Language: C++

#pragma GCC optimize("O3")
#pragma GCC optimize ("unroll-loops")
#pragma GCC target ("avx")
#include <bits/stdc++.h>
constexpr int INF = 2147483647;
constexpr long long int INF_LL = 9223372036854775807;
constexpr int MOD = 1000000007;
constexpr double PI = 3.14159265358979323846;
using namespace std;
typedef long long int ll;
typedef unsigned long long int ull;

//modint
template<typename T> T modinv(T a, T m) {
	T b = m, u = 1, v = 0;
	while (b) {
		T t = a / b;
		a -= t * b; swap(a, b);
		u -= t * v; swap(u, v);
	}
	u %= m;
	if (u < 0) u += m;
	return u;
}
template<typename T> T modpow(T x, int k) {
	if (k == 0)return 1;
	if (k % 2 == 0) return modpow(x * x, k / 2);
	else return x * modpow(x, k - 1);
}

template<int mod> class modint {
public:
	ll val;
	modint(ll a) {
		val = a % mod;
	}
	modint operator+(const modint& r) {
		return modint((this->val + r.val) % mod);
	}
	modint operator-(const modint& r) {
		return modint((this->val - r.val) % mod + ((this->val - r.val) % mod < 0) * mod);
	}
	modint operator*(const modint& r) {
		return modint((((this->val % mod) * (r.val % mod))) % mod);
	}
	modint operator/(const modint& r) {
		return modint(this->val * modinv<ll>(r.val, mod) % mod);
	}
	modint& operator+=(const modint& r) {
		*this = *this + r;
		return *this;
	}
	modint& operator-=(const modint& r) {
		*this = *this - r;
		return *this;
	}
	modint& operator*=(const modint& r) {
		*this = *this * r;
		return *this;
	}
	modint& operator/=(const modint& r) {
		*this = *this / r;
		return *this;
	}

};
modint<MOD> ans = 0;


int main() {
	string L;
	cin >> L;
	
