The COMDAT thing seems to be a bug in the compiler’s handling of a particular module—and quite possibly a clue to the SegFault errors that I kept hitting at run time with native builds of the same module. I have now successfully built half a dozen other modules.
The only thing that still isn’t working is the test executable:
/usr/bin/lld-link: error: entry point must be defined
clang-7: error: linker command failed with exit code 1 (use -v to see invocation)
[5/6] Linking MyPackagePackageTests.xctest
Any idea of a way to work around it?