The diamond operator does not work when passing a value whose type is a type variable

class A<T> {
  void foo() {
    T z = null;
    T x = new B<>(z).f // does not work

    String y = ""
    String k = new B<>(y).f // works
  }
  
}
class B<X> {
  X f;
  B(X f) {
    this.f = f;
  }
}

org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
test.groovy: 4: [Static type checking] - Cannot assign value of type X to variable of type T
 @ line 4, column 11.
       T x = new B<>(z).f // does not work
             ^1 error