[libcxx] Reinstate <string.h> and fix overload sets to be const-correct wherever possible

[Richard Smith via cfe-commits]

Ping.

On Thu, Nov 5, 2015 at 6:32 PM, Richard Smith <richard at metafoo.co.uk> wrote:

> Ping.
>
> On Thu, Oct 29, 2015 at 5:21 PM, Richard Smith <richard at metafoo.co.uk>
> wrote:
>
>> Hi,
>>
>> The attached patch undoes the revert of r249929, and adds an extension to
>> allow <string.h> (and <wchar.h>) to work properly even in environments such
>> as iOS where the underlying libc does not provide C++'s const-correct
>> overloads of strchr and friends.
>>
>> This works as follows:
>>
>>  * The macro _LIBCPP_PREFERRED_OVERLOAD is, where possible, defined by
>> <__config> to an attribute that provides the following semantics:
>>    - A function declaration with the attribute declares a different
>> function from a function declaration without the attribute.
>>    - Overload resolution prefers a function with the attribute over a
>> function without.
>>  * For each of the functions that has a "broken" signature in C, if we
>> don't believe that the C library provided the C++ signatures, and we have a
>> _LIBCPP_PREFERRED_OVERLOAD, then we add the C++ declarations and mark them
>> as preferred over the C overloads.
>>  * The overloads provided in namespace std always exactly match those in
>> ::.
>>
>>
>> This results in the following changes in cases where the underlying libc
>> provides the C signature not the C++ one, compared to the status quo:
>>
>>
>> <string.h>:
>>
>>       char *strchr(const char*, int) // #1
>>       char *strchr(char*, int) // #2
>>       const char *strchr(const char*, int) // #3
>>
>> We used to provide #1 and #2 in namespace std (in <cstring>) and only #1
>> in global namespace (in <string.h>).
>>
>> For a very old clang or non-clang compiler, we now have only #1 in both
>> places (note that #2 is essentially useless). This is unlikely to be a
>> visible change in real code, but it's slightly broken either way and we
>> can't fix it.
>>
>> For newer clang (3.6 onwards?), we now have correct signatures (#2 and
>> #3) in :: and std (depending on header). Taking address of strchr requires
>> ~trunk clang (but it didn't work before either, so this is not really a
>> regression).
>>
>>
>> <wchar.h>:
>>
>>       wchar_t *wcschr(const wchar_t *, wchar_t) // #1
>>       const wchar_t *wcschr(const wchar_t *, wchar_t) // #2
>>       wchar_t *wcschr(wchar_t *, wchar_t) // #3
>>
>> We used to provide #1 in global namespace, and #2 and #3 in namespace
>> std. This broke code that uses 'using namespace std;'.
>>
>> For a very old clang or non-clang compiler, we now have #1 in global
>> namespace and namespace std. This fixes the ambiguity errors, but decreases
>> const-correctness in this case. On the whole, this seems like an
>> improvement to me.
>>
>> For newer clang, we now have correct signatures (#2 and #3) in :: and std
>> (depending on header). As above, taking address doesn't work unless you're
>> using very recent Clang (this is not a regression in ::, but is a
>> regression in namespace std).
>>
>>
>> To summarize, we previously had ad-hoc, inconsistent, slightly broken
>> rules for <cstring> and <cwchar>, and with this patch we fix the overload
>> set to give the exact C++ semantics where possible (for all recent versions
>> of Clang), and otherwise leave the C signatures alone.
>>
>
>
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