Use class name as namespace for type variables

[JukkaL]

This avoids confusion between type variables of two classes, which can
happen at least in some edge cases. Type variables are only the same if
both the numeric id and namespace match (plus meta level).

Fixes #12588 (though the textual presentation of type variables doesn't
take the namespace into consideration yet).

[github-actions]

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

websockets (https://github.com/aaugustin/websockets)
+ src/websockets/legacy/protocol.py:687: error: Argument 1 to "__anext__" of "AsyncIterator" has incompatible type "AsyncIterator[_T_co]"; expected "AsyncIterator[_T_co]"

xarray (https://github.com/pydata/xarray)
- xarray/core/alignment.py:534: error: Argument 1 to "_reindex_one" of "Aligner" has incompatible type "Tuple[DataAlignable, Dict[Tuple[Tuple[Tuple[Hashable, Tuple[Hashable, ...]], ...], Type[Index]], Index]]"; expected "DataAlignable"  [arg-type]

Tanjun (https://github.com/FasterSpeeding/Tanjun)
- tanjun/abc.py:2860: error: Signature of "copy" incompatible with supertype "ExecutableCommand"  [override]
- tanjun/abc.py:2860: note:      Superclass:
- tanjun/abc.py:2860: note:          def copy(self) -> MessageCommand[MessageContext]
- tanjun/abc.py:2860: note:      Subclass:
- tanjun/abc.py:2860: note:          def copy(self, *, parent: Optional[MessageCommandGroup[Any]] = ...) -> MessageCommand[_CommandCallbackSigT]
- tanjun/commands/slash.py:1272: error: Return type "SlashCommand[_CommandCallbackSigT]" of "bind_client" incompatible with return type "SlashCommand[SlashContext]" in supertype "PartialCommand"  [override]
- tanjun/commands/slash.py:1272: error: Return type "SlashCommand[_CommandCallbackSigT]" of "bind_client" incompatible with return type "SlashCommand[SlashContext]" in supertype "ExecutableCommand"  [override]
- tanjun/commands/slash.py:2454: error: Signature of "copy" incompatible with supertype "PartialCommand"  [override]
- tanjun/commands/slash.py:2454: note:      Superclass:
- tanjun/commands/slash.py:2454: note:          def copy(self, *, _new: bool = ...) -> SlashCommand[SlashContext]
- tanjun/commands/slash.py:2454: note:      Subclass:
- tanjun/commands/slash.py:2454: note:          def copy(self, *, _new: bool = ..., parent: Optional[SlashCommandGroup] = ...) -> SlashCommand[_CommandCallbackSigT]
- tanjun/commands/slash.py:2454: error: Signature of "copy" incompatible with supertype "ExecutableCommand"  [override]
- tanjun/commands/slash.py:2454: note:          def copy(self) -> SlashCommand[SlashContext]
- tanjun/commands/slash.py:2454: note:          def copy(self, *, _new: bool = ..., parent: Optional[SlashCommandGroup] = ...) -> SlashCommand[_CommandCallbackSigT]
- tanjun/commands/message.py:249: error: Return type "MessageCommand[_CommandCallbackSigT]" of "bind_client" incompatible with return type "MessageCommand[MessageContext]" in supertype "PartialCommand"  [override]
- tanjun/commands/message.py:249: error: Return type "MessageCommand[_CommandCallbackSigT]" of "bind_client" incompatible with return type "MessageCommand[MessageContext]" in supertype "ExecutableCommand"  [override]
- tanjun/commands/message.py:257: error: Return type "MessageCommand[_CommandCallbackSigT]" of "bind_component" incompatible with return type "MessageCommand[MessageContext]" in supertype "PartialCommand"  [override]
- tanjun/commands/message.py:257: error: Return type "MessageCommand[_CommandCallbackSigT]" of "bind_component" incompatible with return type "MessageCommand[MessageContext]" in supertype "ExecutableCommand"  [override]
- tanjun/commands/message.py:265: error: Signature of "copy" incompatible with supertype "PartialCommand"  [override]
- tanjun/commands/message.py:265: note:      Superclass:
- tanjun/commands/message.py:265: note:          def copy(self, *, _new: bool = ...) -> MessageCommand[MessageContext]
- tanjun/commands/message.py:265: note:      Subclass:
- tanjun/commands/message.py:265: note:          def copy(self, *, parent: Optional[MessageCommandGroup[Any]] = ..., _new: bool = ...) -> MessageCommand[_CommandCallbackSigT]
- tanjun/commands/message.py:265: error: Signature of "copy" incompatible with supertype "ExecutableCommand"  [override]
- tanjun/commands/message.py:265: note:          def copy(self) -> MessageCommand[MessageContext]
