The diagnostic for this program is terrible.

frog:~/workspaces/test/mysterious $ cat -n T.java
     1 class T {
     2 Object x = new A.B();
     3 }
     4
     5
     6 class A {
     7 class B {
     8 B() { }
     9 }
    10 }
frog:~/workspaces/test/mysterious $ newjavac T.java
T.java:2: not an enclosing class: A
    Object x = new A.B();
               ^
1 error
frog:~/workspaces/test/mysterious $