checker: add error for if mut x != none {, when x is an immutable option (fix #24692) (#24694)

vlib/v/checker/if.v

@@ -606,6 +606,10 @@ fn (mut c Checker) smartcast_if_conds(mut node ast.Expr, mut scope ast.Scope, co
 			c.smartcast_if_conds(mut node.right, mut scope, control_expr)
 		} else if node.left in [ast.Ident, ast.SelectorExpr] && node.op == .ne
 			&& node.right is ast.None {
+			if (node.left is ast.Ident && node.left.is_mut)
+				|| (node.left is ast.SelectorExpr && node.left.is_mut) {
+				c.fail_if_immutable(mut node.left)
+			}
 			if node.left is ast.Ident && c.comptime.get_ct_type_var(node.left) == .smartcast {
 				node.left_type = c.type_resolver.get_type(node.left)
 				c.smartcast(mut node.left, node.left_type, node.left_type.clear_flag(.option), mut

vlib/v/checker/tests/option_mut_non_mut_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/option_mut_non_mut_err.vv:3:9: error: `option_1` is immutable, declare it with `mut` to make it mutable
+    1 | fn main() {
+    2 |     option_1 := ?int(10)
+    3 |     if mut option_1 != none {
+      |            ~~~~~~~~
+    4 |         option_1 = 11
+    5 |     }

vlib/v/checker/tests/option_mut_non_mut_err.vv

@@ -0,0 +1,7 @@
+fn main() {
+	option_1 := ?int(10)
+	if mut option_1 != none {
+		option_1 = 11
+	}
+	println('Options: ${option_1}')
+}