I understand that but how can just changing language version change WA to AC? Try to submit my code with C++17 (GNU)

No, it does make perfect sense. I actually wrote a brute force solution and noticed a pattern that (i % a) % b == (i % b) % a only when i lies in the range [ n*LCM , n*LCM + max(a,b) ) where n is any integer and I am counting such blocks in my answer.

I submitted a solution 78241725 that failed on the 48405th query of test case 2. Instead of precalculating all numbers from 0 to a*b, and then using the periodic property of that, I used this idea.

1. (i%a)%b will be equal to (i%b)%a for all i from l to b (l included, b excluded)

2. For every b numbers after i (i included), where i%lcm==0, the value of (i%a)%b will be equal to (i%b)%a.

I computed this count, taking care of left and right limits, but funnily enough my testcase 2 failed at 48405th query, seems some corner case is missing.Any help will be appreciated. PS. I managed to solve the way you did later after the contest, and it worked.

#include <bits/stdc++.h>

// definitions
#define F first
#define S second
#define all(x) x.begin(), x.end()
#define clr(x) memset(x, 0, sizeof(x))
#define rep(i, l, r) for (long long i = (l); i < (long long)(r); i++)
#define repb(i, r, l) for (long long i = (r); i > (long long)(l); i--)
#define pb push_back
#define mod1 998244353
#define mod 1000000007
#define MAX_CHAR 256

using namespace std;
typedef vector<long long> vl;
typedef vector<char> vc;
typedef vector<string> vs;
typedef vector< pair<long long, long long> > vp;
typedef vector<tuple<long long , long long , long long> > vtup;
typedef deque <long long> dq;
typedef long double ld;
typedef long long ll;
typedef unsigned long long ull;


// standard functions
ll gcd(ll x, ll y) {
    if (x < y) return gcd(y, x);
    if (y == 0) return x;
    return gcd(y, x % y);
} 
 


ll modexp(ll x, ll ex, ll modp) {
    ll ans = 1ll;
    while (ex > 0) {
        if (ex & 1ll) ans = (ans * x)%modp;
        ex >>= 1ll;
        x = (x * x)%modp;
    }
    return ans;
} 
 
ll nCrModp(ll n, ll r, ll p) 
{ 
    ll C[r+1]; 
    memset(C, 0, sizeof(C)); 
  
    C[0] = 1; 
    for (ll i = 1; i <= n; i++) 
    { 
        for (ll j = min(i, r); j > 0; j--) 
            C[j] = (C[j] + C[j-1])%p; 
    } 
    return C[r]; 
}
 
bool isPrime(ll n) 
{ 
    if (n <= 1)  return false; 
    if (n <= 3)  return true;  
    if (n%2 == 0 || n%3 == 0) return false; 
  
    for (ll i=5; i*i<=n; i=i+6) 
        if (n%i == 0 || n%(i+2) == 0) 
           return false; 
  
    return true; 
} 
 
vl countFreq(ll arr[], ll n) 
{ 
    unordered_map<ll, ll> mp; 
    vector <ll> v;
    
    for (ll i = 0; i < n; i++) 
        mp[arr[i]]++; 
   
    for (auto x : mp) 
        v.pb(x.second);
    
    return v;
}

ll binarySearch(ll arr[], ll l, ll r, ll x) 
{ 
    while (l <= r) 
    { 
        ll m = l + (r - l) / 2; 
        if (arr[m] == x) 
            return m; 
        if (arr[m] < x) 
            l = m + 1; 
        else
            r = m - 1; 
    }
    return -1; 
}

const ll MAXSIEVE=(1e3);
ll spf[MAXSIEVE];
void sieve() 
{ 
    spf[1] = 1; 
    for (ll i=2; i<MAXSIEVE; i++) 
        spf[i] = i; 

    for (ll i=4; i<MAXSIEVE; i+=2) 
        spf[i] = 2; 
  
    for (ll i=3; i*i<MAXSIEVE; i++) 
    { 
        if (spf[i] == i) 
        { 
            for (ll j=i*i; j<MAXSIEVE; j+=i) 
                if (spf[j]==j) 
                    spf[j] = i; 
        } 
    } 
}

ll countDivisibles(ll A, ll B, ll M) 
{ 
    // Add 1 explicitly as A is divisible by M 
    if (A % M == 0) 
        return (B / M) - (A / M) + 1; 
  
    // A is not divisible by M 
    return (B / M) - (A / M); 
}

// code beigns in main
int main()
{
    
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    ll t;
    cin >> t;
    
    while(t--)
    {
        ll a,b;
        cin >> a >>b;
        
        if(a>b)
            swap(a,b);
        ll lcm=(a*b)/gcd(a,b);
        ll q;
        cin >> q;
        
        rep(i,0,q)
        {
            ll l,r;
            cin >> l >> r;
            
            ll tot=r-l+1;
            ll count=countDivisibles(l, r, lcm);
            
            ll ini=0;
            rep(j,l,r+1)
            {
                if(j<b)
                    ini++;
                else
                    break;
            }
            ll last=r+1;
            repb(j,r,l-1)
                if(j%lcm==0)
                {
                    last=j;
                    break;
                }
            
            ll oth=max(b*(count-1),0ll);
            oth+=min(b,r-last+1);
            tot=tot-(oth+ini);
            
            cout << tot << " ";
        }
        cout << endl;
    }
}