[ty] Add a new property test: all types assignable to Iterable[object] should be considered iterable (#19186)

Author: AlexWaygood

crates/ty_python_semantic/src/types/property_tests.rs

@@ -218,6 +218,7 @@ mod flaky {
     use itertools::Itertools;
 
     use super::{intersection, union};
+    use crate::types::{KnownClass, Type};
 
     // Negating `T` twice is equivalent to `T`.
     type_property_test!(
@@ -311,4 +312,14 @@ mod flaky {
         bottom_materialization_of_type_is_assigneble_to_type, db,
         forall types t. t.bottom_materialization(db).is_assignable_to(db, t)
     );
+
+    // Any type assignable to `Iterable[object]` should be considered iterable.
+    //
+    // Note that the inverse is not true, due to the fact that we recognize the old-style
+    // iteration protocol as well as the new-style iteration protocol: not all objects that
+    // we consider iterable are assignable to `Iterable[object]`.
+    type_property_test!(
+        all_type_assignable_to_iterable_are_iterable, db,
+        forall types t. t.is_assignable_to(db, KnownClass::Iterable.to_specialized_instance(db, [Type::object(db)])) => t.try_iterate(db).is_ok()
+    );
 }