When documenting a typedef (in straight C code) like this:

/**
* The representation of frobo.
*
* The frobo_t type is actually an unsigned integer than can hold 
* the unique identifier for every frobo you could ever have. The value
* of 0 is special, because it indicates something that only frobos
* would understand.
*/

typedef unsigned int frobo_t;

then Doxygen 1.7.1 to emits:

frobo.h:25: warning: return type of member frobo_t is not documented

If the comment is a comment after the typedef, like:

typedef unsigned int frobo_t; //<! This is the frobo type.

then the warning is not generated.

It seems that if you use block comment in front of the typedef, then Doxygen insist of it having a return value. If you specify a \returns in the block, then it is happy.

The 1.6 series did not have this bug.