If on C++ 20 or the Concepts TS is enabled, a boolean concept matching types which have
no_value_policy_type member typedefs; that the type is convertible to
basic_result<value_type, error_type, no_value_policy_type>; that
basic_result<value_type, error_type, no_value_policy_type> is a base of the type.
If without Concepts, a static constexpr bool which is true for types matching the same requirements, using a SFINAE based emulation.
This concept matches any type which provides the same typedefs as a
basic_result<T, E, NoValuePolicy>
, has that
basic_result as a base class, and is implicitly convertible to
basic_result. Whilst not guaranteed, it is very likely that the type is a
basic_result, or inherits publicly from a
basic_result. If you want something which matches any value-or-error type, consider
. If you want something which exactly matches