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/*!
@file
Forward declares `boost::hana::common` and `boost::hana::common_t`.
@copyright Louis Dionne 2013-2017
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
*/
#ifndef BOOST_HANA_FWD_CORE_COMMON_HPP
#define BOOST_HANA_FWD_CORE_COMMON_HPP
#include <boost/hana/config.hpp>
BOOST_HANA_NAMESPACE_BEGIN
//! @ingroup group-core
//! %Metafunction returning the common data type between two data types.
//!
//! `common` is a natural extension of the `std::common_type` metafunction
//! to data types. Given two data types `T` and `U`, we say that they share
//! a common type `C` if both objects of data type `T` and objects of data
//! type `U` may be converted (using `to`) to an object of data type `C`,
//! and if that conversion is equality preserving. In other words, this
//! means that for any objects `t1, t2` of data type `T` and `u1, u2` of
//! data type `U`, the following law is satisfied:
//! @code
//! to<C>(t1) == to<C>(t2) if and only if t1 == t2
//! to<C>(u1) == to<C>(u2) if and only if u1 == u2
//! @endcode
//!
//! The role of `common` is to provide an alias to such a `C` if it exists.
//! In other words, if `T` and `U` have a common data type `C`,
//! `common<T, U>::%type` is an alias to `C`. Otherwise, `common<T, U>`
//! has no nested `type` and can be used in dependent contexts to exploit
//! SFINAE. By default, the exact steps followed by `common` to determine
//! the common type `C` of `T` and `U` are
//! 1. If `T` and `U` are the same, then `C` is `T`.
//! 2. Otherwise, if `true ? std::declval<T>() : std::declval<U>()` is
//! well-formed, then `C` is the type of this expression after using
//! `std::decay` on it. This is exactly the type that would have been
//! returned by `std::common_type`, except that custom specializations
//! of `std::common_type` are not taken into account.
//! 3. Otherwise, no common data type is detected and `common<T, U>` does
//! not have a nested `type` alias, unless it is specialized explicitly.
//!
//! As point 3 suggests, it is also possible (and sometimes necessary) to
//! specialize `common` in the `boost::hana` namespace for pairs of custom
//! data types when the default behavior of `common` is not sufficient.
//! Note that `when`-based specialization is supported when specializing
//! `common` in the `boost::hana` namespace.
//!
//! > #### Rationale for requiring the conversion to be equality-preserving
//! > This decision is aligned with a proposed concept design for the
//! > standard library ([N3351][1]). Also, if we did not require this,
//! > then all data types would trivially share the common data type
//! > `void`, since all objects can be converted to it.
//!
//!
//! Example
//! -------
//! @include example/core/common/common.cpp
//!
//!
//! [1]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3351.pdf
#ifdef BOOST_HANA_DOXYGEN_INVOKED
template <typename T, typename U, optional when-based enabler>
struct common { see documentation };
#else
template <typename T, typename U, typename = void>
struct common;
#endif
//! @ingroup group-core
//! %Metafunction returning whether two data types share a common data type.
//!
//! Given two data types `T` and `U`, this metafunction simply returns
//! whether `common<T, U>::%type` is well-formed.
//!
//!
//! Example
//! -------
//! @include example/core/common/has_common.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
template <typename T, typename U>
struct has_common { whether common<T, U>::type is well-formed };
#else
template <typename T, typename U, typename = void>
struct has_common;
#endif
//! @ingroup group-core
//! Alias to `common<T, U>::%type`, provided for convenience.
//!
//!
//! Example
//! -------
//! @include example/core/common/common_t.cpp
template <typename T, typename U>
using common_t = typename common<T, U>::type;
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_FWD_CORE_COMMON_HPP
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