std::expected::and_then(3)

NAME
std::expected::and_then - std::expected::and_then

Synopsis
   template< class F > (1) (since C++23)
   constexpr auto and_then( F&& f ) &;
   template< class F > (2) (since C++23)
   constexpr auto and_then( F&& f ) const&;
   template< class F > (3) (since C++23)
   constexpr auto and_then( F&& f ) &&;
   template< class F > (4) (since C++23)
   constexpr auto and_then( F&& f ) const&&;
   If *this contains an expected value, invokes f and returns its result; otherwise,
   returns a std::expected object that contains a copy of error().
   If T is not (possibly cv-qualified) void, the contained expected value (obtained
   from operator*) is passed as an argument to f; otherwise f takes no argument.
   1,2) Given type U as
     * std::remove_cvref_t<std::invoke_result_t<F>> if T is (possibly cv-qualified)
       void, or
     * std::remove_cvref_t<std::invoke_result_t<F, decltype(**this)>> otherwise.
   If U is not a specialization of std::expected, or std::is_same_v<U::error_type, E>
   is false, the program is ill-formed.
   The effect is equivalent to
 if (has_value())
 {
     if constexpr (std::is_void_v<T>)
         return std::invoke(std::forward<F>(f));
     else
         return std::invoke(std::forward<F>(f), **this);
 }
 else
     return U(std::unexpect, error());
   These overloads participate in overload resolution only if
   std::is_constructible_v<E, decltype(error())> is true.
   3,4) Given type U as
     * std::remove_cvref_t<std::invoke_result_t<F>> if T is (possibly cv-qualified)
       void, or
     * std::remove_cvref_t<std::invoke_result_t<F, decltype(std::move(**this))>>
       otherwise.
   If U is not a specialization of std::expected, or std::is_same_v<U::error_type, E>
   is false, the program is ill-formed.
   The effect is equivalent to
 if (has_value())
 {
     if constexpr (std::is_void_v<T>)
         return std::invoke(std::forward<F>(f));
     else
         return std::invoke(std::forward<F>(f), std::move(**this));
 }
 else
     return U(std::unexpect, std::move(error()));
   These overloads participate in overload resolution only if
   std::is_constructible_v<E, decltype(std::move(error()))> is true.

Parameters
   f - a suitable function or Callable object that returns a std::expected

Return value
   The result of f or a std::expected object that contains an error value, as described
   above.

Notes
   Feature-test macro Value Std Feature
   __cpp_lib_expected 202211L (C++23) Monadic functions for std::expected

Example
    This section is incomplete
    Reason: no example
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 3938 C++23 and_then was ill-formed if T is not (possibly made well-formed
                       cv-qualified) void and E is not copyable

See also
   unexpect in-place construction tag for unexpected value in expected
   unexpect_t (tag)
   (C++23)
              returns an expected containing the transformed expected value if it
   transform exists; otherwise, returns the expected itself
              (public member function)

Category:
     * Todo no example