To fix we need to add qualifier owner at return type.

    FILE* owner open_file(const char* p) {
        FILE* owner f = …
        return f;
    }

https://thradams.com/cake/playground.html?code=I2luY2x1ZGUgP...

Ownership Feature Strategy (Inspired by stdbool.h)

If the compiler supports ownership checks and qualifiers such as _Owner, _View, _Obj_view, etc., it must define __STDC_OWNERSHIP__.

However, even if the compiler implements ownership, it is not active by default. The objective is to have a smooth transition allowing some files without checks. For instance, a thirty part code inside your project.

For instance, when compiling this file, even if the compiler supports ownership we don't have errors or warnings because the checks are not enabled by default.

    #include <stdlib.h>

    int main() {
      void \* p = malloc(1);
    }

    #include <ownership.h>
    #include <stdlib.h>

    int main() {
      void \* p = malloc(1); //error: missing owner qualifier
    }