cgen: fix assignment to nested maps

vlib/v/gen/cgen.v

@@ -1710,7 +1710,7 @@ fn (g mut Gen) index_expr(node ast.IndexExpr) {
 		} else if sym.kind == .map {
 			info := sym.info as table.Map
 			elem_type_str := g.typ(info.value_type)
-			if g.is_assign_lhs {
+			if g.is_assign_lhs && !g.is_array_set {
 				g.is_array_set = true
 				g.write('map_set(')
 				if !left_is_ptr {

vlib/v/tests/nested_map_test.v

@@ -0,0 +1,17 @@
+fn test_nested_maps() {
+	x := map[string]map[string]int
+	x["a"] = map[string]int
+	assert x["a"]["b"] == 0
+	x["a"]["b"] = 5
+	assert x["a"]["b"] == 5
+	x["a"]["b"] = 7
+	assert x["a"]["b"] == 7
+	y := map[string]map[string]map[string]int
+	y["a"] = map[string]map[string]int
+	y["a"]["b"] = map[string]int
+	assert y["a"]["b"]["c"] == 0
+	y["a"]["b"]["c"] = 5
+	assert y["a"]["b"]["c"] == 5
+	y["a"]["b"]["c"] = 7
+	assert y["a"]["b"]["c"] == 7
+}