Internal error occurs at @typing.overload in large protoc generated files #22257

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1. mjohanse-emr

   added

   crash

   on Oct 21, 2026
2. EmmanuelNiyonshuti commented on Oct 21, 2026

   

   EmmanuelNiyonshuti

   on Oct 21, 2026

   Contributor

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   I've attached source code to reproduce this.

   I'm not seeing the attached source code on the issue?
3. mjohanse-emr commented on Oct 21, 2026

   

   mjohanse-emr

   on Oct 21, 2026

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   Adding attachment that I forgot on creation.

   mypy-bug-repro.zip
4. mjohanse-emr commented on Oct 21, 2026

   

   mjohanse-emr

   on Oct 21, 2026

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   2. > I've attached source code to reproduce this.

   I'm not seeing the attached source code on the issue?
   

   @EmmanuelNiyonshuti sorry about that, I forgot to attach when I created. I just commented with the attachment.
5. MustafaKemal0146 commented on Oct 26, 2026

   

   MustafaKemal0146

   on Oct 26, 2026

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   Thanks for the repro. The crash is a MemoryError from a combinatorial explosion. MetadataStoreServiceStub is generic in 34 value-restricted TypeVars (2 values each). When checking the __init__ overloads, mypy expands every combination of those class TypeVars (2^34) in expand_callable_variants(), even though the overload signatures (explicit concrete self: types) don't use any of them. The same happens again when the overload items are type checked (TypeChecker.expand_typevars()).

   Minimal repro (as a .pyi; with ~16 TypeVars it already takes minutes on master):

   from typing import Generic, TypeVar, overload
   
   T0 = TypeVar("T0", int, str)
   ...  # T1..T33 the same
   
   class C(Generic[T0, ..., T33]):
       @overload
       def __init__(self: C[int, ..., int], x: int) -> None: ...
       @overload
       def __init__(self: C[str, ..., str], x: str) -> None: ...

   Skipping type variables that don't appear in the signature fixes it without changing semantics (substituting an unused type variable is a no-op). I'll open a PR. Related: #20439, nipunn1313/mypy-protobuf#707.