A tricky matter in Geany

I typed these code in Geany:

void copy_fct()
{
    int v1[10] = {0,1,2,3,4,5,6,7,8,9};
    int v2[10] = {0,0,0,0,0,0,0,0,0,0};
    // to become a copy of v1
    
    for(auto i=0; i!=10; ++i) // copy elements
        v2[i]=v1[i];
    // ...
}

while, when I compiled it, there was always a red wave line under "int v2[10]", I tried "int v2[] = {0,0,0,0,0,0,0,0,0,0};" "int v2[];" and "int v2[10] = {};", all the same, I tried declared v2 outside "copy_fct()", it was OK, but, if I do want to declare it inside "copy_fct()", is there a declaration here without descend the warning level (now is default "Wall")?

The whole code was:

#include <iostream>
using namespace std;

int main()
{
    std::cout<<"Hello,World!\n";
    }
    
bool accept3()
{
    int tries = 1;
    while(tries<4){
        cout << "Do you want to proceed (y or n)?\n"; // write question
        char answer = 0;
        cin >> answer; // read answer
        
        switch(answer){
            case 'y':
                return true;
            case 'n':
                return false;
            default:
                cout << "Sorry, I don't understand that.\n";
                ++tries; // increment
            }
        }
        cout << "I'll take taht for a no.\n";
        return false;
    }

void copy_fct()
{
    int v1[10] = {0,1,2,3,4,5,6,7,8,9};
    int v2[10] = {0,0,0,0,0,0,0,0,0,0};
    // to become a copy of v1
    
    for(auto i=0; i!=10; ++i) // copy elements
        v2[i]=v1[i];
    // ...
}