clang, c++17, std::optional, and libc++.dylib

[Ryan Schmidt]

On Jul 5, 2020, at 00:33, Ken Cunningham wrote:

> It is quite simple (I think) to inline a function implementing bad_optional_access into the <optional> header. That could be used for 10.7 to 10.12, and then all those systems could comfortably implement c++17, and no problem. Otherwise we have to install a newer libc++.dylib on older systems, which is a simple but big problem, as it were.
> 
> Here’s the <optional> patch. I only speak basic c++, so improve this if you can.
> 
> So I am proposing to use this patch, and disable Apple’s availability test in <__config> for bad_optional_access.

I can't evaluate the patch, but I just wonder if we have any idea why Apple/LLVM didn't do it that way in the first place. Why did they put it into a new libc++ and cripple the ability to use bad_optional_access on < 10.13? (I wouldn't be surprised if the answer is that they have absolutely no interest in older systems, though that would make me sad.)