checker: check if next() method infers generic type correctly (fix #23927) (#23932)

vlib/v/checker/for.v

@@ -156,6 +156,13 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 			node.cond_type = typ
 			node.kind = sym.kind
 			node.val_type = val_type
+			if node.val_type.has_flag(.generic) {
+				if c.table.sym(c.unwrap_generic(node.val_type)).kind == .any {
+					c.add_error_detail('type parameters defined by `next()` method should be bounded by method owner type')
+					c.error('cannot infer from generic type `${c.table.get_type_name(c.unwrap_generic(node.val_type))}`',
+						node.vv_pos)
+				}
+			}
 			node.scope.update_var_type(node.val_var, val_type)
 
 			if is_comptime {

vlib/v/checker/tests/struct_iter_generic_invalid_infer_err.out

@@ -0,0 +1,8 @@
+vlib/v/checker/tests/struct_iter_generic_invalid_infer_err.vv:25:6: error: cannot infer from generic type `K`
+   23 |     }
+   24 | 
+   25 |     for squared in iter {
+      |         ~~~~~~~
+   26 |         println(squared)
+   27 |     }
+Details: type parameters defined by `next()` method should be bounded by method owner type

vlib/v/checker/tests/struct_iter_generic_invalid_infer_err.vv

@@ -0,0 +1,28 @@
+module main
+
+struct SquareIterator {
+	arr []int
+mut:
+	idx int
+}
+
+fn (mut iter SquareIterator) next[K]() ?K {
+	if iter.idx >= iter.arr.len {
+		return none
+	}
+	defer {
+		iter.idx++
+	}
+	return iter.arr[iter.idx] * iter.arr[iter.idx]
+}
+
+fn main() {
+	nums := [1, 2, 3, 4, 5]
+	iter := SquareIterator{
+		arr: nums
+	}
+
+	for squared in iter {
+		println(squared)
+	}
+}