std::disjunction(3)

NAME
std::disjunction - std::disjunction

Synopsis
   Defined in header <type_traits>
   template<class... B> (since C++17)
   struct disjunction;
   Forms the logical disjunction of the type traits B..., effectively performing a
   logical OR on the sequence of traits.
   The specialization std::disjunction<B1, ..., BN> has a public and unambiguous base
   that is
     * if sizeof...(B) == 0, std::false_type; otherwise
     * the first type Bi in B1, ..., BN for which bool(Bi::value) == true, or BN if
       there is no such type.
   The member names of the base class, other than disjunction and operator=, are not
   hidden and are unambiguously available in disjunction.
   Disjunction is short-circuiting: if there is a template type argument Bi with
   bool(Bi::value) != false, then instantiating disjunction<B1, ..., BN>::value does
   not require the instantiation of Bj::value for j > i
   The behavior of a program that adds specializations for disjunction or disjunction_v
   is undefined.

Template parameters
   B... - every template argument Bi for which Bi::value is instantiated must be usable
          as a base class and define member value that is convertible to bool
  Helper variable template
   template<class... B> (since C++17)
   inline constexpr bool disjunction_v = disjunction<B...>::value;

Possible implementation
   template<class...> struct disjunction : std::false_type { };
   template<class B1> struct disjunction<B1> : B1 { };
   template<class B1, class... Bn>
   struct disjunction<B1, Bn...>
       : std::conditional_t<bool(B1::value), B1, disjunction<Bn...>> { };

Notes
   A specialization of disjunction does not necessarily inherit from of either
   std::true_type or std::false_type: it simply inherits from the first B whose
   ::value, explicitly converted to bool, is true, or from the very last B when all of
   them convert to false. For example, std::disjunction<std::integral_constant<int, 2>,
   std::integral_constant<int, 4>>::value is 2.
   The short-circuit instantiation differentiates disjunction from fold expressions: a
   fold expression like (... || Bs::value) instantiates every B in Bs, while
   std::disjunction_v<Bs...> stops instantiation once the value can be determined. This
   is particularly useful if the later type is expensive to instantiate or can cause a
   hard error when instantiated with the wrong type.
   Feature-test macro: __cpp_lib_logical_traits

Example
// Run this code
 #include <type_traits>
 #include <cstdint>
 #include <string>
 // values_equal<a, b, T>::value is true if and only if a == b.
 template <auto V1, decltype(V1) V2, typename T>
 struct values_equal : std::bool_constant<V1 == V2> {
   using type = T;
 };
 // default_type<T>::value is always true
 template <typename T>
 struct default_type : std::true_type {
   using type = T;
 };
 // Now we can use disjunction like a switch statement:
 template <int I>
 using int_of_size = typename std::disjunction< //
     values_equal<I, 1, std::int8_t>, //
     values_equal<I, 2, std::int16_t>, //
     values_equal<I, 4, std::int32_t>, //
     values_equal<I, 8, std::int64_t>, //
     default_type<void> // must be last!
     >::type;
 static_assert(sizeof(int_of_size<1>) == 1);
 static_assert(sizeof(int_of_size<2>) == 2);
 static_assert(sizeof(int_of_size<4>) == 4);
 static_assert(sizeof(int_of_size<8>) == 8);
 static_assert(std::is_same_v<int_of_size<13>, void>);
 // checking if Foo is constructible from double will cause a hard error
 struct Foo {
     template<class T>
     struct sfinae_unfriendly_check { static_assert(!std::is_same_v<T, double>); };
     template<class T>
     Foo(T, sfinae_unfriendly_check<T> = {} );
 };
 template<class... Ts>
 struct first_constructible {
     template<class T, class...Args>
     struct is_constructible_x : std::is_constructible<T, Args...> {
         using type = T;
     };
     struct fallback {
         static constexpr bool value = true;
         using type = void; // type to return if nothing is found
     };
     template<class... Args>
     using with = typename std::disjunction<is_constructible_x<Ts, Args...>...,
                        fallback>::type;
 };
 // OK, is_constructible<Foo, double> not instantiated
 static_assert(std::is_same_v<first_constructible<std::string, int, Foo>::with<double>,
                        int>);
 static_assert(std::is_same_v<first_constructible<std::string, int>::with<>, std::string>);
 static_assert(std::is_same_v<first_constructible<std::string, int>::with<const char*>,
                        std::string>);
 static_assert(std::is_same_v<first_constructible<std::string, int>::with<void*>, void>);
 int main() { }

See also
   negation logical NOT metafunction
   (C++17) (class template)
   conjunction variadic logical AND metafunction
   (C++17) (class template)