Function inside a function (in C) breaks parser.
A lot of red underlines with "unexpected token" message appears.
Fortunately, parser recovers after "}" that closes outer function.

Code example is:

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

#define assert_true(condition) _assert_true(condition, #condition)

typedef struct {
    int size;
    int capacity;
    void** data;
} array;

void* array_get(array *a, int index);
void array_qsort(array *a, int (*comparator)(const void *element1, const void *element2));

int main(int argc, char** argv) {

    array a;
    
   int string_comparator (const void *element1, const void *element2) {
       const char *str1 = *((char**)element1);
       const char *str2 = *((char**)element2);
       int res = strcmp(str1, str2);
       return res;
   }
   array_qsort(&a, string_comparator);
   
    assert_true(strcmp(array_get(&a, 0), "1") == 0);
    assert_true(strcmp(array_get(&a, 1), "2") == 0);
    assert_true(strcmp(array_get(&a, 2), "3") == 0);
    assert_true(strcmp(array_get(&a, 3), "4") == 0);
    
    return (EXIT_SUCCESS);
}