AuthRequestError enum Null safety

The request was invalid...

The request is missing a required parameter, includes an unsupported parameter value (other than grant type), repeats a parameter, includes multiple credentials, utilizes more than one mechanism for authenticating the client, or is otherwise malformed.

The client was invalid...

Client authentication failed (e.g., unknown client, no client authentication included, or unsupported authentication method). The authorization server MAY return an HTTP 401 (Unauthorized) status code to indicate which HTTP authentication schemes are supported. If the client attempted to authenticate via the "Authorization" request header field, the authorization server MUST respond with an HTTP 401 (Unauthorized) status code and include the "WWW-Authenticate" response header field matching the authentication scheme used by the client.

The grant was invalid...

The provided authorization grant (e.g., authorization code, resource owner credentials) or refresh token is invalid, expired, revoked, does not match the redirection URI used in the authorization request, or was issued to another client.

The requested scope is invalid, unknown, malformed, or exceeds the scope granted by the resource owner.

The authorization grant type is not supported by the authorization server.

The authorization server does not support obtaining an authorization code using this method.

The authenticated client is not authorized to use this authorization grant type.

The resource owner or authorization server denied the request.

The server encountered an error during processing the request.

The server is temporarily unable to fulfill the request.

Indicates that the token is invalid.

This particular error reason is not part of the OAuth 2.0 spec.