-Wall -Wextra -pedantic -std=c++17 -O3 -Wshadow -Wformat=2 -Wfloat-equal -Wconversion -Wlogical-op -Wshift-overflow=2 -Wduplicated-cond -Wcast-qual -Wcast-align -D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC

#include <bits/stdc++.h>
int main() {
    int a, b; std::cin >> a >> b;
    std::cout << a * b << std::endl;
}

On testcase 10000000 10000000 you will see nexr: runtime error: signed integer overflow: 10000000 * 10000000 cannot be represented in type 'int'

#include <bits/stdc++.h>
int64_t val, res(0);
int main() {
    std::cin >> val;
    for (int bit = 0; bit < 60; bit++) {
        if (val & (1 << bit)) res++;
    }
    std::cout << res << std::endl;
    return 0;
}

Testcase 1000000000000. You will see next:

runtime error: shift exponent 32 is too large for 32-bit type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior

#include <bits/stdc++.h>
int main() {
    int arr[10];
    int n; std::cin >> n;
    for (int i = 0; i < n; i++) {
        std::cin >> arr[i];
    }
}

Test: 11 0 1 2 3 4 5 6 7 8 9 0

You will see next:

WRITE of size 4 at 0x1126fd98 thread T0
    #0 0xcb1d96 in std::bas...

#define _GLIBCXX_DEBUG 1
#define _GLIBCXX_DEBUG_PEDANTIC 1
#define _FORTIFY_SOURCE 2

#include <bits/stdc++.h>

int main() {
    std::vector<int> arr(3,1);
    std::cout << arr[3];
}

You can see next:

Error: attempt to subscript container with out-of-bounds index 3, but 
container only holds 3 elements.

It is more useful and have a detailed description in comparison with Clang++17 Disgnostics

Run it

#define _GLIBCXX_DEBUG 1
#define _GLIBCXX_DEBUG_PEDANTIC 1
#define _FORTIFY_SOURCE 2

#include <bits/stdc++.h>

int main() {
    std::set<int> set{1,2,3,4,5};
    auto it = set.begin();
    set.erase(set.begin());
    std::cout << *it << std::endl;
}

We try to save needed iterator, erase it and forgot about it. Then, when we will output a value of iterator, will see next:

Error: attempt to dereference a singular iterator.

Run it

#define _GLIBCXX_DEBUG 1
#define _GLIBCXX_DEBUG_PEDANTIC 1
#define _FORTIFY_SOURCE 2

#include <bits/stdc++.h>

#define all(x) (x).begin(),(x).end()

using vi = std::vector<int>;

int main() {
    vi left = {1,2,3};
    vi right = {3,1,2};
    vi res;
    std::merge(all(left), all(right), std::back_inserter(res));
}

In this example we use standard algo for merging two sorted arrays, but we forgot to sort one of them. No problem, G++ will print next error:

Error: elements in iterator range [__first2, __last2) are not sorted.

Run it

#define _GLIBCXX_DEBUG 1
#define _GLIBCXX_DEBUG_PEDANTIC 1
#define _FORTIFY_SOURCE 2

#include <bits/stdc++.h>

#define all(x) (x).begin(),(x).end()

using vi = std::vector<int>;

int main() {
    vi arr{1,2,3,4};
    auto ptr = arr.begin();
    arr.push_back(5);
    std::cout << *ptr << std::endl;
}

Ok, we forgot, that in resizing of vector if capacity is smaller then needed size, all items will be copied in new allocated memory, old memory wil be deleted. And when we will try to access a deleted memory, will see next:

Error: attempt to dereference a singular iterator.

Run it

#include <iostream>
#include <cassert> // для assert
int main() {
    int a, b; std::cin >> a >> b;
    // calculate a difference
    int c = a - b;
    // check, that difference is calculated right: c = a - b ==> c + b == a
    assert(c + b == a);
    std::cout << c << std::endl;

#include <iostream>
#include <cassert> // для assert
int f(int n) {
// check that argument is always positive integer or zero
    assert(n >= 0);
    if (n == 0) return 1;
    return n * f(n - 1);
}
int main() {
    int n; std::cin >> n; std::cout << f(n);