Apply TypeVar defaults to functions (PEP 696)

[cdce8p]

Use TypeVar defaults to resolve fallback return type of functions.

Note: Defaults for TypeVarTuples don't yet work, probably a result of the limited support for Unpack / TypeVarTuple.

Ref: #14851

[cdce8p]

@hauntsaninja Any idea why the primer comment job is failing? The primer run with artifact looks fine
Primer: https://github.com/python/mypy/actions/runs/5198160649
Comment: https://github.com/python/mypy/actions/runs/5198345718/jobs/9374346954

This seems to have started last night. Strangely enough, everything continues to work fine on my fork with the same config.
It seems the listWorkflowRunArtifacts endpoint might be an issue, but calling it manually works fine 🤷🏻‍♂️

--
Update: Seems the issue resolved itself.

[cdce8p]

@JelleZijlstra This is the next one for PEP 696. Fairly small. The primer result is as expected.

[JelleZijlstra]

I think we actually don't want defaults to be used for functions? https://peps.python.org/pep-0696/#function-defaults

cc @Gobot1234

[Gobot1234]

I think the best way to deal with defaults in functions is to treat them as just normal generic functions without a default (for now) just for reusability and the error should be when dealing with unions etc. (pyright here has the correct behaviour)

[cdce8p]

I actually used the pyright test case here to implement it. Basically the default is ignored unless mypy would return Any.
https://github.com/microsoft/pyright/blob/51d2ff8beb19b60e21732a11e75bb5c6514e62ac/packages/pyright-internal/src/tests/samples/typeVarDefaultFunction1.py

I think the best way to deal with defaults in functions is to treat them as just normal generic functions without a default (for now) just for reusability and the error should be when dealing with unions etc. (pyright here has the correct behaviour)

Mypy detects this as well

Incompatible return value type (got "Union[str, T1]", expected "T1")

[cdce8p]

Moved the validation portion to a separate PR #15442 as that likely uncontroversial.

Although I still think, this PR is the correct move as it matches mypy prior behavior and the way pyright handles it. The default is only used as fallback / last resort instead of Any for functions.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

