unreachable-code-loop-increment warning rules

December 13, 2022

Consider the following code (clang). It has two identical loops explicitly terminated on the first iteration. One of them throws a warning (an error if -Wpedantic -Werror). The other does not. Trying to understand why. The code can be tested on Godbolt here


template <typename K, typename V>
int get(const std::map<K, V>& m) {
        for (const auto& [k, v]: m)
                return v.size(); // No warning
        return 0;
}

int main(int argc, char** argv) {
        std::map<char, std::string>m1 { {'A', "Alex"}, {'B', "Bob"}};
        std::cout << get(m1) << "\n";
        for (const auto& [k, v]: m1)
            return v.size(); // warning: loop will run at most once (loop increment never executed) [-Wunreachable-code-loop-increment] 
        return 0;
}

I expect a warning in both cases, although I don’t really see a value in this warning. If you are wondering, why – I wanted to implement something like:

if (mylist.begin() != mylist.end())
  return mylist.begin();

using a loop iterator.

>Solution :

The difference in behaviour seems to be a side-effect of making get a template. If you make it a regular function then the warning reappears:

int get(const std::map <char, std::string>& m) {
    for (const auto& [k, v]: m)
        return v.size();    // warning: loop will run at most once ...
    return 0;
}

Live demo

Arguably, this is a clang bug.

I am also a bit puzzled when you say: