[BUG]: Argument conversion in dispatcher assumes sequence protocol when casting to list[<something>]

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What version (or hash if on master) of pybind11 are you using?

2.13.6

Problem description

When a derived class implements __getitem__ and there exists an overloaded function that can accept a vector of base instances or the derived instance the argument conversion in the dispatcher logic will try to call derived.__getitem__.
If derived does not implement __len__ this will fail outright, otherwise derived.__getitem__ will be called with integer indices from 0 to len(derived). If derived.__getitem__ does not accept integers (e.g. because it mimics a map) this will also fail.

The resulting error message is extremely confusing because the trace will show that the execution failed at the function call but the message will say that __getitem__ was involved with derived and 0 but supported types are ...

A potential fix / workaround is to manually break the sequence protocol for derived by setting its tp_as_sequence->sq_item to NULL as this will cause it to fail PySequence_Check and avoid this conversion path entirely.

Adding noconvert to the vector argument does not help because this failure already happens in the initial non-converting pass in the dispatcher.

Reproducible example code

I have created a minimal example here: https://github.com/IljaManakov/pybind_arg_cast_issue_repro

Is this a regression? Put the last known working version here if it is.

Not a regression

Start with the minimal reproducer linked in the issue and confirm the failure with pybind11 2.13.6. Trace the dispatcher’s argument-conversion path for overloaded functions accepting a vector of base instances and a derived instance. Done should prevent the unintended sequence-protocol lookup and provide a clear conversion result without breaking valid conversions.