Segfault when not specifying return type of lambda function

A lambda without trailing return type as in:

[&](){return str;};

Is equivalent to:

[&]()->auto{return str;};

So this lambda returns a copy of str.

Calling the std::function object will result in this equivalent code:

const string& std_function_call_operator(){
    // functor = [&]->auto{return str;};

    return functor();
    }

When this function is called, str is copied inside a temporary, the reference is bound to this temporary and then the temporary is destroyed. So you get the famous dangling reference. This is a very classical scenario.