checker: add error for for x in t {, where t.next() returns multiple values

(Fix #9459).

vlib/v/checker/for.v

@@ -61,6 +61,10 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 			if !next_fn.return_type.has_flag(.optional) {
 				c.error('iterator method `next()` must return an optional', node.cond.position())
 			}
+			return_sym := c.table.sym(next_fn.return_type)
+			if return_sym.kind == .multi_return {
+				c.error('iterator method `next()` must not return multiple values', node.cond.position())
+			}
 			// the receiver
 			if next_fn.params.len != 1 {
 				c.error('iterator method `next()` must have 0 parameters', node.cond.position())

vlib/v/checker/tests/for_in_iterator_returning_multiple_values_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/for_in_iterator_returning_multiple_values_err.vv:8:10: error: iterator method `next()` must not return multiple values
+    6 | 
+    7 | t := TestStruct{}
+    8 | for x in t {
+      |          ^
+    9 |     println(x) // ('foo', 'bar')
+   10 |     break

vlib/v/checker/tests/for_in_iterator_returning_multiple_values_err.vv

@@ -0,0 +1,11 @@
+struct TestStruct {}
+
+fn (t TestStruct) next() ?(string, string) {
+	return 'foo', 'bar'
+}
+
+t := TestStruct{}
+for x in t {
+	println(x) // ('foo', 'bar')
+	break
+}