map: fix m[a]++ etc

vlib/builtin/map_test.v

@@ -239,13 +239,40 @@ fn test_map_assign() {
 		's': u16(5)
 		't': 3
 	}
-	d := Mstruct1 {
+	_ := Mstruct1 {
 		{ 'p': 12 }
 	}
-	e := Mstruct2 {
+	_ := Mstruct2 {
 		{ 'q': 1.7 }
 	}
-	f := Mstruct3 {
+	_ := Mstruct3 {
 		{ 'r': u16(6), 's': 5 }
 	}
 }
+
+fn test_postfix_op_directly() {
+	mut a := map[string]int
+	a['aaa']++
+	assert a['aaa'] == 1
+	a['aaa']++
+	assert a['aaa'] == 2
+	a['bbb']--
+	assert a['bbb'] == -1
+	a['bbb']--
+	assert a['bbb'] == -2
+}
+
+fn test_map_push_directly() {
+	mut a := map[string][]string
+	a['aaa'] << ['a', 'b', 'c']
+	assert a['aaa'].len == 3
+	assert a['aaa'] == ['a', 'b', 'c']
+}
+
+fn test_assign_directly() {
+	mut a := map[string]int
+	a['aaa'] += 4
+	assert a['aaa'] == 4
+	a['aaa'] -= 2
+	assert a['aaa'] == 2
+}