Work, as-is, producing the precise existential type that you were expecting. It'd be very helpful if you would open a bug against shapeless so that a fix for this doesn't get forgotten for the next release.

In the meantime, if the type you're really after is List[R[_]] (which seems more likely to be useful to me) then you can ask for it explicitly in a much simpler way than your workaround,

scala> import shapeless._
import shapeless._

scala> class R[T] ; class A ; class B ; class C
defined class R
defined class A
defined class B
defined class C

scala> val s2 = new R[A] :: new R[B] :: new R[C] :: HNil
s2: R[A] :: R[B] :: R[C] :: HNil = R@7a26bc5e :: R@518fdf9 :: R@2bc9e90c :: HNil

scala> s2.toList[R[_]]
res0: List[R[_]] = List(R@7a26bc5e, R@518fdf9, R@2bc9e90c)

Given that this less precise type is likely to be more useful in context, you'll want to continue using the explicit element type argument to toList even after the bug is fixed, so I consider this to be the right answer rather than a workaround.