flag: correct bool logic, add test (#22162)

vlib/flag/flag_to.v

@@ -832,6 +832,7 @@ fn keep_at_max(str string, max int) string {
 pub fn (fm FlagMapper) to_struct[T](defaults ?T) !T {
 	// Generate T result
 	mut result := defaults or { T{} }
+	the_default := defaults or { T{} }
 
 	$if T is $struct {
 		struct_name := T.name
@@ -907,7 +908,7 @@ pub fn (fm FlagMapper) to_struct[T](defaults ?T) !T {
 					if arg := f.arg {
 						return error('can not assign `${arg}` to bool field `${field.name}`')
 					}
-					result.$(field.name) = true
+					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 {
 						'ERROR'

vlib/flag/flag_to_bool_test.v

@@ -0,0 +1,21 @@
+import flag
+
+const gnu_args_bool_flags = ['--no-parallel', '--nocache', '--stay', '--nix']
+
+struct BoolConfig {
+	mix      bool
+	nix      bool
+	parallel bool = true @[long: 'no-parallel']
+	cache    bool @[long: nocache]
+	no_stay  bool @[long: 'stay']
+}
+
+fn test_bool_flags() {
+	bf, _ := flag.to_struct[BoolConfig](gnu_args_bool_flags, style: .long)!
+
+	assert bf.mix == false
+	assert bf.nix == true
+	assert bf.parallel == false
+	assert bf.cache == true
+	assert bf.no_stay == true
+}