datatypes: fix for set - operator, union and intersection, now they…

… no longer change the receiver (fix #21315) (#21362)

vlib/datatypes/set.v

@@ -85,7 +85,7 @@ pub fn (mut set Set[T]) add_all(elements []T) {
 
 // @union returns the union of the two sets.
 pub fn (l Set[T]) @union(r Set[T]) Set[T] {
-	mut set := l
+	mut set := l.copy()
 	for e, _ in r.elements {
 		set.add(e)
 	}
@@ -94,7 +94,7 @@ pub fn (l Set[T]) @union(r Set[T]) Set[T] {
 
 // intersection returns the intersection of sets.
 pub fn (l Set[T]) intersection(r Set[T]) Set[T] {
-	mut set := l
+	mut set := l.copy()
 	for e, _ in l.elements {
 		if !r.exists(e) {
 			set.remove(e)
@@ -110,7 +110,7 @@ pub fn (l Set[T]) intersection(r Set[T]) Set[T] {
 
 // - returns the difference of sets.
 pub fn (l Set[T]) - (r Set[T]) Set[T] {
-	mut set := l
+	mut set := l.copy()
 	for e, _ in l.elements {
 		if r.exists(e) {
 			set.remove(e)

vlib/datatypes/set_test.v

@@ -67,6 +67,7 @@ fn test_union() {
 	first_set.add_all(['b', 'c', 'd'])
 	second_set.add_all(['a', 'e'])
 	mut third_set := first_set.@union(second_set)
+	assert first_set.elements.len == 3
 	assert third_set.exists('a')
 	assert third_set.exists('b')
 	assert third_set.exists('c')
@@ -80,6 +81,7 @@ fn test_intersection() {
 	mut second_set := Set[string]{}
 	second_set.add_all(['bar', 'baz', 'boo'])
 	mut third_set := first_set.intersection(second_set)
+	assert first_set.exists('foo') == true
 	assert third_set.exists('foo') == false
 	assert third_set.exists('bar')
 	assert third_set.exists('baz')
@@ -108,6 +110,20 @@ fn test_difference() {
 	assert third_set.exists('boo')
 }
 
+fn test_diff() {
+	m := ['a', 'b', 'c']
+	n := ['a', 'b', 'c']
+	assert m == n
+	mut set_m := Set[string]{}
+	mut set_n := Set[string]{}
+	set_m.add_all(m)
+	set_n.add_all(n)
+	assert set_m == set_n
+	diff1 := set_m - set_n
+	diff2 := set_n - set_m
+	assert diff1 == diff2
+}
+
 fn test_subset() {
 	mut set := Set[string]{}
 	set.add_all(['a', 'b', 'c'])