EXPERIMENT: skip simplifying Unions of TypedDicts

Author: oconnor663

crates/ty_python_semantic/resources/mdtest/typed_dict.md

@@ -841,10 +841,10 @@ def _(
 
     reveal_type(being["name"])  # revealed: str
 
-    # TODO: A type of `int | None | Unknown` might be better here. `str` is because
-    # `Person | Animal` reduces to `Animal`, and `Animal.__getitem__` can only return `str`.
+    # TODO: A type of `int | None | Unknown` might be better here. The `str` is mixed in
+    # because `Animal.__getitem__` can only return `str`.
     # error: [invalid-key] "Unknown key "age" for TypedDict `Animal`"
-    reveal_type(being["age"])  # revealed: str
+    reveal_type(being["age"])  # revealed: int | None | str
 ```
 
 ### Writing
@@ -1115,7 +1115,8 @@ def combine(p: Person, e: Employee):
     reveal_type(p | p)  # revealed: Person
     reveal_type(e | e)  # revealed: Employee
 
-    reveal_type(p | e)  # revealed: Person
+    # EXPERIMENT: Simplification of TypedDicts in Unions is disabled.
+    reveal_type(p | e)  # revealed: Person | Employee
 ```
 
 When inheriting from a `TypedDict` with a different `total` setting, inherited fields maintain their

crates/ty_python_semantic/src/types/builder.rs

@@ -463,10 +463,11 @@ impl<'db> UnionBuilder<'db> {
             None
         };
 
-        // If an alias gets here, it means we aren't unpacking aliases, and we also
-        // shouldn't try to simplify aliases out of the union, because that will require
-        // unpacking them.
-        let should_simplify_full = !matches!(ty, Type::TypeAlias(_));
+        // If an alias gets here, it means we aren't unpacking aliases, and we also shouldn't try
+        // to simplify aliases out of the union, because that will require unpacking them.
+        // `TypedDict`s also run into trouble here when their fields refer to a recursive union, so
+        // and we need to skip simplification to break cycles.
+        let should_simplify_full = !matches!(ty, Type::TypeAlias(_) | Type::TypedDict(_));
 
         let mut to_remove = SmallVec::<[usize; 2]>::new();
         let ty_negated = if should_simplify_full {

crates/ty_python_semantic/src/types/class.rs

@@ -565,7 +565,7 @@ impl<'db> ClassType<'db> {
                     TypeRelation::Assignability => ConstraintSet::from(!other.is_final(db)),
                 },
 
-                // Protocol and Generic are not represented by a ClassType.
+                // Protocol, Generic, and TypedDict are not represented by a ClassType.
                 ClassBase::Protocol | ClassBase::Generic | ClassBase::TypedDict => {
                     ConstraintSet::from(false)
                 }