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copied!<p>(See also two related questions: <a href="https://stackoverflow.com/questions/6881393/scala-fails-to-infer-the-right-type-arguments">Scala fails to infer the right type arguments</a> and <a href="https://stackoverflow.com/questions/6888136/type-infered-to-nothing-in-scala">Type infered to Nothing in Scala</a>)</p> <p>It looks like a limitation of Scala's type inference, which is intentionally not spec'ed. As work-around, you can get inference by making <code>T</code> a type <em>member</em> of <code>B</code> rather than parameter,</p> <pre><code>abstract class A abstract class B { type T <: A } class ConcreteA extends A class ConcreteB extends B { type T = ConcreteA } class Example[U <: B]( resolver: U ) object Test { new Example( new ConcreteB ) } </code></pre> <p>When using type members, it's useful to know that they can be surfaced as type parameters using refinement, as in Miles Sabin's answer to: <a href="https://stackoverflow.com/questions/6719827/why-is-this-cyclic-reference-with-a-type-projection-illegal">Why is this cyclic reference with a type projection illegal?</a></p> <p>In Jean-Philippe Pellet's answer to <a href="https://stackoverflow.com/questions/6888136/type-infered-to-nothing-in-scala">a related question</a>, type inference was aided by making the type parameter higher kinded. If you <em>introduce an extra type parameter</em> in <code>ConcreteB</code>, then type inference can work,</p> <pre><code>abstract class A abstract class B[T <: A] class ConcreteA extends A class ConcreteB[T <: A] extends B[T] class Example[T <: A, U[T0 <: A] <: B[T0]]( resolver: U[T] ) object Test { new Example( new ConcreteB[ConcreteA] ) } </code></pre> <p>Scala 2.9 gives the mysterious error message below, but Miles Sabin points out it is a bug that will be fixed for 2.9.1</p> <pre><code><console>:15: error: kinds of the type arguments (ConcreteA,ConcreteB[T0]) do not conform to the expected kinds of the type parameters (type T,type U) in class Example. ConcreteB[T0]'s type parameters do not match type U's expected parameters: class ConcreteB has one type parameter, but type U has one new Example( new ConcreteB[ConcreteA] ) ^ </code></pre>

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