Function overloading not work with namespaces (structures) in c

I have in lib.h:

#pragma once
#include <stdio.h>
#include <stdlib.h>

typedef struct LIB {
  int (*a1)(int);
  int (*a2)(int);
} lib;

int a1(int a, int b);
int a2(int a, double b);

#define a0(X, Y) _Generic((X),\
    int:   _Generic((Y),\
        int: a1,\
        double: a2\
)(X, Y)

lib* getLib();

In lib.c:

#include "lib.h"

int a1(int a, int b) {

int a2(int a, double b) {

lib* getLib() {
  lib* ret = (lib*) malloc (sizeof(lib));

  ret->a1 = a1;
  ret->a2 = a2;

  return ret;

In main.c:

#include "lib.h"

int main() {
  lib* l = getLib();
  int l->v = a0(1, 2.4); // error


error: expected identifier
note: expanded from macro 'a0'

Without define, and if a change a0 to a2 it works. How can I overload function in namespace-structure? Why I have ERROR here?

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