In case it's helpful, I'm trying out this __class_getitem__ override to merge the separate TypeVarTuple arguments into a single tuple argument:

    @classmethod
    def __class_getitem__(cls, items: type | tuple[type, ...]):
        # NOTE: Pydantic doesn't support variadic generics yet[1], so we merge any TypeVarTuple
        # argument(s) into a single tuple for now.
        # - 1: https://github.com/pydantic/pydantic/issues/5804
        params = cls.__pydantic_generic_metadata__["parameters"]
        # There can be only a single TypeVarTuple parameter.
        tvt_idx = next((i for i, p in enumerate(params) if isinstance(p, TypeVarTuple)), None)
        if tvt_idx is not None and isinstance(items, tuple):
            # Calculate the number of items for TypeVarTuple
            n_before = len(params[:tvt_idx])
            n_after = len(params[tvt_idx + 1 :])
            n_tuple_items = max(0, len(items) - n_before - n_after)
            tuple_items = items[tvt_idx : tvt_idx + n_tuple_items]
            # If the argument isn't still generic, convert the argument(s) to a tuple so pydantic
            # only sees a single element.
            if n_tuple_items == 1 and get_origin(tuple_items[0]) is Unpack:
                middle = (tuple_items[0],)
            else:
                middle = (tuple[*tuple_items],)  # pyright: ignore # noqa: PGH003 # no Pyright code available
            items = (*items[:n_before], *middle, *items[-n_after:])
        return super().__class_getitem__(items)

This fixes the TypeError when subscripting a class containing a TypeVarTuple with more than a single variadic argument, but I'm not actually sure the effect it has (or not) on field (de)serialization (the TypeVarTuples I'm using are just for methods currently, not fields).

Uh oh!

Support variadic generics #5804

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