Why does this C++ code crash at run time?

Why does the following code crash both on Visual Studio and GCC?

For it to crash it requires the range-based for loop, std::map, std::string and taking a refenrece to the string. If I remove any one of them it will work.

#include <iostream>
#include <string>
#include <map>
using namespace std;

struct S
{
    map<string, string> m;

    S()
    {
        m["key"] = "b";
    }

    const string &func() const
    {
        return m.find("key")->second;
    }
};

int main()
{
    for (char c : S().func())
        cout << c;

    return 0;
}

Ideone link: http://ift.tt/2h6lA78