Rule 2.4 and forward declarations

[michael.metivier]

unless the only occurrence of the tag appears in the typedef declaration.

That is the primary concern. There are no other uses of "test"; all other references to the structure are through the typedef, test_T.

The circular dependency between Callback_T and the structure test complicates these definitions. The example given allows all references to the structure test to be done through the typedef test_T, but is flagged as a violation because test is not used beyond the snippet given.