[bug-gawk] When RS is null, POSIX states \n should be in FS, gawk only does that if FS is single char

[Ed Morton]

I just came across this where setting RS to null causes FS to include 
`\n` if FS is a singe char but not otherwise:

   $ printf '1:2\n3\n' | awk -F':' -v RS= '{for (i=1; i<=NF; i++) print
   i"/"NF, "<"$i">"}'
   1/3 <1>
   2/3 <2>
   3/3 <3>

   $ printf '1::2\n3\n' | awk -F'::' -v RS= '{for (i=1; i<=NF; i++)
   print i"/"NF, "<"$i">"}'
   1/2 <1>
   2/2 <2
   3>

with this gawk version:

   $ awk --version
   GNU Awk 4.2.1, API: 2.0 (GNU MPFR 4.0.2, GNU MP 6.1.2)
   Copyright (C) 1989, 1991-2018 Free Software Foundation.

and that makes sense given the gawk documentation 
(https://www.gnu.org/software/gawk/manual/gawk.html#Multiple-Line) which 
says (red/underline mine):

   When RS is set to the empty string _/and /__FS is set to a single
   character_, the newline character always acts as a field separator.
   This is in addition to whatever field separations result from FS^

but the POSIX spec (http://pubs.opengroup.org/onlinepubs/9699919799/) says:

   *RS*
       The first character of the string value of *RS* shall be the
       input record separator; a <newline> by default. If *RS* contains
       more than one character, the results are unspecified. If *RS* is
       null, then records are separated by sequences consisting of a
       <newline> plus one or more blank lines, leading or trailing
       blank lines shall not result in empty records at the beginning
       or end of the input, and a <newline> shall always be a field
       separator, no matter what the value of *FS* is.

gawk behaves the way I described with or without the `--posix` flag. 
Shouldn't it add `\n` as a separator when RS is null regardless of the 
value of FS like POSIX says? FWIW OSX/BSD awk on MacOS behaves the same 
way that gawk does, idk about other awks.

     Ed.