Returns true if this outcome is unsuccessful and its error compares equal to the error in the failure type sugar. Comparison is done using operator== on error_type and A and on exception_type and B.

Requires: operator== must be a valid expression between error_type and A, or A is void; operator== must be a valid expression between exception_type and B, or B is void. If error_type is void, then so must be A; if exception_type is void, then so must be B.

Complexity: Whatever the underlying operator== has. Constexpr and noexcept of underlying operations is propagated.

Guarantees: None.