However, if you recast to volatile, the compiler will keep it:

    #include <stdlib.h>
    #include <string.h>

    void free(void* ptr);
    void not_free(void* ptr);


    void test_with_free(char* ptr) {
        ptr[5] = 6;
        void *(* volatile memset_v)(void *s, int c, size_t n) = memset;
        memset_v(ptr + 2, 3, 4);
        free(ptr);
    }

    void test_with_other_func(char* ptr) {
        ptr[5] = 6;
        void *(* volatile memset_v)(void *s, int c, size_t n) = memset;
        memset_v(ptr + 2, 3, 4);
        not_free(ptr);
    }