At least since C11 "Any declaration of a library function shall have external linkage." This is important for C++20 modules. Named declarations with internal linkage can't be exported, which prevents using these declarations in the C++23 std and std.compat modules.

Ref: llvm/llvm-project#76980

Doing a quick grep it looks like we have some static inline library functions declared in

• complex.h
• ctype.h
• endian.h
• serenity.h
• arpa/inet.h
• netinet/in.h
• sys/device.h
• sys/time.h