This is a function for developers to use when extending vctrs. It casts
a list to a more specific vectoring type, keeping the length constant.
It does this by discarding (with a warning), any elements after the 1.
It is called from
vec_cast.XYZ.list() methods to preserve symmetry with
vec_list_cast(x, to, ..., x_arg = "", to_arg = "")
Type to coerce to
These dots are for future extensions and must be empty.
vignette("s3-vector") for details.