ldionne added inline comments.
================
Comment at: clang/lib/Headers/stddef.h:1
/*===---- stddef.h - Basic type definitions --------------------------------===
*
----------------
Making a thread out of this:
> The relationship between clang's stddef.h and the C Standard Library stddef.h
> is that there is no relationship. clang's header doesn't #include_next, and
> it is in the search path before the OS's cstdlib.
So in that case what is the purpose of the SDK/system providing a `<stddef.h>`
header? They basically all provide one and it's never used?
================
Comment at: clang/lib/Headers/stddef.h:118-122
+#ifdef __cplusplus
+namespace std {
+typedef decltype(nullptr) nullptr_t;
+}
+using ::std::nullptr_t;
----------------
iana wrote:
> aaron.ballman wrote:
> > iana wrote:
> > > aaron.ballman wrote:
> > > > iana wrote:
> > > > > ldionne wrote:
> > > > > > iana wrote:
> > > > > > > aaron.ballman wrote:
> > > > > > > > Related:
> > > > > > > >
> > > > > > > > https://github.com/llvm/llvm-project/issues/37564
> > > > > > > > https://cplusplus.github.io/LWG/issue3484
> > > > > > > >
> > > > > > > > CC @ldionne
> > > > > > > I don't _think_ this change actually changes the way nullptr_t
> > > > > > > gets defined in C++, does it?
> > > > > > I think we absolutely don't want to touch `std::nullptr_t` from
> > > > > > this header. It's libc++'s responsibility to define that, and in
> > > > > > fact we define it in `std::__1`, so this is even an ABI break (or I
> > > > > > guess it would be a compiler error, not sure).
> > > > > I'm really not touching it though. All I did is move it from
> > > > > `__need_NULL` to `__need_nullptr_t`.
> > > > >
> > > > > The old behavior is that `std::nullptr_t` would only be touched if
> > > > > (no `__need_` macros were set or if `__need_NULL` was set), and
> > > > > (_MSC_EXTENSIONS and _NATIVE_NULLPTR_SUPPORTED are defined).
> > > > >
> > > > > The new behavior is that `std::nullptr_t` will only be touched if
> > > > > ((no `__need_` macros are set) and (_MSC_EXTENSIONS and
> > > > > _NATIVE_NULLPTR_SUPPORTED are defined)) or (the new
> > > > > `__need_nullptr_t` macro is set)
> > > > >
> > > > > So the only change is that C++ code that previously set `__need_NULL`
> > > > > will no longer get `std::nullptr_t`. @efriedma felt like that was a
> > > > > fine change.
> > > > Does libc++ provide the symbols for a freestanding compilation?
> > > >
> > > > I was pointing out those links specifically because the C++ standard
> > > > currently says that stddef.h (the C standard library header) needs to
> > > > provide a definition of `std::nullptr_t`, but that LWG thinks that's
> > > > perhaps not the right way to do that and may be removing that
> > > > requirement.
> > > It is weird the standard puts that in stddef.h and not cstddef. I think
> > > libc++ could provide that in their stddef.h anyway, but the intent in
> > > this review is to not rock the boat and only do the minimal change
> > > discussed above.
> > Yeah, this discussion is to figure out whether we have an existing bug we
> > need to address and if so, where to address it (libc++, clang, or the C++
> > standard). I don't think your changes are exacerbating anything, more just
> > that they've potentially pointed out something related.
> 👍
> Does libc++ provide the symbols for a freestanding compilation?
I don't think we do. We basically don't support `-ffreestanding` right now (we
support embedded and funky platforms via other mechanisms).
But regardless, `<stddef.h>` should never define something in namespace `std`,
that should be libc++'s responsibility IMO. What we could do here instead is
just
```
#ifdef __cplusplus
typedef decltype(nullptr) nullptr_t;
#else
typedef typeof(nullptr) nullptr_t;
#endif
```
and then let libc++'s `<cstddef>` do
```
_LIBCPP_BEGIN_NAMESPACE_STD
using ::nullptr_t;
_LIBCPP_END_NAMESPACE_STD
```
If Clang's `<stddef.h>` did define `::nullptr_t`, we could likely remove
libc++'s `<stddef.h>` and that might simplify things.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D157757/new/
https://reviews.llvm.org/D157757
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