scala - Compile-time Check on Sum of Nat's? -

i attempted write typeclass, sumeq5, such hlist type parameter's first 2 fields add 5:

trait sumeq5[a] object sumeq5 {   def apply[l <: hlist](implicit ev: sumeq5[l]): sumeq5[l] = ev    implicit def sumeq5ev[l <: hlist, <: nat, b <: nat](     implicit hcons: ishcons.aux[l, a, b :: hnil],              ev: sum.aux[a, b, _5]   ): sumeq5[l] = new sumeq5[l] {} } 

but doesn't appear work:

import shapeless._ import shapeless.nat._ import net.sumeq5  scala> sumeq5[_0 :: _5 :: hnil] <console>:19: error: not find implicit value      parameter ev: net.sumeq5[shapeless.::[shapeless.nat._0,shapeless.::        [shapeless.nat._5,shapeless.hnil]]]        sumeq5[_0 :: _5 :: hnil] 

please give me hint why _0 :: _5 :: hnil not have evidence 2 nat's equal 5.

edit

updated question per denis rosca's in shapeless's gitter.

dale wijnand , marcus henry pointing in right direction if want generalize hlists of arbitrary length, if want accommodate 2 element hlists, following rather simpler solution,

scala> import shapeless._, nat._, ops.nat._ import shapeless._ import nat._ import ops.nat._  scala> :paste // entering paste mode (ctrl-d finish)  trait sumeq5[a]  object sumeq5 {   def apply[l <: hlist](implicit ev: sumeq5[l]): sumeq5[l] = ev    implicit def sumeq5ab[a <: nat, b <: nat]     (implicit ev: sum.aux[a, b, _5]): sumeq5[a :: b :: hnil] =       new sumeq5[a :: b :: hnil] {} }  // exiting paste mode, interpreting.  defined trait sumeq5 defined object sumeq5  scala> sumeq5[_0 :: _5 :: hnil] res0: sumeq5[_0 :: _5 :: hnil]] = sumeq5$$anon$1@658c5e59 

the main difference here instance explicitly defined 2 element lists rather being defined lists in general proviso there exists proof list has 2 elements.

following dale's update, can generalize accommodate hlists @ least 2 (rather two) elements, again without additional witnesses,

scala> import shapeless._, nat._, ops.nat._ import shapeless._ import nat._ import ops.nat._  scala> :paste // entering paste mode (ctrl-d finish)  trait sumeq5[a]  object sumeq5 {   def apply[l <: hlist](implicit ev: sumeq5[l]): sumeq5[l] = ev    implicit def sumeq5ab[a <: nat, b <: nat, t <: hlist]     (implicit ev: sum.aux[a, b, _5]): sumeq5[a :: b :: t] =       new sumeq5[a :: b :: t] {} }  // exiting paste mode, interpreting.  defined trait sumeq5 defined object sumeq5  scala> sumeq5[_0 :: _5 :: hnil] res0: sumeq5[_0 :: _5 :: hnil]] = sumeq5$$anon$1@658c5e59