The following Scala program compiles and runs without errors:

trait MyTrait[T] {
    def map[U](t: T, u: MyArgument[U]) : MyArgument[U] = u
}

class MyArgument[U](u: U) {}

class MyClass[U] extends MyTrait[U] {}

class MyClassRenamedTypes[U2] extends MyTrait[U2] {}

object MyClass {
    def main(args: Array[String]) {
        new MyClass[Int].map(1, new MyArgument("String"))
        new MyClassRenamedTypes[Int].map(1, new MyArgument("String"))
    }
}

Consider a Java implementation of the same main main() method:

public class JavaTypeParam {
    public static void main(String[] args) {
        // This should compile:
        new MyClassRenamedTypes<Integer>().map(1, new MyArgument<String>("String"));
        // This fails:
        new MyClass<Integer>().map(1, new MyArgument<String>("String"));
    }
}

Javac fails to compile the last line of this main() method:
{noformat}
JavaTypeParam.java:6: map(U,MyArgument) in MyClass<java.lang.Integer> cannot be applied to (int,MyArgument<java.lang.String>)
new MyClass().map(1, new MyArgument("String"));
^{noformat}

The only difference between MyClass and MyClassRenamedTypes is the name of the type parameter. It looks like type parameters in different namespaces are colliding.

Interestingly, everything compiles if MyArgument[U] is replaced by U. It also compiles if the definition of MyTrait.map() is changed to

{code}
def map[U](u: MyArgument[U]) : MyArgument[U] = u{code}