[PATCH] D146187: [docs] Update the status for coroutines

Mon Mar 20 05:04:21 PDT 2023

aaron.ballman added a comment.

In D146187#4205271 <https://reviews.llvm.org/D146187#4205271>, @ChuanqiXu wrote:

>> Ugh, that does sort of change the calculus for whether we do or don't claim support on Windows. If removing the definition of that macro on Windows causes significant code breakage, that would be a reason we should leave it defined. But do we have evidence of that?
>
> Ah, yes. This is what I want to say. Simply, if we remove the definition for `__cpp_impl_coroutines`, the coroutine header in MS/STL and libstdc++ can't work anymore. For libstdc++, all the coroutine things are only defined if `__cpp_impl_coroutines ` is defined (https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/std/coroutine#L58-L69). And this is the same for MS/STL: https://github.com/microsoft/STL/blob/5116678ea11245b232a6e43824aad25fb60b9c6e/stl/inc/yvals_core.h#L1587-L1589 and https://github.com/microsoft/STL/blob/be29af22c049774ae93f2c33e4eb5d801e12216c/stl/inc/coroutine#L17-L39. So I can image that many uses of coroutines in Windows may get useless after we undefined the `__cpp_impl_coroutines` macros.

Okay, that pretty much paints us into a corner, I guess. If we don't define it any longer, we break existing (working) uses of the feature on Windows, but we defined it prematurely. In this case, let's leave the macro defined so we don't break existing uses -- in the future, I think we should be more conservative with defining feature test macros.

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