message: Rename meta::detail::bool_ to avoid inadvertent use
  ...such as on meta.hpp line 3103.
diff: #if __cpp_concepts <= 201507L
  #define META_CONCEPT concept bool
  // TS concepts subsumption barrier for atomic expressions
  -#define META_CONCEPT_BARRIER(...) ::meta::detail::bool_<__VA_ARGS__>
  +#define META_CONCEPT_BARRIER(...) ::meta::detail::barrier<__VA_ARGS__>
  #else
  #define META_CONCEPT concept
  #define META_CONCEPT_BARRIER(...) __VA_ARGS__
  @@ -267,7 +267,7 @@ namespace meta
  namespace detail
  {
  template <bool B>
  - META_INLINE_VAR constexpr bool bool_ = B;
  + META_INLINE_VAR constexpr bool barrier = B;
  template <class T, T> struct require_constant; // not defined
  }
  