[libcxx-commits] [PATCH] D68364: Implement C++20's P0784 (More constexpr containers)

[Richard Smith - zygoloid via Phabricator via libcxx-commits]

rsmith added inline comments.


================
Comment at: libcxx/include/version:254
 // # define __cpp_lib_concepts                             201806L
+# define __cpp_lib_constexpr_dynamic_alloc              201907L
 // # define __cpp_lib_constexpr_misc                       201811L
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ldionne wrote:
> rsmith wrote:
> > Should this be conditioned on compiler support being available?
> So.. I've decided not to do that in this patch so far.
> 
> The support for constexpr allocation was checked into Clang about a year ago, right? I actually expect this to be a slightly contentious point, but I'd like to assume that we're using a reasonably recent Clang. I don't see a strong point for being able to use new libc++ headers with an old Clang anyway, since vendors usually release the two together. IOW, supporting this would add complexity for virtually no benefit. I do agree it's a slightly more aggressive stance than we've had so far, but this sort of reasonable assumption makes it so much easier to write stuff for libc++.
OK, just a few thoughts then I'm going to bow out of this; this seems like a policy decision for the libc++ maintainers to make.

In favor of dropping support for new libc++ + old clang: we generally don't permit version skew between different components of LLVM. It seems reasonable to expect all wanted parts of a particular LLVM release to be built together.

Against dropping support for new libc++ + old clang: we do support installing more than one version of LLVM (and in particular more than one version of Clang) on the same system, but because libc++ defaults to being installed in `/usr/include/c++/v1`, we don't seem to encourage installing more than one version of libc++, so -- even assuming we only support the *newest* version of libc++ going into `/usr/include/c++/v1` -- new versions of libc++ need to work with old versions of Clang.

I think (largely by accident) Clang will prefer a libc++ installed into `/usr/lib/clang/$VER/include` over one from `/usr/include/c++/v1`. If we switched to installing libc++ there, I don't see any technical barrier to version-locking them, though I'm not sure what story that leaves for use of libc++ with GCC and other compilers. It seems worth noting that this is exactly what libstdc++ does in order to need to support only one version of GCC.


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  https://reviews.llvm.org/D68364/new/

https://reviews.llvm.org/D68364