STC produces compile-time error on encoutering bounded type parameter with conditional

import java.util.function.Supplier;

class A {
  static final <X extends Supplier<Number>> X m(X x, X y) {
    X z = (true) ? x : y
    z
  }
}

org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
test.groovy: 10: [Static type checking] - Incompatible generic argument types. Cannot assign java.util.function.Supplier<java.lang.Object> to: X
 @ line 10, column 5.
       z
       ^

1 error