fork(1) download

copy

#include <iostream>
using namespace std;
 
template<size_t N>
constexpr size_t sum(const char (&ch)[N]) {
	size_t result = 0;
	for(size_t i = 0; i < N; ++i) result += ch[i];
	return result;
}
 
template<size_t N>
size_t calc(size_t add) { return N + add; }
 
template<size_t N>
size_t test(const char (&ch)[N], int add) { return calc<sum<N>(ch)>(add); }
 
int main() {
	cout << calc<sum("00")>(4); // compiles
	cout << test("00", 4);      // fails
	return 0;
}

prog.cpp: In instantiation of 'size_t test(const char (&)[N], int) [with unsigned int N = 3u; size_t = unsigned int]':
prog.cpp:19:22:   required from here
prog.cpp:15:68: error: no matching function for call to 'calc(int&)'
 size_t test(const char (&ch)[N], int add) { return calc<sum<N>(ch)>(add); }
                                                                    ^
prog.cpp:12:8: note: candidate: template<unsigned int N> size_t calc(size_t)
 size_t calc(size_t add) { return N + add; }
        ^
prog.cpp:12:8: note:   template argument deduction/substitution failed:
prog.cpp:15:63:   in constexpr expansion of 'sum<3u>((* & ch))'
prog.cpp:15:68: error: 'ch' is not a constant expression
 size_t test(const char (&ch)[N], int add) { return calc<sum<N>(ch)>(add); }
                                                                    ^
prog.cpp:15:68: note: in template argument for type 'unsigned int' 

Standard output is empty