What makes including inline functions in headers impossible ?

main.c

#include <stdlib.h>
#include <stdio.h>

#include "test.h"

int main(int argc, char *argv[]) {
    printf("inl_fun:%d\n", inl_fun(4));
    printf("ninl_fun:%d\n", ninl_fun(4));

    return 0;
}

test.c

#include <stdlib.h>
#include <stdio.h>

#include "test.h"

int ninl_fun(int arg) {
    return -arg;
}

test.h

#ifndef _TEST_H
#define _TEST_H

#include <assert.h>

static inline int inl_fun(int arg) {
    assert(arg>=0);

    return -arg;
}

int ninl_fun(int arg);

#endif

inline => static inline
perfect!
ty WM