Usernames on registration which fail policies cause conflicting error messages

[MTRNord]

Ideally it only should show the policy error here since the user was not taken in fact. This feels misleading :)

[MTRNord]

It seems that

matrix-authentication-service/crates/handlers/src/views/register.rs

Lines 171 to 180 in d3eb5be

	} else if !homeserver.is_localpart_available(&form.username).await? {
	// The user already exists on the homeserver
	// XXX: we may want to return different errors like "this username is reserved"
	tracing::warn!(
	username = &form.username,
	"User tried to register with a reserved username"
	);
	state.add_error_on_field(RegisterFormField::Username, FieldError::Exists);
	}

is an insufficient check or at least the error returned on the UI isnt granular enough.

Specifically the if case at

matrix-authentication-service/crates/matrix-synapse/src/lib.rs

Lines 271 to 276 in d3eb5be

	} if homeserver_error.errcode() == Some(M_INVALID_USERNAME)
	|| homeserver_error.errcode() == Some(M_USER_IN_USE) =>
	{
	debug!("Username not available: {homeserver_error}");
	Ok(false)
	}

probably is the origin of this as it generalizes

[matrixbot]

For your information, this issue has been copied over to the Element fork of matrix-authentication-service: element-hq/matrix-authentication-service#3132