cgen: fix setting nested map elements fields (#6744)

vlib/v/gen/cgen.v

@@ -3512,7 +3512,8 @@ fn (mut g Gen) index_expr(node ast.IndexExpr) {
 				info := sym.info as table.Map
 				elem_type_str := g.typ(info.value_type)
 				elem_typ := g.table.get_type_symbol(info.value_type)
-				if g.is_assign_lhs && !g.is_array_set && elem_typ.kind != .struct_ {
+				get_and_set_types := elem_typ.kind in [.struct_, .map]
+				if g.is_assign_lhs && !g.is_array_set && !get_and_set_types {
 					g.is_array_set = true
 					g.write('map_set(')
 					if !left_is_ptr {
@@ -3527,7 +3528,7 @@ fn (mut g Gen) index_expr(node ast.IndexExpr) {
 						g.write(', &($elem_type_str[]) { ')
 					}
 				} else if (g.inside_map_postfix || g.inside_map_infix) ||
-					(g.is_assign_lhs && !g.is_array_set && elem_typ.kind == .struct_) {
+					(g.is_assign_lhs && !g.is_array_set && get_and_set_types) {
 					zero := g.type_default(info.value_type)
 					g.write('(*($elem_type_str*)map_get_and_set(')
 					if !left_is_ptr {

vlib/v/tests/nested_map_test.v

@@ -1,20 +1,32 @@
-fn test_nested_maps() {
-	if true{}
-	//
-	else{}
-	mut 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
-	mut 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
+struct Foo {
+mut:
+	name string
+}
+
+fn test_nested_maps() {
+	if true {
+	}
+	//
+	else {
+	}
+	mut 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
+	mut 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
+	mut foos := map[string]map[string]Foo{}
+	foos['a']['b'] = Foo{'bar'}
+	assert foos['a']['b'].name == 'bar'
+	foos['a']['b'].name = 'baz'
+	assert foos['a']['b'].name == 'baz'
 }