cli: ensure that required flags are set (#8826)

examples/cli.v

@@ -42,7 +42,7 @@ fn main() {
 	cmd.parse(os.args)
 }
 
-fn greet_func(cmd Command) {
+fn greet_func(cmd Command) ? {
 	language := cmd.flags.get_string('language') or {
 		panic('Failed to get `language` flag: $err')
 	}
@@ -76,10 +76,10 @@ fn greet_func(cmd Command) {
 	}
 }
 
-fn greet_pre_func(cmd Command) {
+fn greet_pre_func(cmd Command) ? {
 	println('This is a function running before the main function.\n')
 }
 
-fn greet_post_func(cmd Command) {
+fn greet_post_func(cmd Command) ? {
 	println('\nThis is a function running after the main function.')
 }

vlib/cli/command.v

@@ -267,7 +267,7 @@ fn (cmd Command) check_version_flag() {
 
 fn (cmd Command) check_required_flags() {
 	for flag in cmd.flags {
-		if flag.required && flag.value.len > 0 {
+		if flag.required && flag.value.len == 0 {
 			full_name := cmd.full_name()
 			println('Flag `$flag.name` is required by `$full_name`')
 			exit(1)

vlib/cli/command_test.v

@@ -117,6 +117,23 @@ fn test_if_multiple_flags_get_set() {
 	cmd.parse(['command', '-flag', 'value', '-value', '42'])
 }
 
+fn test_if_required_flags_get_set() {
+	mut cmd := cli.Command{
+		name: 'command'
+		execute: flag_should_have_value_of_42
+	}
+	cmd.add_flag(cli.Flag{
+		flag: .string
+		name: 'flag'
+	})
+	cmd.add_flag(cli.Flag{
+		flag: .int
+		name: 'value'
+		required: true
+	})
+	cmd.parse(['command', '-flag', 'value', '-value', '42'])
+}
+
 fn flag_is_set_in_subcommand(cmd cli.Command) ? {
 	flag := cmd.flags.get_string('flag') or { panic(err) }
 	assert flag == 'value'