In Fortran there are three types of INTERFACEs, specific, generic and abstract INTERFACEs. As of r749, doxygen only handles generic interfaces somewhat reasonably, misinterprets specific interfaces and doesn't know about abstract interfaces.

Examples:
 * specific interfaces are listed in an INTERFACE block without a (generic)
   name after INTERFACE; multiple interfaces may be listed in one INTERFACE 
   block. This example, should result in two INTERFACE entries in documentation:

     INTERFACE
       FUNCTION f(x)
         REAL :: f, x
       END FUNCTION
       FUNCTION g(x, y)
         REAL :: g, x, y
       END FUNCTION
     END INTERFACE

 * generic INTERFACEs have a generic name, all INTERFACEs should be listed
   under this name in documentation. Example:

     INTERFACE f
       FUNCTION f4(x)
         REAL(4) :: f4, x
       END FUNCTION
       FUNCTION f8(x)
         REAL(8) :: g, x
       END FUNCTION
     END INTERFACE f

 * like specific interfaces, ABSTRACT interfaces do not have a generic name
   and multiple interfaces may be listed in one interface block. This example,
   should again result in two INTERFACE entries in documentation:

     ABSTRACT INTERFACE
       FUNCTION f(x)
         REAL :: f, x
       END FUNCTION
       SUBROUTINE s
       END SUBROUTINE
     END INTERFACE


Attached patch handles all INTERFACE types. However, a disambiguation of INTERFACEs, e.g. in a listing of a MODULE would be useful, i.e. instead of prefixing INTERFACE to any interface, it would be nice to have "ABSTRACT INTERFACE" and "GENERIC INTERFACE" and use "INTERFACE" for specific interfaces only.

The patch also fixes #521867.