Feature Request: Add C++14 constexpr Support to boost::optional

[alandefreitas]

Currently, boost::optional cannot be used in constexpr contexts in C++14/17 because it lacks constexpr constructors. This limits its usefulness in constexpr-heavy code like compile-time parsers and validators.

constexpr boost::optional<int> g()
{
    return 5;
}

constexpr int y = g().value();

error: constexpr function's return type 'boost::optional<int>' is not a literal type
note: 'optional<int>' is not literal because it is not an aggregate and has 
      no constexpr constructors other than copy or move constructors

https://godbolt.org/z/P667adWE7

Use Case: We're working on adding constexpr support to Boost.URL to enable compile-time URL parsing and validation. Many of the parsing rules naturally return optional values (e.g., optional userinfo, optional port), but we can't use boost::optional in constexpr contexts.

For comparison, boost::system::result already works perfectly in constexpr contexts:

constexpr boost::system::result<int> f()
{
    return 5;
}

constexpr int x = f().value(); // Works fine!