typing.Annotated does not flatten through typing.TypeAliasType

[vberlier]

Bug report

Bug description:

As described in the documentation, nested Annotated types are supposed to be flattened. This works well with legacy type aliases but not with the type statement introduced in python 3.12:

from typing import Annotated, TypeAlias

print(Annotated[Annotated[int, "A"], "B"] == Annotated[int, "A", "B"]) # True

A: TypeAlias = Annotated[int, "A"]
print(Annotated[A, "B"] == Annotated[int, "A", "B"]) # True

type A = Annotated[int, "A"]
print(Annotated[A, "B"] == Annotated[int, "A", "B"]) # False

CPython versions tested on:

3.13

Operating systems tested on:

Windows

Linked PRs

• gh-130117: Document why nested Union, Literal, and Annotated types referenced through a type alias are not flattened #130119

[JelleZijlstra]

This behavior is correct and we will not change it. Flattening through TypeAliasType may force evaluation of the type alias and lead to confusing errors.

I'd welcome a documentation PR clarifying this.

[Viicos]

Note: The typing-inspection library provides utilities to help in this cases:

• typing_inspection.introspection.inspect_annotation(): can be used to parse type aliases inside Annotated.
• typing_inspection.introspection.get_literal_values(): can be used to parse type aliases inside Literal.