Scala generics implementing Ordered[A[B]] gives strange compiler errors -


i have strange error message, , cannot fix asinstanceof[t]

 trait abase {     type <: abase     def compare(that: a): int   }    case class b [a <: abase] (somefield: a) {     //extends ordered[b[a]] {     def compare1(that: b [a]): int = somefield.asinstanceof[a].compare(that.somefield.asinstanceof[a])     /*error: type mismatch;  found   :  required: _1.a val _1:     = somefield.asinstanceof[a].compare(that.somefield.asinstanceof[a])*/     def compare2(that: b [a]): int = somefield.compare(that.somefield.asinstanceof[a])     /*error: type mismatch;  found   :  required: b.this.somefield.a     = somefield.compare(that.somefield.asinstanceof[a])*/     def compare3(that: b [a]): int = somefield.compare(that.somefield)     /*error: type mismatch;  found   : that.somefield.type (with underlying type a)  required: b.this.somefield.a     = somefield.compare(that.somefield)*/   }

error itself

error: type mismatch; found : required: _1.a val _1: a

you can fix casting b's (this) somefield type:

trait abase {   type <: abase   def compare(that: a): int }  case class b [a <: abase] (somefield: a) {    def compare1(that: b [a]): int = somefield.compare(that.somefield.asinstanceof[this.somefield.a])    def compare2(that: b [a]): int = somefield.compare(that.somefield.asinstanceof[this.somefield.a])    def compare3(that: b [a]): int = somefield.compare(that.somefield.asinstanceof[this.somefield.a]) }

you can keep type attribute make more concise:

case class b [a <: abase] (somefield: a) {    type thisa = this.somefield.a    def compare1(that: b [a]): int = somefield.compare(that.somefield.asinstanceof[thisa])    def compare2(that: b [a]): int = somefield.compare(that.somefield.asinstanceof[thisa])    def compare3(that: b [a]): int = somefield.compare(that.somefield.asinstanceof[thisa]) }

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