fix #2002 (#2003)

* fix #2002

`CS AMODE #channel +f nickname` is invalid, but was being accepted
incorrectly.

* simplify logic

irc/chanserv.go

@@ -213,8 +213,17 @@ func csAmodeHandler(service *ircService, server *Server, client *Client, command
 	}
 
 	modeChanges, unknown := modes.ParseChannelModeChanges(params[1:]...)
+	invalid := len(unknown) != 0
+	// #2002: +f takes an argument but is not a channel-user mode,
+	// check for anything valid as a channel mode change that is not valid
+	// as an AMODE change
+	for _, modeChange := range modeChanges {
+		if !utils.SliceContains(modes.ChannelUserModes, modeChange.Mode) {
+			invalid = true
+		}
+	}
 	var change modes.ModeChange
-	if len(modeChanges) > 1 || len(unknown) > 0 {
+	if len(modeChanges) > 1 || invalid {
 		service.Notice(rb, client.t("Invalid mode change"))
 		return
 	} else if len(modeChanges) == 1 {

irc/utils/types.go

@@ -34,3 +34,12 @@ func ReverseSlice[T any](results []T) {
 		results[i], results[j] = results[j], results[i]
 	}
 }
+
+func SliceContains[T comparable](slice []T, elem T) (result bool) {
+	for _, t := range slice {
+		if elem == t {
+			return true
+		}
+	}
+	return false
+}