Walways-true:Warn if a comparison is always true or always false due to the limited range of the data type

["Manuel López-Ibáñez" <lopezibanez at gmail dot com>]

:ADDPATCH c/c++:

A new warning option Walways-true takes over from Wextra the function
of warning about comparison of unsigned variables against zero with <
or >=. To keep current behaviour, Walways-true is also enabled by
Wextra.

Walways-true also includes an unnamed warning for comparisons that are
always true because of type range limits. (This is the warning that
RMS was talking about sometime ago).

This patch affects a few PRs.

PR 29694 and 23587 are about the inconsistency of having some warnings
in Wextra and others issued unconditionally. Thus, this patch fixes
those.

PR 11856 is about warning for templates. This patch is not a
definitive fix, since Gabriel Dos Reis thinks that we should not warn
at all,  but it is a partial fix, since at least it gives to the user
the option of disabling the warning (and it allows the use #pragma
machinery), instead of using -w.


PR 12963 asks for a way to disable the unconditional warning without
using -w. This patch fixes that.


I took testcases from the PRs, bootstrapped and regression tested with
--enable-languages=all for i686-pc-linux-gnu.


Ok for mainline?

Manuel.


2007-03-04 Manuel Lopez-Ibanez <manu@gcc.gnu.org>

PR middle-end/7651
PR c++/11856
PR c/12963
PR c/23587
PR other/29694
* c.opt (Walways-true): New.
* doc/invoke.texi (Walways-true): Document it.
(Wextra): Enabled by -Wextra.
* c-opts.c (c_common_post_options): Enabled by -Wextra.
* c-common.c (shorten_compare): Warn with Walways-true.

testsuite/
2007-03-04  Manuel Lopez-Ibanez  <manu@gcc.gnu.org>

PR middle-end/7651
PR c++/11856
PR c/12963
PR c/23587
PR other/29694
* gcc.dg/compare6.c: Replace Wall with Walways-true.
* gcc.dg/Walways-true.c: New.
* gcc.dg/Walways-true-Wextra.c: New.
* gcc.dg/Walways-true-no.c: New.
* g++.dg/warn/Walways-true.C: New.
* g++.dg/warn/Walways-true-Wextra.C: New.
* g++.dg/warn/Walways-true-no.C: New.

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