cgen: fix map value op-assign modification (#7101)

vlib/builtin/map_test.v

@@ -353,3 +353,11 @@ fn test_map_str_after_delete() {
 	assert osm == "{'first': 1, 'second': 2, 'third': 3}"
 	assert nsm == "{'first': 1, 'third': 3}"
 }
+
+fn test_modify_map_value() {
+	mut m1 := {'foo': 3, 'bar': -7}
+	m1['foo'] += 5
+	m1['bar'] *= -2
+	assert m1['foo'] == 8
+	assert m1['bar'] == 14
+}

vlib/v/gen/cgen.v

@@ -3779,6 +3779,43 @@ fn (mut g Gen) index_expr(node ast.IndexExpr) {
 					} else {
 						g.write(', &($elem_type_str[]) { ')
 					}
+					if g.assign_op != .assign &&
+						g.assign_op in token.assign_tokens && info.value_type != table.string_type {
+						g.write('(*(int*)map_get(')
+						if left_is_ptr && !node.left_type.has_flag(.shared_f) {
+							g.write('*')
+						}
+						g.expr(node.left)
+						if node.left_type.has_flag(.shared_f) {
+							if left_is_ptr {
+								g.write('->val')
+							} else {
+								g.write('.val')
+							}
+						}
+						g.write(', ')
+						g.expr(node.index)
+						zero := g.type_default(info.value_type)
+						if elem_typ.kind == .function {
+							g.write(', &(voidptr[]){ $zero }))')
+						} else {
+							g.write(', &($elem_type_str[]){ $zero }))')
+						}
+						op := match g.assign_op {
+							.mult_assign { '*' }
+							.plus_assign { '+' }
+							.minus_assign { '-' }
+							.div_assign { '/' }
+							.xor_assign { '^' }
+							.mod_assign { '%' }
+							.or_assign { '|' }
+							.and_assign { '&' }
+							.left_shift_assign { '<<' }
+							.right_shift_assign { '>>' }
+							else { '' }
+						}
+						g.write(' $op ')
+					}
 				} else if (g.inside_map_postfix || g.inside_map_infix) ||
 					(g.is_assign_lhs && !g.is_array_set && get_and_set_types) {
 					zero := g.type_default(info.value_type)

vlib/v/tests/shared_map_test.v

@@ -10,7 +10,12 @@ fn incr(shared foo map[string]int, key string, sem sync.Semaphore) {
 }
 
 fn test_shared_array() {
-	shared foo := &{'p': 10, 'q': 20}
+	shared foo := &{'p': 10, 'q': 0}
+	lock foo {
+		unsafe {
+			foo['q'] = 20
+		}
+	}
 	sem := sync.new_semaphore()
 	go incr(shared foo, 'p', sem)
 	go incr(shared foo, 'q', sem)
@@ -19,8 +24,8 @@ fn test_shared_array() {
 	for _ in 0 .. 50000 {
 		lock foo {
 			unsafe {
-				foo['p'] = foo['p'] - 2
-				foo['q'] = foo['q'] + 3
+				foo['p'] -= 2
+				foo['q'] += 3
 			}
 		}
 	}