maps: use explicit map value cloning in maps.merge_in_place and maps.merge (fx #22564) (#22576)

vlib/maps/maps.v

@@ -75,7 +75,11 @@ pub fn from_array[T](array []T) map[int]T {
 // Note that this function modifes `m1`, while `m2` will not be.
 pub fn merge_in_place[K, V](mut m1 map[K]V, m2 map[K]V) {
 	for k, v in m2 {
-		m1[k] = v
+		$if v is $map {
+			m1[k] = v.clone()
+		} $else {
+			m1[k] = v
+		}
 	}
 }
 
@@ -86,7 +90,11 @@ pub fn merge_in_place[K, V](mut m1 map[K]V, m2 map[K]V) {
 pub fn merge[K, V](m1 map[K]V, m2 map[K]V) map[K]V {
 	mut res := m1.clone()
 	for k, v in m2 {
-		res[k] = v
+		$if v is $map {
+			res[k] = v.clone()
+		} $else {
+			res[k] = v
+		}
 	}
 	return res
 }

vlib/maps/maps_clone_test.v

@@ -0,0 +1,36 @@
+import maps
+
+fn test_main() {
+	// mmm map declaration as mutable
+	mut mmm := map[string]map[string]int{}
+
+	// adding values to the map mmm
+	mmm['greet'] = {
+		'hello': 0
+	}
+	mmm['place'] = {
+		'world': 1
+	}
+	mmm['color']['orange'] = 2
+
+	// printing the map mmm
+	assert mmm.str() == "{'greet': {'hello': 0}, 'place': {'world': 1}, 'color': {'orange': 2}}"
+
+	// mmm2 map declaration as const
+	mmm2 := {
+		'name': {
+			'Diego': 3
+		}
+	}
+
+	// printing the map mmm2
+	assert mmm2.str() == "{'name': {'Diego': 3}}"
+
+	// Using the maps module functions
+	// use of maps.merge is commented but its behavior is the same as merge_in_place
+	// mmm = maps.merge(mmm, mmm2)
+	maps.merge_in_place(mut mmm, mmm2)
+
+	// printing again mmm to the standard output
+	assert mmm.str() == "{'greet': {'hello': 0}, 'place': {'world': 1}, 'color': {'orange': 2}, 'name': {'Diego': 3}}"
+}