I noticed a number of errors similar to this when trying to build with Xcode 11 beta 4:
It seems that the issue comes from the Bullet external, but we can fix it here.
The issue is with clang 8+ on any platform, not specifically Xcode. We could try to make the test detect clang 8+ specifically, but instead I think it's better to just port over the fix from bullet itself:
Just that fix is fine, when we upgrade bullet as a whole it'll remain fixed.