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Asked: 2026-05-24T08:04:24+00:00

Consider the following Scala code: abstract class A abstract class B[T <: A] class

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Consider the following Scala code:

abstract class A
abstract class B[T <: A]
class ConcreteA extends A
class ConcreteB extends B[ConcreteA]

class Example[U <: B[T], T <: A]( resolver: U )
object Test {
    new Example( new ConcreteB )
}

The last line new Example( new ConcreteB ) fails to compile with the following error:

error: inferred type arguments [ConcreteB,Nothing] do not conform to class Example’s type parameter bounds [U <: B[T],T <: A]

But ConcreteB has all the necessary data to resolve both U and T. What am I missing here?

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1 Answer

  1. Editorial Team

    Kipton got close with his higher-kinded solution. Unfortunately he tripped over what appears to be a bug in Scala < 2.9.1.RC1. The following works as expected with 2.9.1.RC1 and trunk,

    Welcome to Scala version 2.9.1.RC1 (Java HotSpot(TM) Server VM, Java 1.7.0).
Type in expressions to have them evaluated.
Type :help for more information.

scala> abstract class A
defined class A

scala> abstract class B[T <: A]
defined class B

scala> class ConcreteA extends A
defined class ConcreteA

scala> class ConcreteB[T <: A] extends B[T]
defined class ConcreteB

scala> class Example[T <: A, U[X <: A] <: B[X]](resolver: U[T])
defined class Example

scala> new Example(new ConcreteB[ConcreteA])
res0: Example[ConcreteA,ConcreteB] = Example@ec48e7
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