Returns true if this result is successful or its error does not compare equal to the error in the failure type sugar. Comparison is done using operator!= on error_type and A. If A is void, this call aliases bool has_value() const noexcept .

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

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

Guarantees: None.