If on C++ 20 or the Concepts TS is enabled, a boolean concept matching types which have value_type, error_type and 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 value_or_error<T> . If you want something which exactly matches basic_outcome, use is_basic_outcome<T> .

Namespace: BOOST_OUTCOME_V2_NAMESPACE::concepts

Header: <boost/outcome/basic_result.hpp>