Puzzle #7 (based on C++17)
struct Foo
{
    Foo(int& baz) 
        : m_baz{ baz }
    {}

    void bar() const
    {
        m_baz = 2;
    }

    int& m_baz;
};

int main()
{
    int baz{};
    Foo foo{ baz };
    foo.bar();

    return baz;
}
With given code, pick one answer:
Guaranteed to return 2 from main
Guaranteed to return something other than 2 from main
Undefined bahaviour
Implementation defined
Will not compile