Use anonymous `union`s to implement tagged unions

Reason

A well-designed tagged union is type safe. An anonymous union simplifies the definition of a class with a (tag, union) pair.

Example

This example is mostly borrowed from TC++PL4, pp. 216--218. You can look there for an explanation.

The code is somewhat elaborate. Handling a type with user-defined assignment and destructor is tricky. Saving programmers from having to write such code is one reason for including variant in the standard.

class Value { // two alternative representations represented as a union
private:
    enum class Tag { number, text };
    Tag type; // discriminant

    union { // representation (note: anonymous union)
        int i;
        string s; // string has default constructor, copy operations, and destructor
    };
public:
    struct Bad_entry { }; // used for exceptions

    ~Value();
    Value& operator=(const Value&);   // necessary because of the string variant
    Value(const Value&);
    // ...
    int number() const;
    string text() const;

    void set_number(int n);
    void set_text(const string&);
    // ...
};

int Value::number() const
{
    if (type != Tag::number) throw Bad_entry{};
    return i;
}

string Value::text() const
{
    if (type != Tag::text) throw Bad_entry{};
    return s;
}

void Value::set_number(int n)
{
    if (type == Tag::text) {
        s.~string();      // explicitly destroy string
        type = Tag::number;
    }
    i = n;
}

void Value::set_text(const string& ss)
{
    if (type == Tag::text)
        s = ss;
    else {
        new(&s) string{ss};   // placement new: explicitly construct string
        type = Tag::text;
    }
}

Value& Value::operator=(const Value& e)   // necessary because of the string variant
{
    if (type == Tag::text && e.type == Tag::text) {
        s = e.s;    // usual string assignment
        return *this;
    }

    if (type == Tag::text) s.~string(); // explicit destroy

    switch (e.type) {
    case Tag::number:
        i = e.i;
        break;
    case Tag::text:
        new(&s) string(e.s);   // placement new: explicit construct
    }

    type = e.type;
    return *this;
}

Value::~Value()
{
    if (type == Tag::text) s.~string(); // explicit destroy
}

Enforcement

???