The following commit seems to have broken the display of many icons in
GNOME Shell:

  commit a8cda7f57992e9ce9ae4a694eba54e3eab42c39b
  Author: Raghav Gururajan <raghavgururajan@disroot.org>
  Date:   Thu Dec 26 09:52:55 2019 -0500

  gnu: gnome: Update dependencies.
  
  * gnu/packages/gnome.scm (gnome)[propagated-inputs]: Remove
  font-cantarell, font-dejavu, at-spi2-core, dbus, dconf,
  desktop-file-utils, gnome-defalt-applications, gnome-online-accounts,
  gnome-themes-standard, gst-plugins-base, gst-plugins-good, gucharmap,
  hicolor-icon-theme, pinentry-gnome3, pulseaudio, shared-mime-info,
  system-config-printer, xdg-user-dirs, zenity
  Add accountservice, adwaita-icon-theme, cheese, gdm, gnome-calendar,
  gnome-characters, gnome-clocks, gnome-desktop, gnome-font-viewer,
  gnome-maps, gnome-themes-extra, gsettings-desktop-schema, mutter,
  network-manager, orca, packagekit, simplescan, upower.
  
  Signed-off-by: Efraim Flashner <efraim@flashner.co.il>

Since that commit, on my system *all* application icons are replaced by
a generic icon, e.g. in the "dock" of favorite applications that show up
on the left side in GNOME Shell's "overview" mode, and in the top bar
application menu header.  (See attached screenshot below).  Also, the
gear icon that leads to GNOME settings from the menu on the right side
of GNOME Shell's top bar is missing.

Reverting the above commit, as well as the immediately following commit
8d8c6bffc528b60574f84620bd6c3ee9bfa1173f (gnome: Sort packages
logically), fixes the problem for me.  I'm assuming the latter commit is
harmless based on its commit log, but it has to be reverted as well to
avoid a merge conflict.

I guess the problem was caused by one of the 'propagated-inputs' that
was removed by commit a8cda7f57992e9ce9ae4a694eba54e3eab42c39b, but I
don't have time right now to find out which one.

        Mark