-F fs_val handles backslash-newline differently, compared to -v FS=val and FS=val

[Denys Vlasenko]

GNU awk 5.1.1

gawk -F '\
a' 'BEGIN { print "FS1=" FS }'

gawk -v FS='\
a' 'BEGIN { print "FS2=" FS }'

echo | gawk '{ print "FS3=" FS }' FS='\
a'

The first command treats "backslash+newline" as backslash:

FS1=\a

The second and third commands treat the same as empty string:

FS2=a
FS3=a

I think it would be better if all forms have the same rules.