In some cases, zld is not able to autolink libswiftCompatibility50.a and libswiftCompatibilityDynamicReplacements.a. I'm using Xcode 11.3.1.
Undefined symbols for architecture x86_64:
"__swift_FORCE_LOAD_$_swiftCompatibility50", referenced from:
__swift_FORCE_LOAD_$_swiftCompatibility50_$_foo in foo.a(foo.o)
(maybe you meant: __swift_FORCE_LOAD_$_swiftCompatibility50_$_foo)
"__swift_FORCE_LOAD_$_swiftCompatibilityDynamicReplacements", referenced from:
__swift_FORCE_LOAD_$_swiftCompatibilityDynamicReplacements_$_foo in foo.a(foo.o)
(maybe you meant: __swift_FORCE_LOAD_$_swiftCompatibilityDynamicReplacements_$_foo)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
To reproduce this problem, clone this repo and run ./build.sh. You can compare with default ld by commenting out the line of -fuse-ld.
To work around the issue, we need to append -lswiftCompatibility50 -lswiftCompatibilityDynamicReplacements.