Error using dynamic group() and specialized formats together

[wlandau]

Prework

• Read and abide by drake's code of conduct.
• Search for duplicates among the existing issues, both open and closed.
• Advanced users: verify that the bug still persists in the current development version (i.e. remotes::install_github("ropensci/drake")) and mention the SHA-1 hash of the Git commit you install.

Description

Dynamic group() appears to clash with specialized formats (ropensci-books/drake#155 (comment), cc @adamaltmejd). It is because $mget() does not respect the "decorated" part of drake's decorated storr.

Reproducible example

library(drake)

plan <- drake_plan(
  a = c(1, 2, 3),
  dts = target(
    list(a = a, b = 4),
    dynamic = map(a),
    format = "qs"
  ),
  final = target(
    list(dts),
    dynamic = group(dts),
    format = "qs"
  )
)

make(plan)
#> ▶ target a
#> ▶ dynamic dts
#> > subtarget dts_0a25b599
#> > subtarget dts_6aab08ab
#> > subtarget dts_a150a51a
#> ■ finalize dts
#> ▶ dynamic final
#> > subtarget final_eec507d0
#> Error: Can't find vctrs or base methods for concatenation.
#> vctrs methods must be implemented for class `drake_format_qs`.
#> See <https://vctrs.r-lib.org/articles/s3-vector.html>.

^{Created on 2020-04-06 by the reprex package (v0.3.0)}

Expected result

No error.