Merge 2015-10 LWG Motion 10

Author: zygoloid

source/utilities.tex

@@ -9332,6 +9332,11 @@
   template <class...>
     using void_t            = void;
 
+  // \ref{meta.logical}, logical operator traits:
+  template<class... B> struct conjunction;
+  template<class... B> struct disjunction;
+  template<class B> struct negation;
+
   // \ref{meta.unary.cat}, primary type categories
   template <class T> constexpr bool is_void_v
     = is_void<T>::value;
@@ -9471,6 +9476,11 @@
     = is_base_of<Base, Derived>::value;
   template <class From, class To> constexpr bool is_convertible_v
     = is_convertible<From, To>::value;
+
+  // \ref{meta.logical}, logical operator traits:
+  template<class... B> constexpr bool conjunction_v = conjunction<B...>::value;
+  template<class... B> constexpr bool disjunction_v = disjunction<B...>::value;
+  template<class B> constexpr bool negation_v = negation<B>::value;
 } // namespace std
 \end{codeblock}
 
@@ -10543,6 +10553,88 @@
 \end{codeblock}
 \exitexample
 
+\rSec2[meta.logical]{Logical operator traits}
+
+\pnum
+This subclause describes type traits for applying logical operators
+to other type traits.
+
+\pnum
+\begin{codeblock}
+  template<class... B> struct conjunction : @\seebelow@ { };
+\end{codeblock}
+
+\pnum
+The class template \tcode{conjunction}
+forms the logical conjunction of its template type arguments.
+Every template type argument
+shall be usable as a base class and
+shall have a member \tcode{value} which
+is convertible to \tcode{bool},
+is not hidden, and
+is unambiguously available in the type.
+
+\pnum
+The BaseCharacteristic of a specialization \tcode{conjunction<B1, ..., BN>}
+is the first type \tcode{Bi} in the list \tcode{true_type, B1, ..., BN}
+for which \tcode{Bi::value == false}, or
+if every \tcode{Bi::value != false}, the BaseCharacteristic is \tcode{BN}.
+\enternote This means a specialization of \tcode{conjunction}
+does not necessarily have a BaseCharacteristic
+of either \tcode{true_type} or \tcode{false_type}.
+\exitnote
+
+\pnum
+For a specialization \tcode{conjunction<B1, ..., BN>},
+if there is a template type argument \tcode{Bi} with \tcode{Bi::value == false},
+then instantiating \tcode{conjunction<B1, ..., BN>::value}
+does not require the instantiation of \tcode{Bj::value} for \tcode{j > i}.
+\enternote This is analogous to the short-circuiting behavior of \tcode{\&\&}.
+\exitnote
+
+\pnum
+\begin{codeblock}
+  template<class... B> struct disjunction : @\seebelow@ { };
+\end{codeblock}
+
+\pnum
+The class template \tcode{disjunction}
+forms the logical disjunction of its template type arguments.
+Every template type argument shall be usable as a base class and
+shall have a member \tcode{value} which
+is convertible to \tcode{bool},
+is not hidden, and
+is unambiguously available in the type.
+
+\pnum
+The BaseCharacteristic of a specialization \tcode{disjunction<B1, ..., BN>}
+is the first type \tcode{Bi} in the list \tcode{false_type, B1, ..., BN}
+for which \tcode{Bi::value != false}, or
+if every \tcode{Bi::value == false}, the BaseCharacteristic is \tcode{BN}.
+\enternote This means a specialization of \tcode{disjunction}
+does not necessarily have a BaseCharacteristic
+of either \tcode{true_type} or \tcode{false_type}.
+\exitnote
+
+\pnum
+For a specialization \tcode{disjunction<B1, ..., BN>},
+if there is a template type argument \tcode{Bi} with \tcode{Bi::value != false},
+then instantiating \tcode{disjunction<B1, ..., BN>::value}
+does not require the instantiation of \tcode{Bj::value} for \tcode{j > i}.
+\enternote This is analogous to the short-circuiting behavior of \tcode{||}.
+\exitnote
+
+\pnum
+\begin{codeblock}
+  template<class B> struct negation : bool_constant<!B::value> { };
+\end{codeblock}
+
+\pnum
+The class template \tcode{negation}
+forms the logical negation of its template type argument.
+The type \tcode{negation<B>}
+is a UnaryTypeTrait with a BaseCharacteristic of \tcode{bool_constant<!B::value>}.
+
 \rSec1[ratio]{Compile-time rational arithmetic}
 
 \rSec2[ratio.general]{In general}

source/xref.tex

@@ -915,6 +915,7 @@
 memory.syn\quad\ref{memory.syn}\\
 meta\quad\ref{meta}\\
 meta.help\quad\ref{meta.help}\\
+meta.logical\quad\ref{meta.logical}\\
 meta.rel\quad\ref{meta.rel}\\
 meta.rqmts\quad\ref{meta.rqmts}\\
 meta.trans\quad\ref{meta.trans}\\