[WiP] Remove <experimental/optional> hack from util/optional.h

[uklotzde]

Try to fix the macOS build on the build server caused by #2601.

[daschuer]

I am afraid the optional header is actually not available and this will fail anyway.

If this is not the case this docs here would be void which is very unlikely.
https://libcxx.llvm.org/docs/FeatureTestMacroTable.html
I clearly state that our conditional should work properly.

Can we have a look on the server and verify this before merge?

I think a very easy fix would be to ignore the error in the experimental case.
-Wno-invalid-constexpr

[Be-ing]

We likely need to update the build server anyways to fix https://bugs.launchpad.net/mixxx/+bug/1871238

[daschuer]

OK, so lets go step by step and hold this back until the server was updated.

[daschuer]

This should be work as intermediated fix: #2641

[uklotzde]

I don't think that we need to upgrade the server. The server runs macOS 10.13 and the corresponding XCode version should be compatible.

[uklotzde]

@daschuer Why introduce another hack instead of trying to remove an obsolete one. The macOS failure might be caused by the hack that I once introduced for Xenial.

[uklotzde]

I'm not able able to judge if the current code in util/optional.h is correct. It worked until now on all platforms, that's all what I can say about it. Therefore I propose to remove it.

[Be-ing]

According to https://en.wikipedia.org/wiki/Xcode#Xcode_11.x_(since_SwiftUI_framework) XCode 11.3 which we probably need for the crash bug requires at least macOS 10.14.4

[uklotzde]

Changing the build environment should only be the final step. Upgrading would exclude many macOS users with old hardware. But it is also unacceptable to tolerate Apple's business policy, by letting us do all the work to support their customers they decided to leave behind.

[uklotzde]

Experimental: I have downgraded XCode 11.0 to XCode 9.3 on Travis to match our build server environment.

[uklotzde]

Unfortunately doesn't seem to be available in Xcode 9.3 (and the cmake version is also too old):

src/util/optional.h:3:10: fatal error: 'optional' file not found
#include <optional>
         ^~~~~~~~~~