C11 _Generic basic test

generic.c

#include <stdio.h>

typedef struct {
    double x;
    double y;
} vec2d;

#define sum(X, Y) _Generic((X),                      \
                           int: sum2l((X), (Y)),     \
                           long: sum2l((X), (Y)),    \
                           double: sum2d((X), (Y)),  \
                           float: sum2d((X), (Y)))

// this won't work - because it's wrong
#define sumxWrong(X, Y) _Generic((X),                     \
                            int: sum2l((X), (Y)),         \
                            long: sum2l((X), (Y)),        \
                            double: sum2d((X), (Y)),      \
                            float: sum2d((X), (Y)),       \
                            vec2d: sum2v((X), (Y)))

// this works (thanks to Michael Stoffregen who suggested the right way)
#define sumx(X, Y) _Generic((X),                    \
                            int: sum2l,             \
                            long: sum2l,            \
                            double: sum2d,          \
                            float: sum2d,           \
                            vec2d: sum2v)((X), (Y))



long sum2l(long x, long y)
{
    return x + y/2;
}

double sum2d(double x, double y)
{
    return x + y/2.0;
}

vec2d sum2v(vec2d a, vec2d b)
{
    vec2d r = {a.x + b.x, (a.y + b.y)/2.0};
    return r;
}



int main(void)
{
    printf("%ld %lf\n", sum(5, 3), sum(5.0, 3));

    vec2d a = {1.0, 2.5};
    vec2d b = {3.0, 3.5};
    // calling explicitly the right function: no overloading
    vec2d r = sum2v(a, b);  
    printf("%lf %lf\n", r.x, r.y);

    // let's try with "genericity"
    r = sumx(a, b);
    printf("%lf %lf\n", r.x, r.y);

    /* the following does not work in both compiler: it seems both would choose sum2l, instead of sum2v.

       part of clang error output:

       generic.c:57:9: error: passing 'vec2d' to parameter of incompatible type 'long';
           r = sumx(a, b);
               ^~~~~~~~~~

       part of gcc error output:

       generic.c:57:9: note: in expansion of macro ‘sumx’
            r = sumx(a, b);
                ^
       generic.c:24:6: note: expected ‘long int’ but argument is of type ‘vec2d’       
    */
    //r = sumxWrong(a, b);  // it was sumx, which now works fine ^_^

    /*
        the following does not work neither.

        In both cases I would expect it would choose sum2l; but...

        gcc:

        generic.c:80:15: note: in expansion of macro ‘sumx’
             int res = sumx(1, 2);
                       ^
        generic.c:34:7: note: expected ‘vec2d’ but argument is of type ‘int’
         vec2d sum2v(vec2d a, vec2d b)
               ^


        clang:

        generic.c:89:15: error: passing 'int' to parameter of incompatible type 'vec2d';
            int res = sumx(1, 2);
                      ^~~~~~~~~~

     */
    //int res = sumxWrong(1, 2);
    
    return 0;
}