<numeric>: reduce/scan algorithms aren't enforcing their Mandates

[StephanTLavavej]

Reported on r/cpp: https://www.reddit.com/r/cpp/comments/1c0jld0/extended_use_of_stdreduce/

This is an accepts-invalid bug. None of these calls should compile, but we aren't enforcing the Standard's Mandates. For example, WG21-N4971 [reduce]/5 is:

Mandates: All of

• binary_op(init, *first),
• binary_op(*first, init),
• binary_op(init, init), and
• binary_op(*first, *first)

are convertible to T.

C:\Temp>type meow.cpp

#include <functional>
#include <iostream>
#include <iterator>
#include <numeric>
#include <string>
#include <vector>
using namespace std;

void print_strings(const vector<string>& result) {
    for (const auto& elem : result) {
        cout << "\"" << elem << "\", ";
    }
    cout << endl;
}

int main() {
    const vector<int> v{1, 2, 3, 4, 5};
    auto string_plus_int = [](const string& a, int b) { return a + to_string(b); };
    auto square          = [](int x) { return x * x; };

    { // N4971 [reduce]/5
        const auto str = reduce(v.begin(), v.end(), string{}, string_plus_int);
        cout << str << endl;
    }

    { // N4971 [transform.reduce]/3
        const auto str = transform_reduce(v.begin(), v.end(), v.begin(), string{}, string_plus_int, plus<int>{});
        cout << str << endl;
    }

    { // N4971 [transform.reduce]/7
        const auto str = transform_reduce(v.begin(), v.end(), string{}, string_plus_int, square);
        cout << str << endl;
    }

    { // N4971 [exclusive.scan]/3
        vector<string> result;
        (void) exclusive_scan(v.begin(), v.end(), back_inserter(result), string{}, string_plus_int);
        print_strings(result);
    }

    { // N4971 [inclusive.scan]/4
        vector<string> result;
        (void) inclusive_scan(v.begin(), v.end(), back_inserter(result), string_plus_int, string{});
        print_strings(result);
    }

    { // N4971 [transform.exclusive.scan]/1
        vector<string> result;
        (void) transform_exclusive_scan(v.begin(), v.end(), back_inserter(result), string{}, string_plus_int, square);
        print_strings(result);
    }

    { // N4971 [transform.inclusive.scan]/2
        vector<string> result;
        (void) transform_inclusive_scan(v.begin(), v.end(), back_inserter(result), string_plus_int, square, string{});
        print_strings(result);
    }
}

C:\Temp>cl /EHsc /nologo /W4 /std:c++17 /MTd /Od meow.cpp && meow
meow.cpp
12345
246810
1491625
"", "1", "12", "123", "1234",
"1", "12", "123", "1234", "12345",
"", "1", "14", "149", "14916",
"1", "14", "149", "14916", "1491625",

[frederick-vs-ja]

This seemingly supersedes #891. But some requirements seem bogus (#891 (comment)).

[StephanTLavavej]

Thanks for the link! There was significant analysis in that issue, so let's keep both of them open for now.