Scala - Lower bound inference in path-dependent types -

i'm trying understand why can't scala compiler infer following restriction on path-dependent type:

trait mytrait class mytraitimpl extends mytrait trait mytrait2[a <: mytrait] {   type mytraittype = } class mytrait2impl[a <: mytrait] extends mytrait2[a]  val obj: mytrait2[_] = new mytrait2impl[mytraitimpl]  def mymethod[a <: mytrait](t2: mytrait2[a]) = println("hi!")  mymethod[obj.mytraittype](obj) // <console>:14: error: type arguments [obj.mytraittype] not conform method mymethod's type parameter bounds [a <: mytrait] //               mymethod[obj.mytraittype](obj) 

for me, intuitively, mytraittype can't other subclass of mytrait, bound right on a in mytrait2. if there is, can give me example or point me code snippet wrong?

if scala compiler limitation, can show me way achieve using type system? note that:

• i not have mytrait object, nor mymethod receive one;
• i not need mymethod know concrete type of a; needs know a subtype of mytrait , t2 parametrized on a;
• the underscore in obj intentional; call mymethod, don't know concrete type of a (or else not problem);
• i prefer solutions don't have modify mymethod.

you should use constraints on type member instead of bounds on type parameter in mytrait2 declaration:

trait mytrait class mytraitimpl extends mytrait trait mytrait2 { // remove [a <: mytrait]   type mytraittype <: mytrait // add <: mytrait } class mytrait2impl[a <: mytrait] extends mytrait2 { type mytraittype = }  val obj: mytrait2 = new mytrait2impl[mytraitimpl]  def mymethod[a <: mytrait](t2: mytrait2{ type mytraittype = }) = println("hi!")  mymethod[obj.mytraittype](obj) 

you'll compilation error on wrong types, expected:

scala> val otherobj: mytrait2 = new mytrait2impl[mytraitimpl] otherobj: mytrait2 = mytrait2impl@8afcd0c  scala> mymethod[obj.mytraittype](otherobj) <console>:15: error: type mismatch;  found   : otherobj.type (with underlying type mytrait2)  required: mytrait2{type mytraittype = obj.mytraittype}               mymethod[obj.mytraittype](otherobj)                                         ^ 

proof works list[mytrait2]:

scala> {      |   obj <- list[mytrait2](      |            new mytrait2impl[mytraitimpl],      |            new mytrait2impl[mytraitimpl]      |          )      | } mymethod[obj.mytraittype](obj) hi! hi!