steam.py (https://github.com/Gobot1234/steam.py)
- steam/id.py:576: error: Need type annotation for "id"  [var-annotated]
- steam/app.py:195: error: Need type annotation for "app"  [var-annotated]
- steam/app.py:207: error: Need type annotation for "licenses"  [var-annotated]
- steam/app.py:1435: error: Need type annotation for "partial_dlc"  [var-annotated]
- steam/app.py:1447: error: List comprehension has incompatible type List[PartialPackage[<nothing>]]; expected List[FetchedAppPackage]  [misc]
+ steam/app.py:1447: error: List comprehension has incompatible type List[PartialPackage[str | None]]; expected List[FetchedAppPackage]  [misc]
- steam/package.py:357: error: Need type annotation for "master_package"  [var-annotated]
- steam/game_server.py:319: error: Need type annotation for "app"  [var-annotated]
- steam/profile.py:119: error: Need type annotation for "app"  [var-annotated]
- steam/profile.py:144: error: Need type annotation for "msg"  [var-annotated]
- steam/manifest.py:552: error: Need type annotation for "app"  [var-annotated]
- steam/manifest.py:967: error: Need type annotation for "parent"  [var-annotated]
- steam/manifest.py:1126: error: Need type annotation for "_apps"  [var-annotated]
+ steam/manifest.py:1158: error: Incompatible return value type (got "list[PartialApp[str | None]]", expected "list[PartialApp[None]] | list[PartialApp[str]]")  [return-value]
- steam/user.py:266: error: Need type annotation for "new_message"  [var-annotated]
- steam/published_file.py:210: error: Need type annotation for "created_with"  [var-annotated]
- steam/event.py:77: error: Need type annotation for "app"  [var-annotated]
- steam/event.py:276: error: Incompatible types in assignment (expression has type "App[Any] | None", variable has type "PartialApp[Any] | None")  [assignment]
+ steam/event.py:276: error: Incompatible types in assignment (expression has type "App[Any] | PartialApp[str | None] | None", variable has type "PartialApp[str | None] | None")  [assignment]
- steam/clan.py:542: error: Need type annotation for "event"  [var-annotated]
- steam/state.py:304: error: Need type annotation for "steam_id"  [var-annotated]
- steam/state.py:304: error: Argument "type" to "ID" has incompatible type "int"; expected "None"  [arg-type]
+ steam/state.py:304: error: Argument "type" to "ID" has incompatible type "int"; expected "Type | None"  [arg-type]
- steam/state.py:617: error: Need type annotation for "msg"  [var-annotated]
- steam/state.py:677: error: Need type annotation for "msg"  [var-annotated]
- steam/state.py:700: error: Need type annotation for "msg"  [var-annotated]
- steam/state.py:721: error: Need type annotation for "msg"  [var-annotated]
- steam/state.py:731: error: Need type annotation for "msg"  [var-annotated]
- steam/state.py:739: error: Need type annotation for "msg"  [var-annotated]
- steam/state.py:777: error: Need type annotation for "msg"  [var-annotated]
- steam/state.py:788: error: Need type annotation for "msg"  [var-annotated]
- steam/state.py:801: error: Need type annotation for "msg"  [var-annotated]
- steam/state.py:1388: error: Need type annotation for "id"  [var-annotated]
- steam/state.py:1430: error: Incompatible types in assignment (expression has type "ID[Any]", variable has type "Clan")  [assignment]
+ steam/state.py:1430: error: Incompatible types in assignment (expression has type "Clan | ID[Type]", variable has type "Clan")  [assignment]
- steam/state.py:1432: error: Incompatible types in assignment (expression has type "ID[Any]", variable has type "Clan")  [assignment]
+ steam/state.py:1432: error: Incompatible types in assignment (expression has type "ID[Type]", variable has type "Clan")  [assignment]
- steam/state.py:1720: error: Need type annotation for "msg"  [var-annotated]
- steam/state.py:1813: error: Need type annotation for "msg"  [var-annotated]
- steam/state.py:1852: error: Need type annotation for "msg"  [var-annotated]
- steam/state.py:1872: error: Need type annotation for "steam_id"  [var-annotated]
- steam/state.py:1915: error: Need type annotation for "license_"  [var-annotated]
- steam/state.py:2352: error: Need type annotation for "msg"  [var-annotated]
- steam/state.py:2359: error: Need type annotation for "msg"  [var-annotated]
- steam/state.py:2377: error: Need type annotation for "msg"  [var-annotated]
- steam/state.py:2388: error: Need type annotation for "msg"  [var-annotated]
- steam/state.py:2398: error: Need type annotation for "msg"  [var-annotated]
- steam/state.py:2418: error: Need type annotation for "msg"  [var-annotated]
- steam/bundle.py:98: error: Need type annotation for "_apps"  [var-annotated]
- steam/bundle.py:99: error: Need type annotation for "_packages"  [var-annotated]
- steam/store.py:75: error: Need type annotation for "package"  [var-annotated]
- steam/store.py:76: error: Need type annotation for "bundle"  [var-annotated]
- steam/store.py:171: error: Need type annotation for "_apps"  [var-annotated]
- steam/store.py:186: error: Need type annotation for "related_parent_app"  [var-annotated]
- steam/store.py:327: error: Need type annotation for "package"  [var-annotated]
- steam/client.py:572: error: Need type annotation for "steam_id"  [var-annotated]
- steam/client.py:572: error: Argument "type" to "ID" has incompatible type "int"; expected "None"  [arg-type]
+ steam/client.py:572: error: Argument "type" to "ID" has incompatible type "int"; expected "Type | None"  [arg-type]
- steam/client.py:646: error: Need type annotation for "steam_id"  [var-annotated]
- steam/client.py:646: error: Argument "type" to "ID" has incompatible type "int"; expected "None"  [arg-type]
+ steam/client.py:646: error: Argument "type" to "ID" has incompatible type "int"; expected "Type | None"  [arg-type]
- steam/client.py:658: error: Need type annotation for "steam_id"  [var-annotated]
- steam/client.py:658: error: Argument "type" to "ID" has incompatible type "int"; expected "None"  [arg-type]
+ steam/client.py:658: error: Argument "type" to "ID" has incompatible type "int"; expected "Type | None"  [arg-type]
- steam/client.py:1154: error: Need type annotation for "trades"  [var-annotated]

[hauntsaninja]

Thanks for working on this!