flag: fix tail/single bool flag error condition logic in flag_to.v (fix #25166) (#25172)

vlib/flag/flag_to.v

@@ -934,8 +934,10 @@ pub fn (fm FlagMapper) to_struct[T](defaults ?T) !T {
 						.f64()
 				} $else $if field.typ is bool {
 					if arg := f.arg {
+						if arg != '' {
 							return error('can not assign `${arg}` to bool field `${field.name}`')
 						}
+					}
 					result.$(field.name) = !the_default.$(field.name)
 				} $else $if field.typ is string {
 					trace_dbg_println('${@FN}: assigning (string) ${struct_name}.${field.name} = ${f.arg or {

vlib/flag/flag_to_tail_bool_test.v

@@ -0,0 +1,42 @@
+import flag
+
+const args_bool_short = ['some.exe', '-h']
+const args_bool_long = ['some.exe', '-help']
+
+struct CliOptions {
+	show_help bool @[long: 'help'; short: h]
+}
+
+fn test_short_tail_bool() {
+	cli_options, unmatched := flag.to_struct[CliOptions](args_bool_short,
+		skip:  1
+		style: .v
+		mode:  .relaxed
+	)!
+
+	if unmatched.len > 0 {
+		assert false
+	}
+	if cli_options.show_help {
+		assert true
+	} else {
+		assert false
+	}
+}
+
+fn test_long_tail_bool() {
+	cli_options, unmatched := flag.to_struct[CliOptions](args_bool_long,
+		skip:  1
+		style: .v
+		mode:  .relaxed
+	)!
+
+	if unmatched.len > 0 {
+		assert false
+	}
+	if cli_options.show_help {
+		assert true
+	} else {
+		assert false
+	}
+}