Returns true if this result 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. If A is void, this call aliases bool has_error() 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.