> # Bare names
> vars_select(letters, foo)
Error: Can't subset columns that don't exist.
x The column `foo` doesn't exist.

> vars_select(letters, -foo)
Error: Can't subset columns that don't exist.
x The column `foo` doesn't exist.

> # Names
> vars_select(letters, "foo")
Error: Can't subset columns that don't exist.
x The column `foo` doesn't exist.

> vars_select(letters, a:"foo")
Error: Can't subset columns that don't exist.
x The column `foo` doesn't exist.

> # Locations
> vars_select(letters, 30, 50, 100)
Error: Can't subset columns that don't exist.
x The locations 30, 50, and 100 don't exist.
i There are only 26 columns.

> vars_select(letters, -100)
Error: Can't subset columns that don't exist.
x The location 100 doesn't exist.
i There are only 26 columns.

> vars_select(letters, !100)
Error: Can't subset columns that don't exist.
x The location 100 doesn't exist.
i There are only 26 columns.

