Problem p00109 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef long double ld;
typedef tuple<int, int> duo;
typedef string::const_iterator state;

const int dx[] = {0, 0, 1, -1, 1, 1, -1, -1};
const int dy[] = {1, -1, 0, 0, 1, -1, 1, -1};
const int Mod = 1000000000 + 0;
//{{{ templates
#define TT_ template <typename T> inline
#define TTF_ template <typename T, typename F> inline
TT_ T sq(T x) { return x * x; }
TT_ T In() {
  T x;
  cin >> x;
  return x;
}
TT_ void Out(T &x) { cout << x; }
TT_ void sort(T &v) { sort(begin(v), end(v)); }
TT_ void revs(T &v) { reverse(begin(v), end(v)); }
TT_ void uniq(T &v) {
  sort(v);
  v.erase(unique(begin(v), end(v)), end(v));
}
TT_ int ubnd(T &v, typename T::value_type &x) {
  return upper_bound(begin(v), end(v), x) - begin(v);
}
TT_ int lbnd(T &v, typename T::value_type &x) {
  return lower_bound(begin(v), end(v), x) - begin(v);
}
TTF_ void inpt(T &v, int n, F f) {
  for (v.reserve(n); n--; v.emplace_back(f()))
    ;
}
TTF_ void show(T &v, F f, string d = " ", string e = "\\n") {
  int i = 0;
  for (auto &x : v)
    i++ && (cout << d), f(x);
  cout << e;
}
TT_ typename T::iterator minel(T &v) { return min_element(begin(v), end(v)); }
TT_ typename T::iterator maxel(T &v) { return max_element(begin(v), end(v)); }

inline void fast_io() {
  ios::sync_with_stdio(0);
  cin.tie(0);
}
inline int in() {
  int x;
  scanf("%d", &x);
  return x;
}
inline ll pow_mod(ll a, ll k, ll m) {
  ll r = 1;
  for (; k > 0; a = a * a % m, k >>= 1)
    if (k & 1)
      r = r * a % m;
  return r;
}
inline ll mod_inv(ll a, ll p) { return pow_mod(a, p - 2, p); }
//}}} priority_queue queue deque front stringstream max_element min_element
//insert count make_tuple

template <typename T, bool Check = false> struct LexParser {
  typedef T expr_t;
  typedef string::const_iterator state_t;
  LexParser() {}
  expr_t eval(const string &expr) {
    auto s = begin(expr);
    auto r = expression(s);
    consume(s, '=');
    return r;
  }
  void consume(state_t &s, char expected) {
    if (!Check || *s == expected) {
      s++;
      return;
    }
    cerr << "Expected '" << expected << "' but got '" << *s << "'" << endl;
    cerr << "Rest string is '";
    while (*s)
      cerr << *s;
    cerr << "'" << endl;
  }
  expr_t number(state_t &s) {
    expr_t r = 0;
    while (isdigit(*s)) {
      r = r * 10 + (*s - '0');
      s++;
    }
    return r;
  }
  expr_t term(state_t &s) {
    expr_t r = factor(s);
    while (1) {
      if (*s == '*') {
        consume(s, '*');
        r *= factor(s);
      } else if (*s == '/') {
        consume(s, '/');
        r /= factor(s);
      } else
        break;
    }
    return r;
  }
  expr_t factor(state_t &s) {
    expr_t r;
    if (*s == '(') {
      consume(s, '(');
      r = expression(s);
      consume(s, ')');
    } else
      r = number(s);
    return r;
  }
  expr_t expression(state_t &s) {
    expr_t r = term(s);
    while (1) {
      if (*s == '+') {
        consume(s, '+');
        r += term(s);
      } else if (*s == '-') {
        consume(s, '-');
        r -= term(s);
      } else
        break;
    }
    return r;
  }
};

int main() {
  LexParser<int> parser;
  for (int n = in(); n--;) {
    string expr;
    cin >> expr;
    cout << parser.eval(expr) << endl;
  }
  return 0;
}

Error: WA

Fixed submission:

#include <iostream>
#include <string>

using namespace std;

int main() {
  int n;
  string expr;

  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> expr;
    cerr << expr << endl;
  }
  return (0);
}