cstdlib declarations of ::abs

[Dave Gittins]

On Mon, Mar 6, 2017 at 10:44 AM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> It's not the case that most are there, for <cmath> it's around a third
> of them (e.g. it declares three overloads of std::sin but only one of
> the three is also in the global namespace).

Fair enough. Then the perception is maybe even less accurate (but still there).

>> Alternatively, I wonder if gcc could warn for this? Presumably the
>> compiler will be aware that I am calling a function that was
>> "indirectly" declared in a C-library header, and therefore outside the
>> realm of standard-controlled behaviour? I don't know how hard that
>> would be to achieve, just wonder if it's a possibility.
>
> I think the compiler would have to have special cases for every
> function. I don't think your heuristic would work, because if you
> #include <stdlib.h> and call ::abs then it's still declared in a
> C-library header but there's no problem and no need to warn in that
> case.

But in that case the compiler could figure out that stdlib.h was
included "directly" from user code, rather than "indirectly" being
included by a libstdc++ header. So the rule would be something like
this: if we're compiling C++, and the function was declared in a
C-library foo.h, and the *only* #include <foo.h> (or #include_next
<foo.h>) directives in this TU are in lines from headers on the
libstdc++ default search path, then the user code never explicitly
asked for foo.h and we should warn. Does that work in theory...?