arrays: fix arrays.fold, when the init value in the call, is an array (#21921)

vlib/arrays/arrays.v

@@ -331,7 +331,12 @@ pub fn filter_indexed[T](array []T, predicate fn (idx int, elem T) bool) []T {
 // assert r == 5
 // ```
 pub fn fold[T, R](array []T, init R, fold_op fn (acc R, elem T) R) R {
-	mut value := init
+	mut value := R{}
+	$if R is $array {
+		value = init.clone()
+	} $else {
+		value = init
+	}
 
 	for e in array {
 		value = fold_op(value, e)

vlib/arrays/arrays_fold_test.v

@@ -0,0 +1,21 @@
+import arrays
+
+type MyInt = int
+
+fn test_main() {
+	assert arrays.fold[int, []int]([1, 2, 3, 4], []int{}, fn (r []int, t int) []int {
+		return arrays.merge(r, [t])
+	}) == [1, 2, 3, 4]
+	assert arrays.fold[string, []string](['a', 'b', 'c', 'd'], []string{}, fn (r []string, t string) []string {
+		return arrays.merge(r, [t])
+	}) == ['a', 'b', 'c', 'd']
+	assert arrays.fold[bool, []bool]([true, false], []bool{}, fn (r []bool, t bool) []bool {
+		return arrays.merge(r, [t])
+	}) == [false, true]
+	assert arrays.fold[MyInt, []MyInt]([MyInt(0), 1], []MyInt{}, fn (r []MyInt, t MyInt) []MyInt {
+		return arrays.merge(r, [t])
+	}) == [MyInt(0), 1]
+	assert arrays.fold([1, 2, 3, 4], [5, 6, 7], fn (r []int, t int) []int {
+		return r.map(it * t)
+	}) == [120, 144, 168]
+}