[ty] Add attribute assignment tests for unions

Author: thejchap

crates/ty_python_semantic/resources/mdtest/attributes.md

@@ -1628,6 +1628,86 @@ date.year = 2025
 date.tz = "UTC"
 ```
 
+### Setting attributes on unions
+
+Setting attributes on unions where all elements of the union have the attribute is acceptable
+
+```py
+from typing import Union
+
+class A:
+    x: int
+
+class B:
+    x: int
+
+C = Union[A, B]
+
+a: C = A()
+a.x = 42
+```
+
+Setting attributes on unions where any element of the union does not have the attribute reports
+possibly unbound
+
+```py
+from typing import Union, Sequence
+
+class A:
+    pass
+
+class B:
+    x: int
+
+C = Union[A, B]
+
+def _(a: C):
+    a.x = 42  # TODO: error: [possibly-unbound-attribute]
+```
+
+The same goes for
+
+```py
+from dataclasses import dataclass
+from typing import Union, Sequence
+from abc import ABC
+
+class Base(ABC):
+    x: Sequence[bytes] = ()
+
+class Derived(Base):
+    pass
+
+class Other:
+    pass
+
+D = Union[Derived, Other]
+
+d: D = Other()
+
+# TODO: error: [possibly-unbound-attribute]
+# error: [unresolved-attribute]
+d.x = None
+```
+
+Setting attributes on a generic where the upper bound is a union, and not all elements of the union
+have the attribute, also reports possibly unbound:
+
+```py
+from typing import Union, TypeVar
+
+class A:
+    pass
+
+class B:
+    x: int
+
+C = TypeVar("C", bound=Union[A, B])
+
+def _(a: C):
+    a.x = 42  # error: [possibly-unbound-attribute]
+```
+
 ### `argparse.Namespace`
 
 A standard library example of a class with a custom `__setattr__` method is `argparse.Namespace`: