Same experience here. I haven't checked the history of Git's completion script in the distribution, but it looks like—among other things—a wrapper function name changed from _git to __git.

Here's a fix that works for me, git and hub completion seem to work correctly independently and with the alias git='hub'.

$ diff -u /usr/local/Cellar/hub/2.14.2/etc/bash_completion.d/hub.bash_completion.sh{.orig,}

--- /usr/local/Cellar/hub/2.14.2/etc/bash_completion.d/hub.bash_completion.sh.orig	2021-01-05 21:11:04.000000000 +0700
+++ /usr/local/Cellar/hub/2.14.2/etc/bash_completion.d/hub.bash_completion.sh	2021-01-05 21:12:39.000000000 +0700
@@ -2,12 +2,12 @@
 # This script complements the completion script that ships with git.

 # If there is no git tab completion, but we have the _completion loader try to load it
-if ! declare -F _git > /dev/null && declare -F _completion_loader > /dev/null; then
+if ! declare -F __git > /dev/null && declare -F _completion_loader > /dev/null; then
   _completion_loader git
 fi

 # Check that git tab completion is available and we haven't already set up completion
-if declare -F _git > /dev/null && ! declare -F __git_list_all_commands_without_hub > /dev/null; then
+if declare -F __git > /dev/null && ! declare -F __git_list_all_commands_without_hub > /dev/null; then
   # Duplicate and rename the 'list_all_commands' function
   eval "$(declare -f __git_list_all_commands | \
         sed 's/__git_list_all_commands/__git_list_all_commands_without_hub/')"
@@ -382,6 +382,5 @@
   }

   # Enable completion for hub even when not using the alias
-  complete -o bashdefault -o default -o nospace -F _git hub 2>/dev/null \
-    || complete -o default -o nospace -F _git hub
+  __git_complete hub __git_main
 fi

Sharing here before I submit a PR because I'm not sure this is the "right way" to fix it going forward (and backward…), in particular the __git_complete function in git-completion.bash says

# This is NOT a public function; use at your own risk.

Seeing same issue. hub completions work normally using the hub command (when adding the fix above), but when setting the alias it is not working.

Dunno if relevant, but there was recently a bash update as well.
GNU bash, version 5.1.4(1)-release (x86_64-apple-darwin19.6.0)

Yeah, looks like _git went away in git/git@441ecda

@ches I think your fix looks right to me.

@ches Thanks for diving into this! Your diff looks good and you are welcome to submit it as a PR. Any chance that you can make it backwards-compatible, meaning that if _git is detected, use that; otherwise use __git?

Having the same issue, uninstalled hub until it's fixed

completion from git package:

$ complete -p git
complete -o bashdefault -o default -o nospace -F __git_wrap__git_main git

completion from hub package

$ complete -p git
complete -F _minimal git

Sharing here before I submit a PR because I'm not sure this is the "right way" to fix it going forward (and backward…), in particular the __git_complete function in git-completion.bash says

# This is NOT a public function; use at your own risk.

FYI: __git_complete went public in git/git@5a067ba.

Also it seems __git_list_all_commands is long gone: git/git@3301d36.
This seems to help: liskin/dotfiles@6ca7aa9#diff-fb2d256214467c25c8e55a2180f2ea13de24f6d62529ebceb523bba0711403e1

Thanks for writing this up.

==> bash-completion@2: stable 2.11 (bottled), HEAD
==> git: stable 2.41.0 (bottled), HEAD
==> hub: stable 2.14.2 (bottled), HEAD

I noticed that https://github.com/scop/bash-completion introduces a few nuances to the problem:

eval $(/opt/homebrew/bin/brew shellenv)
. /opt/homebrew/etc/bash_completion.d/git-completion.bash
. /opt/homebrew/etc/bash_completion.d/hub.bash_completion.sh
complete | grep '\bgit\b'
# => complete -o bashdefault -o default -o nospace -F __git_wrap__git_main git

eval $(/opt/homebrew/bin/brew shellenv)
. "/opt/homebrew/etc/profile.d/bash_completion.sh"
complete | grep '\bgit\b'
# => complete -F _minimal git

Also it seems __git_list_all_commands is long gone: git/git@3301d36. This seems to help: liskin/dotfiles@6ca7aa9#diff-fb2d256214467c25c8e55a2180f2ea13de24f6d62529ebceb523bba0711403e1

This is not a full solution AFAICT since internally git now depends on git --list-cmds=... which hub does wrap and extend, so some things work, but not everything seemingly? e.g. hub help <TAB> should likely complete custom commands but seemingly does not because hub --list-cmds=.... only lists its own when seeing "others"?

Also it seems __git_list_all_commands is long gone: git/git@3301d36. This seems to help: liskin/dotfiles@6ca7aa9#diff-fb2d256214467c25c8e55a2180f2ea13de24f6d62529ebceb523bba0711403e1

This is not a full solution AFAICT since internally git now depends on git --list-cmds=... which hub does wrap and extend, so some things work, but not everything seemingly? e.g. hub help <TAB> should likely complete custom commands but seemingly does not because hub --list-cmds=.... only lists its own when seeing "others"?

I don't think hub help <Tab> ever did work, as there's no mention of help in https://github.com/mislav/hub/blob/master/etc/hub.bash_completion.sh, so it just falls back to git help completion.