Resolve Self to the TypeVar when subscripting a class-bounded TypeVar

[kinto0]

Summary:
Subscripting a value whose type is a TypeVar bounded by a class (seq[0:1] with S: SupportsSlice) inferred the bound instead of S when __getitem__ returns Self. Calling the method explicitly (seq.__getitem__(slice(0, 1))) already inferred S.

The subscript path handled a bounded TypeVar by subscripting its bound, so __getitem__ was looked up and called with the bound as the receiver, and Self resolved to the bound. Binary operators don't have this problem because they call the dunder on the TypeVar itself.

For a TypeVar bounded by a class whose subscripts use the plain __getitem__ call, we now call __getitem__ on the TypeVar, the same way an explicit method call does. Attribute lookup on a bounded TypeVar binds Self to the TypeVar. Bounds with builtin subscript handling (lists, tuples, shaped arrays, nn.ModuleDict) still subscript the bound, so their precise indexing is kept.

fixes #5257

Differential Revision: D125829160

[meta-codesync]

@kinto0 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D125829160.

[github-actions]

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

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SUMMARY
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Total: +1 new errors, -2 fixed errors
By preset: +1/-2 (default), +1/-2 (strict)

Projects with changes (2):
  hydpy: +1 -1
  pandas: +0 -1
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FULL DIFF DETAILS
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hydpy (https://github.com/hydpy-dev/hydpy)
- ERROR hydpy/auxs/iuhtools.py:516:13-20: Cannot index into `float` [bad-index]
+ ERROR hydpy/auxs/iuhtools.py:516:13-20: Cannot index into `VectorFloatFlex` [bad-index]

pandas (https://github.com/pandas-dev/pandas)
- ERROR pandas/core/dtypes/missing.py:655:16-43: Returned type `ndarray | Unknown` is not assignable to declared return type `_ArrLikeT` [bad-return]