Conflict between FPAT and empty RS

[luciole75w]

Hello,

I'm having an odd behavior with GNU awk when using FPAT and RS together. 
Here is the code.

$ awk '{ print ; $2 = "-" ; print }' RS='' FPAT='\\w+' <<<'a b c d'
a b c d
a -

$ ... | xxd -g1 -c8
00000000: 61 20 62 20 63 20 64 0a  a b c d.
00000008: 61 20 2d 20 20 20 20 0a  a -    .

I'd expect the 2nd print to output "a - c d" but as you can see, 
modifying the field 2 replaces fields 3 and 4 with spaces in $0. When RS 
is set to something other than the empty string (or just not set), the 
output is correct.

Accessing a field is a partial workaround only up to that field, not for 
fields above. Accessing NF works for all fields. However the workaround 
must be used *before* $0 is referenced, otherwise the missing fields 
seem really lost.

$ awk '{ $2 = "-" ; $3 ; print }' RS='' FPAT='\\w+' <<<'a b c d'
a - c

$ awk '{ $2 = "-" ; NF ; print }' RS='' FPAT='\\w+' <<<'a b c d'
a - c d

$ awk '{ $2 = "-" ; $0 ; NF ; print }' RS='' FPAT='\\w+' <<<'a b c d'
a -

Another strange thing which may be related is that setting FS='' before 
FPAT fixes the behavior.

$ awk '{ $2 = "-" ; print }' RS='' FS='' FPAT='\\w+' <<<'a b c d'
a - c d

But according to the documentation FPAT should override FS, so I'd 
actually expect FS to have no effect in this case.

Tested with gawk 4.1.4 and 5.0.1 on Linux Mint 19.3.

Regards,

luciole