checker: fix return type of alias primitive operator overloading

vlib/v/checker/infix.v

@@ -435,7 +435,25 @@ fn (mut c Checker) infix_expr(mut node ast.InfixExpr) ast.Type {
 					c.check_div_mod_by_zero(node.right, node.op)
 				}
 
-				return_type = promoted_type
+				left_sym = c.table.sym(unwrapped_left_type)
+				right_sym = c.table.sym(unwrapped_right_type)
+				if left_sym.info is ast.Alias
+					&& c.table.sym(left_sym.info.parent_type).is_primitive() {
+					if left_sym.has_method(node.op.str()) {
+						if method := left_sym.find_method(node.op.str()) {
+							return_type = method.return_type
+						}
+					}
+				} else if right_sym.info is ast.Alias
+					&& c.table.sym(right_sym.info.parent_type).is_primitive() {
+					if right_sym.has_method(node.op.str()) {
+						if method := left_sym.find_method(node.op.str()) {
+							return_type = method.return_type
+						}
+					}
+				} else {
+					return_type = promoted_type
+				}
 			}
 		}
 		.gt, .lt, .ge, .le {

vlib/v/checker/tests/method_op_alias_err.out

@@ -5,7 +5,7 @@ vlib/v/checker/tests/method_op_alias_err.vv:4:18: error: expected `Foo` not `Foo
       |                  ~~~~
     5 |     return Foo2(f + f1)
     6 | }
-vlib/v/checker/tests/method_op_alias_err.vv:5:17: error: infix expr: cannot use `string` (right expression) as `string`
+vlib/v/checker/tests/method_op_alias_err.vv:5:17: error: infix expr: cannot use `Foo2` (right expression) as `Foo`
     3 | 
     4 | fn (f Foo) + (f1 Foo2) Foo2 {
     5 |     return Foo2(f + f1)

vlib/v/tests/alias_primitive_operator_overloading_test.v

@@ -0,0 +1,31 @@
+type Alias = u8
+
+fn new_alias() Alias {
+	return 0
+}
+
+fn (a Alias) add(b Alias) Alias {
+	return new_alias()
+}
+
+fn (a Alias) mul(b Alias) Alias {
+	return new_alias()
+}
+
+fn (a Alias) + (b Alias) Alias {
+	return a.add(b)
+}
+
+fn (a Alias) * (b Alias) Alias {
+	return a.mul(b)
+}
+
+fn test_alias_primitive_operator_overloading() {
+	a := new_alias()
+	b := new_alias()
+
+	c := a + b
+	d := a.add(b)
+	assert typeof(c).name == 'Alias'
+	assert typeof(c).name == 'Alias'
+}

vlib/v/tests/alias_voidptr_operator_overloading_test.v

@@ -0,0 +1,30 @@
+type AF_ARRAY = voidptr
+
+fn (a AF_ARRAY) add(b AF_ARRAY) AF_ARRAY {
+	mut y := AF_ARRAY(0)
+	return y
+}
+
+fn (a AF_ARRAY) mul(b AF_ARRAY) AF_ARRAY {
+	mut y := AF_ARRAY(0)
+	return y
+}
+
+fn (a AF_ARRAY) + (b AF_ARRAY) AF_ARRAY {
+	return a.add(b)
+}
+
+fn (a AF_ARRAY) * (b AF_ARRAY) AF_ARRAY {
+	return a.mul(b)
+}
+
+fn test_alias_voidptr_operator_overloading() {
+	a := AF_ARRAY(0)
+	b := AF_ARRAY(0)
+
+	c := a + b
+	y := a * a
+
+	assert c == a.add(b)
+	assert y == a.mul(a)
+}