Clang is reporting

Apple clang version 11.0.0 (clang-1100.0.33.17)
Target: x86_64-apple-darwin18.7.0
Thread model: posix
InstalledDir: /Applications/Xcode_11.3.1.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

which is a bit strange (xcode 11.3.1 but clang v11.0.0). This is sufficient to compile the avx512f loops.

XCode 10 is from June 2018, XCode 11 from June 2019, and Xcode 11 requires macOS 10.14.4 or later. So there's people that will still have XCode 10, and with pre-2012 hardware one cannot upgrade. So this isn't ideal - is it possible to work around this compiler issue instead?

We should definitely update our wheel build repos to the latest XCode/Clang though, if not already done.

We may want to update to macos-10.15 soon too.

MacOS 10.15 is a trainwreck of a release, so many devs want to stick to 10.14 -

The PR worked around the compiler issue with an #ifdef, so the code should run on both versions. The question here is what version should CI be running. Maybe the answer should be "oldest and newest", much like we have a gcc4.8 run for linux.

Oldest and newest would be ideal, not sure about slowing down CI too much though. The average macOS dev is pretty good about upgrading, so if we have to choose I think keeping oldest would be more useful - newest we're more likely to catch ourselves.

Or: oldest here, newest in wheel builds.

This is working "as advertised": the 64-bit OpenBLAS run is using xcode 10

LGTM, merged. Thanks @mattip