diff 2.8.7, -y and ')'

[Duncan Moore]

I'm using diff 2.8.7.
If I have 2 files,

% cat -n x
     1  a
     2  b
     3  c

% cat -n y
     1  a
     2  b
     3
     4  c

where the 3rd line of y is empty, I get

% diff -B -y -W 20 x y
a       a
b       b
c
      ) c

Is this really correct? From what the manual says about the meaning of
the right parenthesis (only the second file contains the line, but the
difference is ignored), I would have expected:

% diff -B -y -W 20 x y
a       a
b       b
      )
c       c

(A similar thing happens if -I is used instead of -B.)

When --left-column is used, a right parenthesis seems to have a
different meaning - both files contain the line, but the right hand
side line isn't shown, although I can't find this meaning of ')'
explicitly stated in the manual.