The following code is incorrectly accepted by GCC:

int main(){
    void*p=int(0);
}

This code is invalid because int(0) is not a null pointer constant. Clang and MSVC (with /permissive-) reject this program for this reason. Also, GCC rejects this as a null pointer constant in some other contexts, for example:

#include<iostream>
void foo(void*){
    std::cout<<"1\n";
}
void foo(...){
    std::cout<<"2\n";
}
int main(){
    foo(0);
    foo(int(0));
}

GCC prints "1 2" here (correctly) since int(0) isn't treated as a null pointer constant. Though not all other contexts handle this correctly, for example static_cast<void*>(int(0)) is also incorrect accepted.