Link error triggered by `dub test` but not by `dub build --unittest`

[Per Nordlöw]

On Tuesday, 16 June 2020 at 12:21:26 UTC, Per Nordlöw wrote:
> All the linker errors originate from zio.d but zio.d has its 
> unittests disabled so how come this fails to link?


zio.d is not the source of the problem. Something else is.

The common denominator seems to be that the builtins xopEqual and 
xopCmp aren't defined upon linking. These seem to be generated by 
dmd. Can anybody explain what's going on here?