Cast a list to vector of specific type
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
Type to coerce to
vignette("s3-vector") for details.