checker: disallow Optional and Result high val in a `for x in lo…

…w..high {` loop (#21043)

vlib/v/checker/for.v

@@ -57,6 +57,9 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 				node.cond.pos().extend(node.high.pos()))
 		} else if typ_idx !in ast.integer_type_idxs {
 			c.error('range type can only be an integer type', node.cond.pos().extend(node.high.pos()))
+		} else if high_type.has_option_or_result() {
+			c.error('the `high` value in a `for x in low..high {` loop, cannot be Result or Option',
+				node.high.pos())
 		}
 		if high_type in [ast.int_type, ast.int_literal_type] {
 			node.val_type = typ

vlib/v/checker/tests/for_in_range_result_optional_err.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/for_in_range_result_optional_err.vv:11:23: error: the `high` value in a `for x in low..high {` loop, cannot be Result or Option
+    9 | fn main() {
+   10 |     arr := [1, 2, 3, 4, 5, 6, 7, 8, 9]
+   11 |     for _ in 0 .. arrays.idx_min(arr) {
+      |                          ~~~~~~~~~~~~
+   12 |     }
+   13 |
+vlib/v/checker/tests/for_in_range_result_optional_err.vv:14:16: error: the `high` value in a `for x in low..high {` loop, cannot be Result or Option
+   12 |     }
+   13 | 
+   14 |     for _ in 0 .. foo() {
+      |                   ~~~~~
+   15 |     }
+   16 | }

vlib/v/checker/tests/for_in_range_result_optional_err.vv

@@ -0,0 +1,16 @@
+module main
+
+import arrays
+
+fn foo() ?int {
+	return 12
+}
+
+fn main() {
+	arr := [1, 2, 3, 4, 5, 6, 7, 8, 9]
+	for _ in 0 .. arrays.idx_min(arr) {
+	}
+
+	for _ in 0 .. foo() {
+	}
+}