Questions & Answers

If you change the union so that the field names are identical to the enum constants:

typedef union person_param_val
{
    char* NAME;
    unsigned int AGE;
} person_param_val_t;

Then you can create a macro which will pass a properly initialized compound literal:

#define set_person_param_ext(k,v) \
        set_person_param(k, (person_param_val_t){.k=v})

So then this:

set_person_param_ext(NAME, "Alex");
set_person_param_ext(AGE, 5);

Will expand to this:

set_person_param(NAME, (person_param_val_t){.NAME="Alex"});
set_person_param(AGE, (person_param_val_t){.AGE=5});

You do not need any magic. This macro is enough

#define set_person_param(param, val)  g_person_##param.param = (val)

And this sample function:

int foo(void)
{
    person_param_val_t g_person_name, g_person_age;

    set_person_param(name, "Alex");
    set_person_param(age, 5);
}

will be preprocessed to:

int foo(void)
{
    person_param_val_t g_person_name, g_person_age;

    g_person_name.name = "Alex";
    g_person_age.age = 5;
}

As I understand it was something you wanted to archive. Your enum type is not needed.

If you want val to be the same union type then:

#define set_person_param(param, val)  g_person_##param = (val)

example:

    set_person_param(name, (person_param_val_t){.name="Alex"});

I could see:

typedef enum person_param_name {
    NAME,
    AGE,
} person_param_name_t;

typedef union person_param_val
{
    char* name;
    unsigned int age;
} person_param_val_t;

person_param_val_t person_param_val_init_charp(char *name) {
    return (person_param_val_t){.name=name};
}
person_param_val_t person_param_val_init_u(unsigned age) {
    return (person_param_val_t){.age=age};
}
#define MAKE_PERSON_PARAM_VAL(x)  _Generic((x) \
    , unsigned: person_param_val_init_u \
    , char *:person_param_val_init_charp \
    )(x)

int set_person_param(person_param_name_t param_name, person_param_val_t param_val);

#define set_person_param(a, b) \
    set_person_param(a, MAKE_PERSON_PARAM_VAL(b))
    
int main() {
    set_person_param(NAME, "Alex");
    set_person_param(AGE, 5u);
}

With GCC with extension, you will get away with just:

#define set_person_param(a, b) \
     set_person_param(a, (person_param_val_t)(b))

But I would not write such code. This is C. In C, you would write it all explicitly. I do not see a value in person_param_name. You still have to enumerate all types explicitly inside set_person_param. I would just write set_person_param_age(unsigned age) and set_person_param_name(char *name) explicitly. If not, I would consider rethinking the whole approach, as most probably you want to implement virtual function. I would advise, strongly consider not writing an interface with endless number of cases in enums, because you might end up with this. Instead, create objects with a pointer to the interface stored with a vtable.