Funny observation about (recursive) structural types in Scala

Question

I needed some recursive structural type in some piece of code using with traits and the structural type as type parameter constraint. It worked fine, but later I learned Scala does not support recursive structural types.

So can someone explain me why this works fine:

scala> trait Test[M[A] <: { def map[B](f: A => B) : M[B] } ] {}
defined trait Test

and this not:

scala> def test[M[A] <: { def map[B](f: A => B) : M[B] } ] = null
<console>:5: error: illegal cyclic reference involving type M
       def test[M[A] <: { def map[B](f: A => B) : M[B] } ] = null