- tanjun/commands/message.py:265: note:          def copy(self, *, parent: Optional[MessageCommandGroup[Any]] = ..., _new: bool = ...) -> MessageCommand[_CommandCallbackSigT]
- tanjun/commands/message.py:423: error: Signature of "copy" incompatible with supertype "PartialCommand"  [override]
- tanjun/commands/message.py:423: note:      Superclass:
- tanjun/commands/message.py:423: note:          def copy(self, *, _new: bool = ...) -> MessageCommandGroup[MessageContext]
- tanjun/commands/message.py:423: note:      Subclass:
- tanjun/commands/message.py:423: note:          def copy(self, *, parent: Optional[MessageCommandGroup[Any]] = ..., _new: bool = ...) -> MessageCommandGroup[_CommandCallbackSigT]
- tanjun/commands/message.py:423: error: Signature of "copy" incompatible with supertype "ExecutableCommand"  [override]
- tanjun/commands/message.py:423: note:          def copy(self) -> MessageCommandGroup[MessageContext]
- tanjun/commands/message.py:423: note:          def copy(self, *, parent: Optional[MessageCommandGroup[Any]] = ..., _new: bool = ...) -> MessageCommandGroup[_CommandCallbackSigT]
- tanjun/commands/message.py:493: error: Return type "MessageCommandGroup[_CommandCallbackSigT]" of "bind_client" incompatible with return type "MessageCommandGroup[MessageContext]" in supertype "PartialCommand"  [override]
- tanjun/commands/message.py:493: error: Return type "MessageCommandGroup[_CommandCallbackSigT]" of "bind_client" incompatible with return type "MessageCommandGroup[MessageContext]" in supertype "ExecutableCommand"  [override]
- tanjun/commands/message.py:501: error: Return type "MessageCommandGroup[_CommandCallbackSigT]" of "bind_component" incompatible with return type "MessageCommandGroup[MessageContext]" in supertype "PartialCommand"  [override]
- tanjun/commands/message.py:501: error: Return type "MessageCommandGroup[_CommandCallbackSigT]" of "bind_component" incompatible with return type "MessageCommandGroup[MessageContext]" in supertype "ExecutableCommand"  [override]
- tanjun/commands/menu.py:448: error: Return type "MenuCommand[_MenuCommandCallbackSigT, _MenuTypeT]" of "set_tracked_command" incompatible with return type "MenuCommand[MenuContext, _MenuTypeT]" in supertype "AppCommand"  [override]
- tanjun/commands/menu.py:484: error: Signature of "copy" incompatible with supertype "PartialCommand"  [override]
- tanjun/commands/menu.py:484: note:      Superclass:
- tanjun/commands/menu.py:484: note:          def copy(self, *, _new: bool = ...) -> MenuCommand[MenuContext, _MenuTypeT]
- tanjun/commands/menu.py:484: note:      Subclass:
- tanjun/commands/menu.py:484: note:          def copy(self, *, _new: bool = ..., parent: Optional[SlashCommandGroup] = ...) -> MenuCommand[_MenuCommandCallbackSigT, _MenuTypeT]
- tanjun/commands/menu.py:484: error: Signature of "copy" incompatible with supertype "ExecutableCommand"  [override]
- tanjun/commands/menu.py:484: note:          def copy(self) -> MenuCommand[MenuContext, _MenuTypeT]
- tanjun/commands/menu.py:484: note:          def copy(self, *, _new: bool = ..., parent: Optional[SlashCommandGroup] = ...) -> MenuCommand[_MenuCommandCallbackSigT, _MenuTypeT]

[JelleZijlstra]

The mypy-primer output looks suspicious, though I haven't investigated in detail. This error message won't be fun for anyone: + src/websockets/legacy/protocol.py:687: error: Argument 1 to "__anext__" of "AsyncIterator" has incompatible type "AsyncIterator[_T_co]"; expected "AsyncIterator[_T_co]".

[hauntsaninja]

cc @A5rocks since this PR accomplishes similar things as #11657

[A5rocks]

I think this namespacing is a better approach than my own PR (which essentially mutated the typevars on an existing class to be at least larger than every other in its inheritance tree). Should its test also be carried over, though?

[JukkaL]

It looks like websockets didn't have an error more or less accidentally before (one mypy bug hid another mypy bug). Mypy infers types with unbound type variables (which it shouldn't do), and previously two types with unbound type variables were considered compatible, whereas now there's an error. I'm going to try to improve the error message in a follow-up PR, however (or fix the leaking unbound type variables).

@A5rocks I'll add the test case in a follow-up PR.