checker: check unsigned integer compared with negative value (#13670)

vlib/v/checker/checker.v

@@ -819,6 +819,28 @@ pub fn (mut c Checker) infix_expr(mut node ast.InfixExpr) ast.Type {
 				} else if need_overload && !gen_sym.has_method('<') && node.op == .gt {
 					c.error('cannot use `>` as `<=` operator method is not defined', left_right_pos)
 				}
+			} else if left_type in ast.integer_type_idxs && right_type in ast.integer_type_idxs {
+				is_left_type_signed := left_type in ast.signed_integer_type_idxs
+					|| left_type == ast.int_literal_type_idx
+				is_right_type_signed := right_type in ast.signed_integer_type_idxs
+					|| right_type == ast.int_literal_type_idx
+				if is_left_type_signed != is_right_type_signed {
+					if is_right_type_signed {
+						if mut node.right is ast.IntegerLiteral {
+							if node.right.val.int() < 0 {
+								c.error('unsigned integer cannot be compared with negative value',
+									node.right.pos)
+							}
+						}
+					} else if is_left_type_signed {
+						if mut node.left is ast.IntegerLiteral {
+							if node.left.val.int() < 0 {
+								c.error('unsigned integer cannot be compared with negative value',
+									node.left.pos)
+							}
+						}
+					}
+				}
 			}
 		}
 		.left_shift {

vlib/v/checker/tests/infix_unsigned_and_signed_int_err.out

@@ -0,0 +1,13 @@
+vlib/v/checker/tests/infix_unsigned_and_signed_int_err.vv:2:14: error: unsigned integer cannot be compared with negative value
+    1 | fn main() {
+    2 |     if u32(1) < -1 {
+      |                 ~~
+    3 |         println('unexpected')
+    4 |     }
+vlib/v/checker/tests/infix_unsigned_and_signed_int_err.vv:6:5: error: unsigned integer cannot be compared with negative value
+    4 |     }
+    5 |
+    6 |     if -1 > u32(1) {
+      |        ~~
+    7 |         println('unexpected')
+    8 |     }

vlib/v/checker/tests/infix_unsigned_and_signed_int_err.vv

@@ -0,0 +1,9 @@
+fn main() {
+	if u32(1) < -1 {
+		println('unexpected')
+	}
+
+	if -1 > u32(1) {
+		println('unexpected')
+	}
+}