Spurious inference error when return type of generic method requires unchecked conversion to target

This code doesn't compile in JDK 5/6/7:

import java.util.List;

interface B { <X> List m(); }

class Test {
  void test(B ab) { List<?> n = ab.m(); }
}


Test.java:26: incompatible types; no instance(s) of type variable(s) X exist so that java.util.List conforms to java.util.List<?>
found : <X>java.util.List
required: java.util.List<?>
  void test(B ab) { List<?> n = ab.m(); }
                                    ^
1 error