imihu

imihu

$ scala
Welcome to Scala version 2.9.1.final (Java HotSpot(TM) Client VM, Java 1.6.0_29).
Type in expressions to have them evaluated.
Type :help for more information.

scala> trait A[T] { def t : T }
defined trait A

scala> trait B extends A[Int]
defined trait B

scala> def f[C[_] <: A[_]](a : C[_]) = a
f: [C[_] <: A[_]](a: C[_])C[_]

scala> f(new B { def t = 1 } : B)
res0: A[_] = $anon$1@b1cb25

scala> f(new B { def t = 1 })
res1: A[_] = $anon$1@10010ec

scala> def g[T, C[T] <: A[T]](a : C[T]) = a
g: [T, C[T] <: A[T]](a: C[T])C[T]

scala> g(new B { def t = 1 } : B)
res2: A[Int] = $anon$1@1ac7fbb

scala> g(new B { def t = 1 })
<console>:11: error: type mismatch;
 found   : java.lang.Object with B
 required: ?C[?T]
Note that implicit conversions are not applicable because they are ambiguous:
 both method any2Ensuring in object Predef of type [A](x: A)Ensuring[A]
 and method any2ArrowAssoc in object Predef of type [A](x: A)ArrowAssoc[A]
 are possible conversion functions from java.lang.Object with B to ?C[?T]
              g(new B { def t = 1 })