I don't use the macOS toolchain, but several others who do use it have filed issues that they see assertion failures in the OSS 6.0.3 or earlier toolchains for macOS, ie not just the snapshots.
When I tried reproducing with 6.0.3 or other final releases on linux, I cannot, because assertions are disabled for such tagged releases on linux, but the same assertions trigger on the linux development snapshots. This leads me to believe they're right about the OSS 6.0.3 toolchain for macOS and earlier releases having assertions enabled, even though I've never used those macOS toolchains. 