in_place_init & friends could be constexpr to enforce constant initialization

[ldionne]

In https://github.com/boostorg/optional/blob/develop/include/boost/optional/optional.hpp#L93-L106, we have:

struct in_place_init_t
{
  struct init_tag{};
  explicit in_place_init_t(init_tag){}
};
const in_place_init_t in_place_init ((in_place_init_t::init_tag()));

struct in_place_init_if_t
{
  struct init_tag{};
  explicit in_place_init_if_t(init_tag){}
};
const in_place_init_if_t in_place_init_if ((in_place_init_if_t::init_tag()));

This results in static initializers in binaries (although trivial ones). Instead, it should be valid to change their definition to the following, which would definitely get rid of static initializers in >= C++11:

struct in_place_init_t
{
  struct init_tag{};
  BOOST_CONSTEXPR explicit in_place_init_t(init_tag){}
};
BOOST_CONSTEXPR_OR_CONST in_place_init_t in_place_init ((in_place_init_t::init_tag()));

struct in_place_init_if_t
{
  struct init_tag{};
  BOOST_CONSTEXPR explicit in_place_init_if_t(init_tag){}
};
BOOST_CONSTEXPR_OR_CONST in_place_init_if_t in_place_init_if ((in_place_init_if_t::init_tag()));

[akrzemi1]

Implemented in develop.