cgen: fix map clone (#7366)

vlib/builtin/map.v

@@ -573,7 +573,7 @@ pub fn (d &DenseArray) clone() DenseArray {
 }
 
 [unsafe]
-pub fn (m map) clone() map {
+pub fn (m &map) clone() map {
 	metasize := int(sizeof(u32) * (m.cap + 2 + m.extra_metas))
 	res := map{
 		key_bytes: m.key_bytes

vlib/builtin/map_test.v

@@ -426,3 +426,17 @@ fn test_modify_map_value() {
 	assert m1['foo'] == 8
 	assert m1['bar'] == 14
 }
+
+fn test_map_clone() {
+	mut nums := {
+		'foo': 1,
+		'bar': 2
+	}
+	mut nums2 := nums.clone()
+	nums2['foo']++
+	nums2['bar'] *= 4
+	assert nums['foo'] == 1
+	assert nums['bar'] == 2
+	assert nums2['foo'] == 2
+	assert nums2['bar'] == 8
+}

vlib/v/gen/fn.v

@@ -382,6 +382,9 @@ fn (mut g Gen) method_call(node ast.CallExpr) {
 		g.gen_str_for_type(node.receiver_type)
 	}
 	mut has_cast := false
+	if left_sym.kind == .map && node.name == 'clone' {
+		receiver_type_name = 'map'
+	}
 	// TODO performance, detect `array` method differently
 	if left_sym.kind == .array && node.name in
 		['repeat', 'sort_with_compare', 'free', 'push_many', 'trim', 'first', 'last', 'pop', 'clone', 'reverse', 'slice'] {