dynamicReplacement.swift

// a.swift
public dynamic func f() {
    print("original")
}

// b.swift
import a

@_dynamicReplacement(for: f())
public func f_b() {
    print("replacement in b")
}

// c.swift
import Foundation
import a

f()

let file = URL(fileURLWithPath: "/path/to/libb.dylib")

guard let handle = dlopen(file.path, RTLD_NOW) else {
    fatalError("dlopen failed")
}

f()

/*
$ swiftc -emit-module -module-name a a.swift
$ swiftc -emit-library -module-name a a.swift
$ swiftc -emit-library -module-name b -I. liba.dylib b.swift
$ swiftc -emit-executable -module-name c -I . liba.dylib c.swift
$ ./c
*/