// The unit type in ML or Python, (), is a tuple of zero items.  It
// has size zero and one possible value.  GCC supports this as a GNU
// extension to standard C:
// <https://gcc.gnu.org/onlinedocs/gcc-15.2.0/gcc/Empty-Structures.html>
#include <stdio.h>

typedef struct {} unit;

static unit f(unit *p)
{
    return (unit){};
}

static void g(unit u)
{
}

int main()
{
    unit a = {}, b = {};
    a = b;
    f(&a);
    g(b);
    // In GCC Debian 12.2.0-14 these two variables get the same
    // address; in Debian clang version 14.0.6 they get different
    // addresses unless you turn on optimization.
    printf("a is at %lx, b is at %lx, "
            "sizeof unit is %d\n",
            (unsigned long)&a, (unsigned long)&b,
            (int)sizeof(unit));
    return 0;
}