Option for eval_select(allow_positional = FALSE)

[DavisVaughan]

It would be nice if we had a way to disallow positional tidyselection entirely in some cases.

For example, in tidyr here tidyverse/tidyr#1541 (comment), we remove columns generated from names_from and values_from before evaluating the id_cols tidyselection to prevent the user from selecting an ID column that collides with the pivot columns. This works quite well unless you use positions in id_cols, in which case the positions you supply don't align with the data frame it is actually evaluated on.

We don't think id_cols is a great place to be providing positions anyways, so we'd like to completely turn off the ability to use positional selection here.

We think allow_positional = FALSE would affect a few places:

• all_of() and any_of() technically allow positional inputs, so we'd need a peek_positional() helper like we have for peek_vars() to dynamically determine if we should disallow them there
• last_col() works by position and has the potential to be quite confusing if used as an id_cols, so it should probably be completely disallowed. It would also need peek_positional().
• In walk_data_tree() we'd check for a literal that is a numeric value and error when allow_positional = FALSE. Hopefully that catches all cases that aren't selection helpers, because everything recurses through here. Even 1:5, which does eval_colon() and recurses through walk_data_tree() with the LHS and RHS.