`unspecified()` and scalar ptype2 symmetry is wrong

[DavisVaughan]

With current master:

library(vctrs)

foobar <- function(x = list()) structure(x, class = "vctrs_foobar")

vec_ptype2(unspecified(), foobar())
#> list()
#> attr(,"class")
#> [1] "vctrs_foobar"

vec_ptype2(foobar(), unspecified())
#> Error: `x` must be a vector, not a `vctrs_foobar` object

^{Created on 2020-02-11 by the reprex package (v0.3.0)}

[DavisVaughan]

vec_ptype2.vctrs_unspecified() may need to call vec_assert() on y

[romainfrancois]

Is this the same issue as this ?

library(vctrs)
library(tibble)
df <- tibble(x = NA)
vec_ptype(df)$x
#> logical(0)
vec_ptype2(df, df)$x
#> <unspecified> [0]

^{Created on 2020-02-17 by the reprex package (v0.3.0.9000)}

[lionel-]

@romainfrancois Both ptype and ptype2 return will unfinalised prototypes across all columns once Davis' changes in #816 are merged.

This particular issue is just about a missing vec_assert() though.

[DavisVaughan]

@romainfrancois with current master, because of #816

library(vctrs)
library(tibble)

df <- tibble(x = NA)

vec_ptype(df)$x
#> <unspecified> [0]
vec_ptype2(df, df)$x
#> <unspecified> [0]

vec_ptype_common(df)$x
#> logical(0)
vec_ptype_common(df, df)$x
#> logical(0)

^{Created on 2020-02-17 by the reprex package (v0.3.0)}