I think the following patch addresses all requirements and in the
process it fixes some small deficiencies of the current status, such
as talking about C99 when invoked from C++.

Bootstrapped and regression tested on x86_64-unknown-linux-gnu with
--enable-languages=all,obj-c++ --enable-decimal-float and
--target_board=\{-m32,-m64\}

OK for trunk?

2008-10-23  Manuel Lopez-Ibanez  <manu@gcc.gnu.org>

	PR 13358
	* doc/invoke.texi (-Wlong-long): Update description.
	* c-lex (interpret_integer): Only warn if there was no previous
	overflow and -Wlong-long is enabled.
	* c-decl.c (declspecs_add_type): Drop redundant flags.
	* c.opt (Wlong-long): Init to -1.
	* c-opts.c (sanitize_cpp_opts): Synchronize cpp's warn_long_long
	and front-end warn_long_long. Wlong-long only depends on other
	flags if it is uninitialized.
	* c-parser.c (disable_extension_diagnostics): warn_long_long is
	the same for CPP and FE.
	(restore_extension_diagnostics): Likewise.
libcpp/
	* init.c (cpp_create_reader): Wlong_long is disabled by default.
	* expr.c (cpp_classify_number): Give different messages for C and
	C++ front-ends.
cp/
	* parser.c (cp_parser_check_decl_spec): Drop redundant flags.
	* error.c (pedwarn_cxx98): New.
	* cp-tree.h (pedwarn_cxx98): Declare.
	
testsuite/
	* gcc.dg/wtr-int-type-1.c: Use two dg-warning to match two
	messages. Test for "long long" in system headers.
	* gcc.dg/c99-longlong-2.c: New.
	* g++.dg/warn/pr13358.C: New.
	* g++.dg/warn/pr13358-2.C: New.
	* g++.dg/warn/pr13358-3.C: New.
	* g++.dg/warn/pr13358-4.C: New.



2008/8/29 Manuel López-Ibáñez <lopezibanez@gmail.com>:
> 2008/8/29 Joseph S. Myers <joseph@codesourcery.com>:
>> On Fri, 29 Aug 2008, Manuel López-Ibáñez wrote:
>>
>>> CPP and the FE were using different semantics. invoke.texi said that
>>> Wlong-long was the default. This was not correct. It was the default
>>> for CPP but -Wno-long-long was the default for the FE. Now there is
>>> only one semantic: the one from the FE.
>>>
>>> However, we were inhibiting warning even if the user requested it
>>> through Wlong-long. Now explicit Wlong-long (or Wno-long-long)
>>> overrides everything else, except in system headers.
>>
>> If you allow -Wlong-long to have an effect in C99 mode, -std=c99
>> -pedantic-errors -Wlong-long should give the diagnostics as warnings not
>> errors.  Does it?
>
> No but I can make it do that just using pedwarn_c90 instead of pedwarn
> and add a testcase. I'll do the same for c++98. I''ll need to add an
> equivalent of pedwarn_c90 to C++ and CPP. In C++ is probably easy.
>
> Is there a way to tell in CPP if we are being called from the C++ FE?
> and which standard? I cannot find anything in the sources apart from
> CPP_OPTION (pfile, c99).
>
> Cheers,
>
> Manuel.
>