Should `tbl_vars()` add grouping metadata with grouped tibble?

[lionel-]

Currently select() uses the unexported sel_vars() to create variables passed to tidyselect. These variables contain the metadata necessary for group_cols() to figure out the grouping variables.

While this works for select(), group_cols() can't be used in other selecting functions like tidyr::gather(). Support for grouped tibbles in tbl_vars() would solve this.

[romainfrancois]

Do we still need this ?

It's kind of a mess:

• tbl_vars() is generic with at least implementations in dplyr and dbplyr
• group_vars() too
• tbl_nongroup_vars() calls tbl_vars() and group_vars()
• sel_vars() calls tbl_vars() and group_vars()

Perhaps we can export sel_vars() instead of changing tbl_vars() ?

[lionel-]

Yes we still need this. Not sure what's the best API for it.

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