r224012 - Emit warning if define or undef reserved identifier or keyword.

[Nico Weber]

On Wed, Dec 17, 2014 at 7:05 PM, Richard Smith <richard at metafoo.co.uk>
wrote:
>
> On Tue, Dec 16, 2014 at 1:17 PM, Nico Weber <thakis at chromium.org> wrote:
>>
>> Ok, I moved -Wkeyword-macro into -pedantic in r224371. I also removed
>> -Wreserved-id-macro in the same revision, as it's not covered by the
>> standards bit you cited
>>
>
> This seems backwards?
>
> -Wreserved-id-macro is (was) warning on code that is ill-formed.
> -Wkeyword-macro is warning on code that has undefined behavior (only) if a
> standard library header is #included.
>
> C++ [lex.name]p3 (introduced by DR, so applying retroactively to all C++
> standards) says:
>
> "[S]ome identifiers are reserved for use by C ++ implementations and shall
> not be used otherwise; no diagnostic is required.
> — Each identifier that contains a double underscore __ or begins with an
> underscore followed by an uppercase letter is reserved to the
> implementation for any use.
> — Each identifier that begins with an underscore is reserved to the
> implementation for use as a name in the global namespace."
>
> Aaron's quote of [macro.names]p2 applies only to "C++ programs that use
> the facilities of the C ++ standard library." (per
> [constraints.overview]p1), and "If a program declares or defines a name in
> a context where it is reserved, other than as explicitly allowed by
> this Clause, its behavior is undefined." (per [reserved.names]p1).
>

I'm not sure I understand the point here – do you mean the warning should
only fire if a standard library has been included? In pedantic mode, I
would expect that people would want to know about stuff that's undefined
behavior even if they don't happen to include a standard library header at
the moment.


> In your commit message for r224371 (which hasn't yet hit the mailing
> list), you say:
>
> "-Wreserved-id-macro warns on
>
>     #define __need_size_t
>
> which is more or less public api for glibc headers."
>
> I don't agree with your example; these __need_* macros exist only to allow
> communication between the /usr/include headers and the compiler-provided
> <stddef.h>, because /usr/include needs to include a subset of <stddef.h>
> depending on which of its headers is included. They're not for end-users,
> and these warnings should be suppressed in system headers.
>

Some end user code uses them, for example to work around bugs in
/usr/include headers. I know of one example in Chromium (
https://code.google.com/p/chromium/codesearch#chromium/src/chrome/browser/ui/libgtk2ui/gtk2_ui.cc&q=__need_Null&sq=package:chromium&type=cs&l=72
) and I found
https://github.com/asfernandes/firebird/blob/35b06c7892e2440c4f4528b9ecc99969927728e0/src/common/utils.cpp#L35
(there might be more, but github's search is somewhat hard to operate.)

Having said that, I'm happy with the warning coming back in an improved
form.


> That said, there are a bunch of reserved macro names that *is* part of the
> glibc public API, such as the _*_SOURCE macro family. If we reinstate this
> warning, we should whitelist those ones at least.
>
> On Mon, Dec 15, 2014 at 10:39 AM, Joerg Sonnenberger <
>> joerg at britannica.bec.de> wrote:
>>>
>>> On Mon, Dec 15, 2014 at 10:24:22AM -0800, Nico Weber wrote:
>>> > For the "final(a,b,c)" define: should we warn on that? It doesn't
>>> change
>>> > the meaning of the keyword "final" since that doesn't have parameters.
>>>
>>> For C++11 and newer: IMO yes. Otherwise, no.
>>>
>>> Joerg
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>>
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>>
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