mypy: TransportProtocol.undefined types as int, so four base-template defaults disagree with their annotations

[JarryShaw]

Describe the bug

Four [assignment] errors in pcapkit/const/reg/apptype/apptype.py's base-template code, every one
about TransportProtocol.undefined used as a default where a TransportProtocol is annotated. They
were masked by a file-wide # mypy: disable-error-code=assignment pragma until #769 removed it.

Reproduction

$ mypy --follow-imports=silent --ignore-missing-imports --show-column-numbers --show-error-codes \
       pcapkit/const/reg/apptype/apptype.py          # with the pragma
Success: no issues found in 1 source file

$ # same command, pragma removed
:2258:42: error: Incompatible types in assignment (expression has type "int", variable has type "TransportProtocol")  [assignment]
:2282:46: error: Incompatible default for parameter "proto" (default has type "int", parameter has type "TransportProtocol")  [assignment]
:2371:48: error: Incompatible default for parameter "proto" (default has type "int", parameter has type "TransportProtocol | str")  [assignment]
:2429:52: error: Incompatible default for parameter "proto" (default has type "int", parameter has type "TransportProtocol | str")  [assignment]
Found 4 errors in 1 file (checked 1 source file)

Whole-package: 112 errors in 38 files → 116 in 39.

Expected behavior

TransportProtocol.undefined should type as TransportProtocol, so the four defaults agree with
their annotations and no suppression is needed.

Additional context

Root cause isolated: undefined = 0 is a bare int literal, so mypy infers its attribute type as
int, while tcp/udp/sctp/dccp are auto()-valued and infer as Any — which is why only
undefined trips it. mypy has no aenum plugin, so the class is treated as a plain class rather
than an enum. Pre-existing and independent of #769: stripping only the pragma on unmodified
main reproduces all four, and none of the ~12,391 per-member lines contributes one.

The four sites are __new__, _dispatch, get and get_all; the fix belongs in the BASE
template in pcapkit/vendor/reg/apptype/apptype.py, not in the generated file. Whether it is
achievable without an aenum stub is not established.

Not urgent — mypy is advisory in CI (continue-on-error: true in lint.yml).

One follow-up this creates. lint.yml's header pins mypy at 112 errors / 38 files measured at
932cb48d1 (#753, #761). That pin stays honest because it names its commit, but once #769 merges
the current figure is 116/39. Either re-pin it or note the delta here, whichever the maintainer
prefers — flagging it so the two PRs do not quietly leave each other stale.

[JarryShaw]

Blocked on #769, which is open and owns pcapkit/vendor/reg/apptype/apptype.py — the BASE template where the fix for these four errors belongs. #769 is also what unmasks them, by removing the file-wide # mypy: disable-error-code=assignment pragma, so this issue cannot be actioned before it lands.

Unblocks when #769 merges. Note the follow-up recorded in the body: lint.yml's mypy pin moves 112/38 -> 116/39 at that point.

[JarryShaw]

Correction to the site list above — one of the four is misattributed, and it is my error.

apptype.py:2258 is not _dispatch. It is the class-level attribute

__transport__: 'TransportProtocol' = TransportProtocol.undefined

so it is an attribute assignment rather than a default argument, which is also why its message reads "Incompatible types in assignment" while the other three read "Incompatible default for parameter". _dispatch has no proto default and contributes nothing here. The correct four sites are __transport__ (:2258), __new__ (:2282), get (:2371) and get_all (:2429).

One further measurement worth recording: the pragma removal is wider than #744 requires. The per-member annotation lives only in the crawler's TRANSPORT template — the base module has had no members since #754 — so keeping the pragma in the base module while removing it from TRANSPORT returns mypy to exactly Found 112 errors in 38 files (checked 506 source files). That is a live minimal alternative to fixing the inference, measured clean. It does not change the ruling already given (option 1, leave them visible), but it belongs on the record here.

[JarryShaw]

Unblocked — #769 merged as 83b58ebda, so pcapkit/vendor/reg/apptype/apptype.py (the BASE template, where the fix belongs) is free.

The four errors are now live on main: whole-package mypy is 116 in 39 files, up from 112/38, because #769 removed the file-wide # mypy: disable-error-code=assignment. Two measured notes carried over from the review. The site list in the body is corrected in the comment above — :2258 is the class attribute __transport__: 'TransportProtocol' = TransportProtocol.undefined, not _dispatch. And there is a minimal alternative that measures clean: keeping the pragma in the base module alone, while leaving it off the four per-transport modules, restores Found 112 errors in 38 files (checked 506 source files) exactly — because the per-member annotation only ever lived in the TRANSPORT template. The maintainer ruled (option 1) to leave the errors visible rather than re-suppress, so that alternative is recorded, not recommended.

Also still open here: lint.yml pins mypy at 112/38 measured at 932cb48d1. That pin stays literally true because it names its commit, but the live figure is now 116/39.