checker: fix negative values for unsigned integers (#10976)

vlib/v/checker/checker.v

@@ -3949,6 +3949,14 @@ pub fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 										left.obj.is_auto_heap = true
 									}
 								}
+								if left_type in ast.unsigned_integer_type_idxs {
+									if right is ast.IntegerLiteral {
+										if right.val[0] == `-` {
+											c.error('Cannot assign negative value to unsigned integer type',
+												right.pos)
+										}
+									}
+								}
 							}
 							ast.GlobalField {
 								left.obj.typ = left_type

vlib/v/checker/tests/negative_assign_to_unsigned.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/negative_assign_to_unsigned.vv:3:9: error: Cannot assign negative value to unsigned integer type
+    1 | fn main() {
+    2 |     mut u := u32(10)
+    3 |     u = -10
+      |         ~~~
+    4 |     eprintln(u)
+    5 | }

vlib/v/checker/tests/negative_assign_to_unsigned.vv

@@ -0,0 +1,5 @@
+fn main() {
+    mut u := u32(10)
+    u = -10
+    eprintln(u)
+}