This issue is reproducible with Xcode14, but is an error in Xcode15. The issue is that when using cmake -DCMAKE_BUILD_TYPE=Debug it will add "-g" to the options passed to swiftc and when you have more than 128 swift source files in your library the .swiftmodule file seems to be added to the linker input files. If you have less than 128 files the .swiftmodule is not add or the linker input file is not used. Xcode14 and possibly earlier only considered this a warning, but Xcode15 reports it as an error. Removing "-g" from the CMAKE_Swift_FLAGS_DEBUG will allow the library to compile, but without debug info.
make clean && make ... ninja: Entering directory `.build' [1/1] Linking Swift shared library lib/libDummyLibrary.dylib ld: warning: ignoring file swift/DummyLibrary.swiftmodule, building for macOS-arm64 but attempting to link with file built for unknown-unsupported file format ( 0xE2 0x9C 0xA8 0x0E 0x01 0x08 0x00 0x00 0x7D 0x0B 0x00 0x00 0x07 0x01 0xB2 0xC0 ) make clean && make ... error: link command failed with exit code 1 (use -v to see invocation) ld: unknown file type in 'swift/DummyLibrary.swiftmodule' clang: error: linker command failed with exit code 1 (use -v to see invocation) error: fatalError ninja: build stopped: subcommand failed.