Change error conditions

crates/ty_python_semantic/resources/mdtest/loops/for.md

@@ -362,6 +362,45 @@ def _(x: Sequence[int], y: object):
             reveal_type(item)  # revealed: int
 ```
 
+## Intersection where some elements are not iterable
+
+When iterating over an intersection type, we should only fail if all positive elements fail to
+iterate. If some elements are iterable and some are not, we should iterate over the iterable ones
+and intersect their element types.
+
+```py
+from ty_extensions import Intersection
+
+class NotIterable:
+    pass
+
+def _(x: Intersection[list[int], NotIterable]):
+    # `list[int]` is iterable (yielding `int`), but `NotIterable` is not.
+    # We should still be able to iterate over the intersection.
+    for item in x:
+        reveal_type(item)  # revealed: int
+```
+
+## Intersection where all elements are not iterable
+
+When iterating over an intersection type where all positive elements are not iterable, we should
+fail to iterate.
+
+```py
+from ty_extensions import Intersection
+
+class NotIterable1:
+    pass
+
+class NotIterable2:
+    pass
+
+def _(x: Intersection[NotIterable1, NotIterable2]):
+    # error: [not-iterable]
+    for item in x:
+        reveal_type(item)  # revealed: Unknown
+```
+
 ## Possibly-not-callable `__iter__` method
 
 ```py

crates/ty_python_semantic/src/types.rs

@@ -6615,11 +6615,19 @@ impl<'db> Type<'db> {
                 Type::Intersection(intersection) => {
                     // For intersections, we iterate over each positive element and intersect
                     // the resulting element types. Negative elements don't affect iteration.
-                    let mut elements_iter = intersection.positive_elements_or_object(db);
-                    let first_element_spec = elements_iter.next()?.try_iterate_with_mode(db, EvaluationMode::Sync).ok()?;
-                    let mut builder = TupleSpecBuilder::from(&*first_element_spec);
-                    for element in elements_iter {
-                        builder = builder.intersect(db, &*element.try_iterate_with_mode(db, EvaluationMode::Sync).ok()?);
+                    // We only fail if all elements fail to iterate; as long as at least one
+                    // element can be iterated over, we can produce a result.
+                    let mut specs_iter = intersection
+                        .positive_elements_or_object(db)
+                        .filter_map(|element| {
+                            element
+                                .try_iterate_with_mode(db, EvaluationMode::Sync)
+                                .ok()
+                        });
+                    let first_spec = specs_iter.next()?;
+                    let mut builder = TupleSpecBuilder::from(&*first_spec);
+                    for spec in specs_iter {
+                        builder = builder.intersect(db, &spec);
                     }
                     Some(Cow::Owned(builder.build()))
                 }