git: allow alias-shadowing deprecated builtins

git-whatchanged(1) is deprecated and you need to pass
`--i-still-use-this` in order to force it to work as before.
There are two affected users, or usages:

1. people who use the command in scripts; and
2. people who are used to using it interactively.

For (1) the replacement is straightforward.[1]  But people in (2) might
like the name or be really used to typing it.[3]

An obvious first thought is to suggest aliasing `whatchanged` to the
git-log(1) equivalent.[1]  But this doesn’t work and is awkward since you
cannot shadow builtins via aliases.

Now you are left in an uncomfortable limbo; your alias won’t work until
the command is removed for good.

Let’s lift this limitation by allowing *deprecated* builtins to be
shadowed by aliases.

The only observed demand for aliasing has been for git-whatchanged(1),
not for git-pack-redundant(1).  But let’s be consistent and treat all
deprecated commands the same.

[1]:

        git log --raw --no-merges

     With a minor caveat: you get different outputs if you happen to
     have empty commits (no changes)[2]
[2]: https://lore.kernel.org/git/20250825085428.GA367101@coredump.intra.peff.net/
[3]: https://lore.kernel.org/git/BL3P221MB0449288C8B0FA448A227FD48833AA@BL3P221MB0449.NAMP221.PROD.OUTLOOK.COM/

Based-on-patch-by: Jeff King <peff@peff.net>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 40 insertions, 0 deletions

diff --git a/t/t0014-alias.sh b/t/t0014-alias.sh
index 854d59ec58..1b196ed9d6 100755
--- a/t/t0014-alias.sh
+++ b/t/t0014-alias.sh
@@ -27,6 +27,20 @@ test_expect_success 'looping aliases - internal execution' '
 	test_grep "^fatal: alias loop detected: expansion of" output
 '
 
+test_expect_success 'looping aliases - deprecated builtins' '
+	test_config alias.whatchanged pack-redundant &&
+	test_config alias.pack-redundant whatchanged &&
+	cat >expect <<-EOF &&
+	${SQ}whatchanged${SQ} is aliased to ${SQ}pack-redundant${SQ}
+	${SQ}pack-redundant${SQ} is aliased to ${SQ}whatchanged${SQ}
+	fatal: alias loop detected: expansion of ${SQ}whatchanged${SQ} does not terminate:
+	  whatchanged <==
+	  pack-redundant ==>
+	EOF
+	test_must_fail git whatchanged -h 2>actual &&
+	test_cmp expect actual
+'
+
 # This test is disabled until external loops are fixed, because would block
 # the test suite for a full minute.
 #
@@ -55,4 +69,30 @@ test_expect_success 'tracing a shell alias with arguments shows trace of prepare
 	test_cmp expect actual
 '
 
+can_alias_deprecated_builtin () {
+	cmd="$1" &&
+	# some git(1) commands will fail for `-h` (the case for
+	# git-status as of 2025-09-07)
+	test_might_fail git status -h >expect &&
+	test_file_not_empty expect &&
+	test_might_fail git -c alias."$cmd"=status "$cmd" -h >actual &&
+	test_cmp expect actual
+}
+
+test_expect_success 'can alias-shadow deprecated builtins' '
+	for cmd in $(git --list-cmds=deprecated)
+	do
+		can_alias_deprecated_builtin "$cmd" || return 1
+	done
+'
+
+test_expect_success 'can alias-shadow via two deprecated builtins' '
+	# some git(1) commands will fail... (see above)
+	test_might_fail git status -h >expect &&
+	test_file_not_empty expect &&
+	test_might_fail git -c alias.whatchanged=pack-redundant \
+		-c alias.pack-redundant=status whatchanged -h >actual &&
+	test_cmp expect actual
+'
+
 test_done