[ty] Infer subscript types through aliases

crates/ty_python_semantic/resources/mdtest/pep695_type_aliases.md

@@ -172,6 +172,25 @@ def _(x: X, y: tuple[Literal[1], Literal[3]]):
     reveal_type(x < y)  # revealed: Literal[True]
 ```
 
+## In subscripts
+
+Subscript operations should work through type aliases.
+
+```py
+class C:
+    def __getitem__(self, index: int) -> int:
+        return 1
+
+class D:
+    def __getitem__(self, index: int) -> int:
+        return 1
+
+type CD = C | D
+
+def _(x: CD):
+    reveal_type(x[1])  # revealed: int
+```
+
 ## `TypeAliasType` properties
 
 Two `TypeAliasType`s are distinct and disjoint, even if they refer to the same type

crates/ty_python_semantic/src/types/infer/builder.rs

@@ -12368,6 +12368,13 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                 Some(todo_type!("Subscript expressions on intersections"))
             }
 
+            (Type::TypeAlias(alias), _) => Some(self.infer_subscript_expression_types(
+                subscript,
+                alias.value_type(db),
+                slice_ty,
+                expr_context,
+            )),
+
             // Ex) Given `("a", "b", "c", "d")[1]`, return `"b"`
             (Type::NominalInstance(nominal), Type::IntLiteral(i64_int)) => nominal
                 .tuple_spec(db)