VS2013下LNK2019错误复现求助：测试代码存类似问题

I'm currently stuck on an LNK2019 unresolved external symbol error, so I threw together a minimal test project outside my main codebase to try and replicate the issue. The error pops up in a different spot than my original problem, but it feels like the same root cause. I'm only using Visual Studio 2013 right now—haven't messed with other IDEs yet.

Here's the test code I put together:

main.cpp #

#include "functions.hpp"
int main(int argc, char *argv[]){
    core::function1();
    return 0;
}

functions.hpp #

#ifndef _functions_hpp
#define _functions_hpp
#include "core.hpp"
namespace core{
    void function1();
}
#endif

core.hpp #

#ifndef _core_hpp
#define _core_hpp
// Note: The full content of this header was not provided
#endif

内容的提问来源于stack exchange，提问作者Ystalio Di Staniovich