documentation fix for gawk manual discussion of unary operators

[Paul Eggert]

I'm following up to an earlier bug report
<http://lists.gnu.org/archive/html/bug-gnu-utils/2006-01/msg00036.html>
where I noted a minor incompatibility of Gawk and POSIX with respect
to unary operators.  On further thought I decided this was a bug in
POSIX not a bug in Gawk's behavior, so I filed a bug report with the
Open Group and was recently notified that the bug will be fixed in the
next POSIX version.

However, the Gawk manual still needs fixing, since it incorrectly claims
that the precedence of prefix unary operators does not matter as long
as only unary operators are involved, and this claim is incorrect when
_postfix_ unary operators are also involved.

Here is a proposed patch, relative to Gawk 3.1.5.

2006-07-29  Paul Eggert  <address@hidden>

        * gawk.texi: Document that `$$0++--' isn't valid even though it
        is unambiguous according to the Awk grammar.  This is in response
        to Open Group XCU ERN 86
        <http://www.opengroup.org/austin/aardvark/latest/xcubug2.txt>.

--- gawk-3.1.5-orig/doc/gawk.texi       2005-07-25 15:28:03.000000000 -0400
+++ gawk-3.1.5/doc/gawk.texi    2006-07-29 08:37:35.000000000 -0400
@@ -9093,14 +9093,20 @@ exponentiation operators, which group in
 Thus, @samp{a - b + c} groups as @samp{(a - b) + c} and
 @samp{a = b = c} groups as @samp{a = (b = c)}.
 
-The precedence of prefix unary operators does not matter as long as only
-unary operators are involved, because there is only one way to interpret
+Normally the precedence of prefix unary operators does not matter,
+because there is only one way to interpret
 them: innermost first.  Thus, @samp{$++i} means @samp{$(++i)} and
 @samp{++$x} means @samp{++($x)}.  However, when another operator follows
 the operand, then the precedence of the unary operators can matter.
 @samp{$x^2} means @samp{($x)^2}, but @samp{-x^2} means
 @samp{-(x^2)}, because @samp{-} has lower precedence than @samp{^},
 whereas @samp{$} has higher precedence.
+Also, operators cannot be combined in a way that violates the
+precedence rules; for example, @samp{$$0++--} is not a valid
+expression because the first @samp{$} has higher precedence than the
address@hidden; to avoid the problem the expression can be rewritten as
address@hidden($0++)--}.
+
 This table presents @command{awk}'s operators, in order of highest
 to lowest precedence: