weak import __darwin_check_fd_set_overflow breaks -no_weak_imports linker option

Number:rdar://FB7647406 Date Originated:04/01/2020
Status:Open Resolved:
Product:Xcode Product Version:Xcode 11.4 (11E146)
Classification:Incorrect/Unexpected Behavior Reproducible:yes
Please describe the issue:
The newly added weak import of `__darwin_check_fd_set_overflow` in `usr/include/sys/_types/_fd_def.h` breaks the `-no_weak_imports` linker option.

Please list the steps you took to reproduce the issue:
see https://gist.github.com/lewurm/210b320cb6037ca8ad4a370adf1bde11

Copy/paste here:

$ make set_xcode11.3
sudo xcode-select -s /Applications/Xcode113.app/Contents/Developer
$ xcodebuild -version
Xcode 11.3
Build version 11C29

$ cat c.c
#include <sys/select.h>

int foo (void) {
        fd_set s;
        FD_SET (0, &s);
        return FD_ISSET (0, &s);

$ make c.dylib
clang -c -mmacosx-version-min=10.9 -o c.o c.c
clang -dynamiclib -mmacosx-version-min=10.9 -Wl,-no_weak_imports -o c.dylib c.o
$ nm c.o
0000000000000060 t ___darwin_fd_isset
0000000000000000 T _foo
$ make clean
rm -f c.dylib c.o

$ make set_xcode11.4
sudo xcode-select -s /Applications/Xcode114.app/Contents/Developer
$ xcodebuild -version
Xcode 11.4
Build version 11E146

$ make c.dylib
clang -c -mmacosx-version-min=10.9 -o c.o c.c
clang -dynamiclib -mmacosx-version-min=10.9 -Wl,-no_weak_imports -o c.dylib c.o
ld: weak import of symbol '___darwin_check_fd_set_overflow' not supported because of option: -no_weak_imports for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [c.dylib] Error 1
$ nm c.o
                 U ___darwin_check_fd_set_overflow
0000000000000000 T _foo

What did you expect to happen?
Behave as Xcode 11.3 and successfully link.

What actually happened?
Linking fails due to weak import of `__darwin_check_fd_set_overflow`.



We have added availability annotiations to _darwincheck_fd_set_overflow to avoid the use of weak. It is now required to build with current deployment target if you want to use -Wl,-no_weak_imports, building with older deployment target fundamentally requires weak imports, it is not supported to combine that with that linker option. Please try building for macOS 11 target to verify that -Wl,-no_weak_imports works.

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