[J3] IEEE supported formats?

[Steven G. Kargl]

A bug has been reported with gfortran's IEEE support, but it is
unclear (to me) whether it is a compiler bug or a defect in the
Fortran standard.

On some architectures, gfortran supports 4 REAL kinds with a radix
of 2.  For this discussion call these types REAL(4), REAL(8), REAL(10),
and REAL(16).  The types with kind=4, 8, and 16 map to IEEE Std 754-2008's
binary32, binary64, and binary128 basic formats.  REAL(10) maps to an
extended binary64 format (sometimes called extended double or Intel
80-bit format).  This program

use ieee_arithmetic
print *, precision(1._4),  selected_real_kind(6),  ieee_selected_real_kind(6)
print *, precision(1._8),  selected_real_kind(8),  ieee_selected_real_kind(8)
print *, precision(1._10), selected_real_kind(16), ieee_selected_real_kind(16)
print *, precision(1._16), selected_real_kind(21), ieee_selected_real_kind(21)
end

outputs 

           6           4           4
          15           8           8
          18          10          10
          33          16          16

It is claimed, that for the 3rd line, gfortran's ieee_selected_real_kind(16)
should return 16. That is, it should select the binary128 basic format.

Is the intent of the Fortran standard to have Section 17 apply only to
the IEEE Std 754-2008's basic formats?  If yes, should the Fortran standard
explicitly state this?  If no, and extended formats can be supported by
a processor, should the Fortran standard state that extended formats 
are allowed.  A search of the WD 1539-1 J3/18-007r1 did not turn up 
any restrictions of the supported format.  I'll note that it is possible
that I missed the restrictions or in the absences of any restrictions
gfortran result conforms to the Fortran standard.

-- 
Steve