	modint<MOD> mul = 1;
	for (int i = 0; i < L.size(); i++){
		if (i == 0 && L[i] == '1')mul *= 2;
		else if (L[i] == '1') {
			ans += mul * modpow(3, L.size() - i - 1);
			mul *= 2;
		}
		if (i == L.size() - 1)ans += mul;
	}
	for (int i = 1; i < L.size(); i++) {
		ans += 2 * modpow(modint<MOD>(3), i - 1);
	}
	cout << ans.val + 1 << endl;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:84:26: error: no match for 'operator*' (operand types are 'int' and 'modint<1000000007>')
   84 |                 ans += 2 * modpow(modint<MOD>(3), i - 1);
      |                        ~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                        |         |
      |                        int       modint<1000000007>
In file included from /usr/include/c++/14/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:127,
                 from a.cc:4:
/usr/include/c++/14/complex:400:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator*(const complex<_Tp>&, const complex<_Tp>&)'
  400 |     operator*(const complex<_Tp>& __x, const complex<_Tp>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:400:5: note:   template argument deduction/substitution failed:
a.cc:84:56: note:   mismatched types 'const std::complex<_Tp>' and 'int'
   84 |                 ans += 2 * modpow(modint<MOD>(3), i - 1);
      |                                                        ^
/usr/include/c++/14/complex:409:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator*(const complex<_Tp>&, const _Tp&)'
  409 |     operator*(const complex<_Tp>& __x, const _Tp& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:409:5: note:   template argument deduction/substitution failed:
a.cc:84:56: note:   mismatched types 'const std::complex<_Tp>' and 'int'
   84 |                 ans += 2 * modpow(modint<MOD>(3), i - 1);
      |                                                        ^
/usr/include/c++/14/complex:418:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator*(const _Tp&, const complex<_Tp>&)'
  418 |     operator*(const _Tp& __x, const complex<_Tp>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:418:5: note:   template argument deduction/substitution failed:
a.cc:84:56: note:   'modint<1000000007>' is not derived from 'const std::complex<_Tp>'
   84 |                 ans += 2 * modpow(modint<MOD>(3), i - 1);
      |                                                        ^
In file included from /usr/include/c++/14/valarray:605,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:166:
/usr/include/c++/14/bits/valarray_after.h:407:5: note: candidate: 'template<class _Dom1, class _Dom2> std::_Expr<std::__detail::_BinClos<std::__multiplies, std::_Expr, std::_Expr, _Dom1, _Dom2>, typename std::__fun<std::__multiplies, typename _Dom1::value_type>::result_type> std::operator*(const _Expr<_Dom1, typename _Dom1::value_type>&, const _Expr<_Dom2, typename _Dom2::value_type>&)'
  407 |     _DEFINE_EXPR_BINARY_OPERATOR(*, struct std::__multiplies)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:407:5: note:   template argument deduction/substitution failed:
a.cc:84:56: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'int'
   84 |                 ans += 2 * modpow(modint<MOD>(3), i - 1);
      |                                                        ^
/usr/include/c++/14/bits/valarray_after.h:407:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__multiplies, std::_Expr, std::_Constant, _Dom, typename _Dom::value_type>, typename std::__fun<std::__multiplies, typename _Dom1::value_type>::result_type> std::operator*(const _Expr<_Dom1, typename _Dom1::value_type>&, const typename _Dom::value_type&)'
  407 |     _DEFINE_EXPR_BINARY_OPERATOR(*, struct std::__multiplies)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:407:5: note:   template argument deduction/substitution failed:
a.cc:84:56: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'int'
   84 |                 ans += 2 * modpow(modint<MOD>(3), i - 1);
      |                                                        ^
/usr/include/c++/14/bits/valarray_after.h:407:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__multiplies, std::_Constant, std::_Expr, typename _Dom::value_type, _Dom>, typename std::__fun<std::__multiplies, typename _Dom1::value_type>::result_type> std::operator*(const typename _Dom::value_type&, const _Expr<_Dom1, typename _Dom1::value_type>&)'
  407 |     _DEFINE_EXPR_BINARY_OPERATOR(*, struct std::__multiplies)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:407:5: note:   template argument deduction/substitution failed:
a.cc:84:56: note:   'modint<1000000007>' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
   84 |                 ans += 2 * modpow(modint<MOD>(3), i - 1);
      |                                                        ^
/usr/include/c++/14/bits/valarray_after.h:407:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__multiplies, std::_Expr, std::_ValArray, _Dom, typename _Dom::value_type>, typename std::__fun<std::__multiplies, typename _Dom1::value_type>::result_type> std::operator*(const _Expr<_Dom1, typename _Dom1::value_type>&, const valarray<typename _Dom::value_type>&)'
  407 |     _DEFINE_EXPR_BINARY_OPERATOR(*, struct std::__multiplies)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:407:5: note:   template argument deduction/substitution failed:
a.cc:84:56: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'int'
   84 |                 ans += 2 * modpow(modint<MOD>(3), i - 1);
      |                                                        ^
/usr/include/c++/14/bits/valarray_after.h:407:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__multiplies, std::_ValArray, std::_Expr, typename _Dom::value_type, _Dom>, typename std::__fun<std::__multiplies, typename _Dom1::value_type>::result_type> std::operator*(const valarray<typename _Dom::value_type>&, const _Expr<_Dom1, typename _Dom1::value_type>&)'
  407 |     _DEFINE_EXPR_BINARY_OPERATOR(*, struct std::__multiplies)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:407:5: note:   template argument deduction/substitution failed:
a.cc:84:56: note:   'modint<1000000007>' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
   84 |                 ans += 2 * modpow(modint<MOD>(3), i - 1);
      |                                                        ^
/usr/include/c++/14/valarray:1198:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__multiplies, std::_ValArray, std::_ValArray, _Tp, _Tp>, typename std::__fun<std::__multiplies, _Tp>::result_type> std::operator*(const valarray<_Tp>&, const valarray<_Tp>&)'
 1198 | _DEFINE_BINARY_OPERATOR(*, __multiplies)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1198:1: note:   template argument deduction/substitution failed:
a.cc:84:56: note:   mismatched types 'const std::valarray<_Tp>' and 'int'
   84 |                 ans += 2 * modpow(modint<MOD>(3), i - 1);
      |                                                        ^
/usr/include/c++/14/valarray:1198:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__multiplies, std::_ValArray, std::_Constant, _Tp, _Tp>, typename std::__fun<std::__multiplies, _Tp>::result_type> std::operator*(const valarray<_Tp>&, const typename valarray<_Tp>::value_type&)'
 1198 | _DEFINE_BINARY_OPERATOR(*, __multiplies)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1198:1: note:   template argument deduction/substitution failed:
a.cc:84:56: note:   mismatched types 'const std::valarray<_Tp>' and 'int'
   84 |                 ans += 2 * modpow(modint<MOD>(3), i - 1);
      |                                                        ^
/usr/include/c++/14/valarray:1198:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__multiplies, std::_Constant, std::_ValArray, _Tp, _Tp>, typename std::__fun<std::__multiplies, _Tp>::result_type> std::operator*(const typename valarray<_Tp>::value_type&, const valarray<_Tp>&)'
 1198 | _DEFINE_BINARY_OPERATOR(*, __multiplies)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1198:1: note:   template argument deduction/substitution failed:
a.cc:84:56: note:   'modint<1000000007>' is not derived from 'const std::valarray<_Tp>'
   84 |                 ans += 2 * modpow(modint<MOD>(3), i - 1);
      |                                                        ^