bug#42512: "guix refresh --list-updaters" fails

[Danny Milosavljevic]

$ LC_ALL=C guix describe
Generation 108  Jul 16 2020 11:03:13    (current)
  guix d57ce20
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: d57ce2024b0bdec0f8d05258c32c7bc7275a17a6
  heads 9cc77db
    repository URL: https://github.com/daym/heads-guix.git
    branch: wip-musl
    commit: 9cc77db51d994dc3ea03298e3714e2a0c7b929a5

$ LC_ALL=C guix refresh  --list-updaters
[...]
  - cpan: Updater for CPAN packages (5.3% coverage)
  - kernel.org: Updater for packages hosted on kernel.org (.3% coverage)
Backtrace:
          12 (primitive-load "/home/dannym/.config/guix/current/bin/?")
In guix/ui.scm:
  1974:12 11 (run-guix-command _ . _)
In guix/scripts/refresh.scm:
    502:4 10 (guix-refresh . _)
In ice-9/boot-9.scm:
  1731:15  9 (with-exception-handler #<procedure 7fefd641f2d0 at ic?> ?)
In srfi/srfi-37.scm:
   201:16  8 (next-arg)
   113:18  7 (invoke-option-processor _ _ _ _ _)
In unknown file:
           6 (_ #<procedure 7fefd641f240 at srfi/srfi-37.scm:114:22?> ?)
In guix/scripts/refresh.scm:
    285:4  5 (_ . _)
In srfi/srfi-1.scm:
   460:18  4 (fold #<procedure 7fefdaecbf60 at guix/scripts/refresh?> ?)
In guix/scripts/refresh.scm:
   287:29  3 (_ #<<upstream-updater> name: gnu-ftp description: "Up?> ?)
In unknown file:
           2 (filter #<procedure pred (package)> (#<package heads?> ?))
In guix/memoization.scm:
    101:0  1 (_ #<hash-table 7fefdaedbce0 12/31> #<package heads-u-?> ?)
In guix/gnu-maintenance.scm:
   182:20  0 (_)

guix/gnu-maintenance.scm:182:20: In procedure struct-vtable: Wrong type 
argument in position 1 (expecting struct): #f

(define gnu-package?
  (let ((official-gnu-packages (memoize official-gnu-packages)))
    (mlambdaq (package)
      "Return true if PACKAGE is a GNU package.  This procedure may access the
network to check in GNU's database."
      (define (mirror-type url)
        (let ((uri (string->uri url))) ; <------------------------

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