add is_logic

[gpeterhoff]

What do I mean?
bool is always a logic type, of course. But there can be more logic types, e.g. boost::triboool.
is_logic must be adapted in the relevant libraries.
is_logic.hpp

//	Distributed under the Boost Software License Version 1.0 https://www.boost.org/LICENSE_1_0.txt
//	Copyright Gero Peterhoff

#ifndef BOOST_TYPE_TRAITS_IS_LOGIC_HPP
#define BOOST_TYPE_TRAITS_IS_LOGIC_HPP

#include <boost/type_traits/is_bool.hpp>
#include <type_traits>



namespace boost
{
template <typename Type>
struct is_logic : public integral_constant
<
	bool,
	is_bool<Type>::value
>	{};



#if !defined(BOOST_NO_CXX17_INLINE_VARIABLES)
template <typename Type> inline constexpr bool is_logic_v = is_logic<Type>::value;
#endif
}	//	boost

#endif	//	BOOST_TYPE_TRAITS_IS_LOGIC_HPP

[jzmaddock]

This is not what type_traits is for - we do not provide specialization points for other libraries, we introspect the type system, nothing more or less.

[gpeterhoff]

tribool is a component of boost. As I wrote, such libraries must of course set is_logic (to true) themselves. But first a basis has to be created to represent this.
As I wrote ages ago, I would like to provide classes for 4-value logic https://en.wikipedia.org/wiki/Many-valued_logic#Belnap_logic_(B4) and for 3-value logic (which could replace the old tribool).
These would of course contain an is_logic.
Now you have to decide:

• is_logic is not implemented
  you are stuck with the stone old code (tribool), without the possibility to extend it
• is_logic is implemented
  various logic classes can be categorized (there are even more than 2-, 3- and 4-value logics)

please open this issues.