Fix descriptor access for complex generics#21506
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Use the unbound __get__ type and call it with all three arguments (self, instance, owner) so that type inference can jointly solve all type variables. The previous approach called bind_self first and then passed only two arguments, which failed when the __get__ self annotation contained type variables shared with the instance parameter.
get_method returns FuncBase | Decorator; unwrap Decorator.func before passing to function_type (which expects FuncBase). Annotate dunder_get_type as Type so the transform_callee_type (-> Type) assignment is compatible. Fixes python#21505
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Diff from mypy_primer, showing the effect of this PR on open source code: steam.py (https://github.com/Gobot1234/steam.py)
- steam/chat.py:402: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[Chat[ChatMessageT, ClanT, GroupT]]"; expected "type[Never]" [arg-type]
+ steam/chat.py:402: error: Argument 3 to "__get__" of "classproperty" has incompatible type "type[Chat[ChatMessageT, ClanT, GroupT]]"; expected "type[Never]" [arg-type]
- steam/chat.py:589: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[ChatGroup[MemberT, ChatT, ChatGroupTypeT]]"; expected "type[Never]" [arg-type]
+ steam/chat.py:589: error: Argument 3 to "__get__" of "classproperty" has incompatible type "type[ChatGroup[MemberT, ChatT, ChatGroupTypeT]]"; expected "type[Never]" [arg-type]
- steam/chat.py:608: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[ChatGroup[MemberT, ChatT, ChatGroupTypeT]]"; expected "type[Never]" [arg-type]
+ steam/chat.py:608: error: Argument 3 to "__get__" of "classproperty" has incompatible type "type[ChatGroup[MemberT, ChatT, ChatGroupTypeT]]"; expected "type[Never]" [arg-type]
- steam/chat.py:643: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[ChatGroup[MemberT, ChatT, ChatGroupTypeT]]"; expected "type[Never]" [arg-type]
+ steam/chat.py:643: error: Argument 3 to "__get__" of "classproperty" has incompatible type "type[ChatGroup[MemberT, ChatT, ChatGroupTypeT]]"; expected "type[Never]" [arg-type]
- steam/chat.py:688: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[ChatGroup[MemberT, ChatT, ChatGroupTypeT]]"; expected "type[Never]" [arg-type]
+ steam/chat.py:688: error: Argument 3 to "__get__" of "classproperty" has incompatible type "type[ChatGroup[MemberT, ChatT, ChatGroupTypeT]]"; expected "type[Never]" [arg-type]
- steam/chat.py:696: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[ChatGroup[MemberT, ChatT, ChatGroupTypeT]]"; expected "type[Never]" [arg-type]
+ steam/chat.py:696: error: Argument 3 to "__get__" of "classproperty" has incompatible type "type[ChatGroup[MemberT, ChatT, ChatGroupTypeT]]"; expected "type[Never]" [arg-type]
- steam/chat.py:869: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[ChatGroup[MemberT, ChatT, ChatGroupTypeT]]"; expected "type[Never]" [arg-type]
+ steam/chat.py:869: error: Argument 3 to "__get__" of "classproperty" has incompatible type "type[ChatGroup[MemberT, ChatT, ChatGroupTypeT]]"; expected "type[Never]" [arg-type]
- steam/state.py:1473: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[GroupChannel]"; expected "type[Never]" [arg-type]
+ steam/state.py:1473: error: Argument 3 to "__get__" of "classproperty" has incompatible type "type[GroupChannel]"; expected "type[Never]" [arg-type]
- steam/state.py:1473: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[ClanChannel]"; expected "type[Never]" [arg-type]
+ steam/state.py:1473: error: Argument 3 to "__get__" of "classproperty" has incompatible type "type[ClanChannel]"; expected "type[Never]" [arg-type]
- steam/state.py:1512: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[GroupChannel]"; expected "type[Never]" [arg-type]
+ steam/state.py:1512: error: Argument 3 to "__get__" of "classproperty" has incompatible type "type[GroupChannel]"; expected "type[Never]" [arg-type]
- steam/state.py:1512: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[ClanChannel]"; expected "type[Never]" [arg-type]
+ steam/state.py:1512: error: Argument 3 to "__get__" of "classproperty" has incompatible type "type[ClanChannel]"; expected "type[Never]" [arg-type]
- steam/state.py:2280: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[Commentable]"; expected "type[Never]" [arg-type]
+ steam/state.py:2280: error: Argument 3 to "__get__" of "classproperty" has incompatible type "type[Commentable]"; expected "type[Never]" [arg-type]
- steam/state.py:2292: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[Commentable]"; expected "type[Never]" [arg-type]
+ steam/state.py:2292: error: Argument 3 to "__get__" of "classproperty" has incompatible type "type[Commentable]"; expected "type[Never]" [arg-type]
- steam/state.py:2307: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[OwnerT]"; expected "type[Never]" [arg-type]
+ steam/state.py:2307: error: Argument 3 to "__get__" of "classproperty" has incompatible type "type[OwnerT]"; expected "type[Never]" [arg-type]
- steam/state.py:2331: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[Commentable]"; expected "type[Never]" [arg-type]
+ steam/state.py:2331: error: Argument 3 to "__get__" of "classproperty" has incompatible type "type[Commentable]"; expected "type[Never]" [arg-type]
- steam/state.py:2342: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[Commentable]"; expected "type[Never]" [arg-type]
+ steam/state.py:2342: error: Argument 3 to "__get__" of "classproperty" has incompatible type "type[Commentable]"; expected "type[Never]" [arg-type]
- steam/state.py:2438: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[Awardable[int]]"; expected "type[Never]" [arg-type]
+ steam/state.py:2438: error: Argument 3 to "__get__" of "classproperty" has incompatible type "type[Awardable[int]]"; expected "type[Never]" [arg-type]
- steam/state.py:2451: error: Argument 2 to "__get__" of "classproperty" has incompatible type "type[Awardable[int]]"; expected "type[Never]" [arg-type]
+ steam/state.py:2451: error: Argument 3 to "__get__" of "classproperty" has incompatible type "type[Awardable[int]]"; expected "type[Never]" [arg-type]
operator (https://github.com/canonical/operator)
- ops/_private/harness.py:429: error: Argument 1 to "__get__" of "ObjectEvents" has incompatible type "None"; expected "Object" [arg-type]
+ ops/_private/harness.py:429: error: Argument 2 to "__get__" of "ObjectEvents" has incompatible type "None"; expected "Object" [arg-type]
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When mypy resolved a descriptor's
__get__return type inanalyze_descriptor_access(mypy/checkmember.py), it first calledbind_selfto produce a bound-method type and then checked the call with only(instance, owner)arguments. This failed for descriptors whose__get__carries aSelf-like type variable that also appears in theinstanceparameter, becausebind_selfcould not unify those variables against complex callable types, producing incorrect or missing inferred types.The fix replaces that two-step approach with a single call using the raw unbound function type and three positional arguments
(self, instance, owner), letting the type-inference engine solve all type variables jointly in one pass.To verify, write a descriptor class whose
__get__annotatesselfwith a type variable shared withinstance, runmypyon it, and confirm the inferred return type is correct rather thanAnyor an error.Fixes #21505