fizzbuzz.cpp

#include <vector>
#include <iostream>
#include <numeric>
#include <string>
#include <optional>

void printStringLine(std::string s)
{
    std::cout << s << '\n';
}

template<typename T>
concept Magma = requires(T a) 
{
    { a + a } -> T; // a binary operator that returns the same Type
                    // define operator+ for your Type if you get an error here
};

template<Magma T>  
std::optional<T> operator+(std::optional<T> first, std::optional<T> second)
{
    if(first) 
        if(second)
            return std::make_optional(first.value() + second.value());
        else
            return first;
    else
        return second;
}

std::string fizzBuzzOrNumber(int n)
{
    // what should this function be called?
    auto maybeStringWhenNMatchesDivisor = [n](int divisor, std::string s) {
        return (n % divisor) == 0 ? std::make_optional(s) : std::nullopt;
    };
    auto maybeFizz = maybeStringWhenNMatchesDivisor(3, "Fizz");
    auto maybeBuzz = maybeStringWhenNMatchesDivisor(5, "Buzz");
    return (maybeFizz + maybeBuzz).value_or(std::to_string(n));
}

int main()
{
    std::vector<int> nums(100);
    std::iota(begin(nums),
              end(nums),
              1);

    for(auto n : nums)
        printStringLine(fizzBuzzOrNumber(n));
        
    return 0;
}

requires compiler options: -std=c++2a -fconcepts

Which compilers have you tested this on? I am assuming gcc. clang and/or MSVC? I can test and report.

https://godbolt.org/z/zm5hPn clang (expermental concepts) works.

I've tested it on clang and gcc. I've not tried on MSVC.

The code doesn't compile on MSVC as it is. You get the following error message:
Error C7593: a return type requirement shall not be a trailing return type '-> T'. Consider using '-> std::convertible_to' instead.

If the line:
{ a + a } -> T
is changed to (as the error message suggests):
{ a + a } -> std::convertible_to

It works fine.