pref: error if unknown argument passed to v (#6487)

vlib/v/pref/pref.v

@@ -147,7 +147,13 @@ pub fn parse_args(args []string) (&Preferences, string) {
 				res.only_check_syntax = true
 			}
 			'-v' {
-				res.is_verbose = true
+				// `-v` flag is for setting verbosity, but without any args it prints the version, like Clang
+				if args.len > 1 {
+					res.is_verbose = true
+				} else {
+					command = 'version'
+					command_pos = i
+				}
 			}
 			'-silent' {
 				res.output_mode = .silent
@@ -313,21 +319,31 @@ pub fn parse_args(args []string) (&Preferences, string) {
 				i++
 			}
 			else {
-				mut should_continue := false
-				for flag_with_param in list_of_flags_with_param {
-					if '-$flag_with_param' == arg {
-						should_continue = true
+				if arg[0] == `-` {
+					if arg[1..] in list_of_flags_with_param {
+						// skip parameter
 						i++
-						break
+						continue
+					}
+				} else {
+					if command == '' {
+						command = arg
+						command_pos = i
 					}
-				}
-				if should_continue {
 					continue
 				}
-				if !arg.starts_with('-') && command == '' {
-					command = arg
+				if arg in ['-V', '-version', '--version'] {
+					command = 'version'
 					command_pos = i
+					continue
 				}
+				if command !in ['', 'run', 'build', 'build-module'] {
+					// arguments for e.g. fmt are checked elsewhere
+					continue
+				}
+				eprint('Unknown argument `$arg`')
+				eprintln(if command.len == 0 {''} else {' for command `$command`'})
+				exit(1)
 			}
 		}
 	}