[Moussa-Mikhail]

fixes #22181

This PR expands type aliases in annotations of self and cls so that is_expected_self_type can handle them like any other annotation. It also refactors some repetitive if conditions by early returning when they aren't true. Using mypy's own type substitution means type aliases with strange shapes (e.g. type Extra[T, S] = T, UnionSame[T] = T | T) work without special-casing them.

The code in this PR was written by Claude and reviewed by me.

[chrikrah]

Requesting changes for one crash. The expansion substitutes a plain Self for every alias parameter, and a TypeVarTuple parameter needs an unpacked argument, so this input ends in an INTERNAL ERROR where main prints two ordinary errors.

$ cat self3.py
from typing import Self
type Vt[*Ts] = tuple[*Ts]
class C:
    def vt(self: Vt[Self]) -> None: ...
$ python -m mypy --python-version 3.12 --no-incremental --show-traceback self3.py   # head 5a326cd6c
self3.py:4: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
RuntimeError: Invalid type replacement to expand: Self?
$ <same command>   # base f501cd0b7
self3.py:4: error: Method cannot have explicit self annotation and Self type  [misc]
self3.py:4: error: The erased type of self "tuple[Any]" is not a supertype of its class "self3.C"  [misc]
$ python -m pytest mypy/test/testcheck.py -n 4 -q -k "SelfType or selftype or Self or Alias or TypeAlias"   # Python 3.12.6
648 passed, 1 skipped, 1 xfailed       # base
651 passed, 1 skipped, 1 xfailed       # head
2 failed, 1 passed                     # head with semanal.py reverted, -k testTypingSelfIdentityTypeAlias

blocking, @Moussa-Mikhail: add or not all(isinstance(tv, TypeVarType) for tv in node.alias_tvars) to the early return in expand_type_alias_for_self_check. With that line self3.py prints the base errors again and the same selection still gives 651 passed.

non-blocking: the UnionSame case in testTypingSelfIdentityTypeAliasSubstitution can go. It pins self: Self | Self in a reveal, which nobody writes, and a later union simplification would break it.

Excavation: #14041 added the error to keep Self and an explicit self annotation apart, and #21928 carved out Annotated[Self, ...]. This extends the carve-out to aliases that collapse to Self, so the old intent still holds.

@ilevkivskyi the general expansion also covers the Anno[T] alias from the issue without a second special case, so it stays. Is the general expansion acceptable for this PR?

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