[ty] fix PEP 695 type aliases in with statement (#24395)

Author: carljm

crates/ty_python_semantic/resources/mdtest/with/sync.md

@@ -40,6 +40,46 @@ def _(flag: bool):
         reveal_type(f)  # revealed: str | int
 ```
 
+## Type aliases preserve context manager behavior
+
+```toml
+[environment]
+python-version = "3.12"
+```
+
+```py
+from typing import Self, TypeAlias
+from typing_extensions import TypeAliasType
+
+class A:
+    def __enter__(self) -> Self:
+        return self
+
+    def __exit__(self, exc_type, exc_value, traceback) -> None: ...
+
+class B:
+    def __enter__(self) -> Self:
+        return self
+
+    def __exit__(self, exc_type, exc_value, traceback) -> None: ...
+
+UnionAB1: TypeAlias = A | B
+type UnionAB2 = A | B
+UnionAB3 = TypeAliasType("UnionAB3", A | B)
+
+def f1(x: UnionAB1) -> None:
+    with x as y:
+        reveal_type(y)  # revealed: A | B
+
+def f2(x: UnionAB2) -> None:
+    with x as y:
+        reveal_type(y)  # revealed: A | B
+
+def f3(x: UnionAB3) -> None:
+    with x as y:
+        reveal_type(y)  # revealed: A | B
+```
+
 ## Context manager without an `__enter__` or `__exit__` method
 
 ```py

crates/ty_python_semantic/src/types.rs

@@ -3285,6 +3285,10 @@ impl<'db> Type<'db> {
                     .member_lookup_with_policy(db, name, policy)
             }
 
+            Type::TypeAlias(alias) => alias
+                .value_type(db)
+                .member_lookup_with_policy(db, name, policy),
+
             _ if policy.no_instance_fallback() => self.invoke_descriptor_protocol(
                 db,
                 name_str,
@@ -3293,10 +3297,6 @@ impl<'db> Type<'db> {
                 policy,
             ),
 
-            Type::TypeAlias(alias) => alias
-                .value_type(db)
-                .member_lookup_with_policy(db, name, policy),
-
             Type::LiteralValue(literal)
                 if literal.as_enum().is_some()
                     && matches!(name_str, "name" | "_name_" | "value" | "_value_") =>