Constness of return-by-value temporaries

It's perfectly legitimate to use "const_cast", and subsequently alter an
object, but only if the object was defined as non-const in the first place.
Example 1:

#include <string>

void Func( const std::string &cstr )
{
    std::string &str = const_cast<std::string&>(cstr);

    str += "monkey";
}

int main()
{
    std::string str("ape");

    Func(str);
}

I've seen many examples of where a temporary object is non-const. Example 2:

#include <string>
#include <vector>

std::vector<std::string> Func()
{
    std::vector<std::string> vec;
    
    vec.push_back("One");
    vec.push_back("Two");
    
    return vec;
}

int main()
{
    Func().push_back("Three"); // <-- Non-const member function
}

(I'll admit that I'm not certain if ALL temporaries are non-const throughout
the entire language. Could someone please clarify that?)

We can use a const reference to "extend the lifetime" of a temporary
returned-by-value from a function. Example 3:

#include <string>
#include <vector>

std::vector<std::string> Func()
{
    std::vector<std::string> vec;

    vec.push_back("One");
    vec.push_back("Two");
    
    return vec;
}

int main()
{
    const std::vector<std::string> &cvec = Func();
    
    std::string str( cvec.at(0) );

    str += "monkey";
}

We saw in Example 2, that the temporary object returned-by-value from
"Func" was non-const.

I have a question, which I have asked on comp.lang.c++, but which no-one has
seemed willing to answer. Is the following translation unit well-formed, and
absent of undefined behaviour?

#include <string>
using std::string;

#include <vector>
using std::vector;

vector<string> Func()
{
    vector<string> vec;

    vec.push_back("One");
    vec.push_back("Two");

    return vec;
}

#include <iostream>
using std::cout; using std::endl;

int main()
{
    const vector<string> &cvec = Func();

    vector<string> &vec = const_cast< vector<string>& >(cvec);

    vec.push_back("Three");

    cout << vec.at(0) << endl
         << vec.at(1) << endl
         << vec.at(2) << endl;
}

-Tom1s

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