The object ID parsing machinery is aware of "@" as a synonym for "HEAD"
and this is documented accordingly in gitrevisions(7).  The push
documentation describes the source portion of a refspec as "any
arbitrary 'SHA-1 expression'"; however, "@" is not allowed on the
left-hand side of a refspec, since we attempt to check for it being a
valid ref name and fail (since it is not).

Teach the refspec machinery about this alias and silently substitute
"HEAD" when we see "@".  This handles the fact that HEAD is a symref and
preserves its special behavior.  We need not handle other arbitrary
object ID expressions (such as "@^") when pushing because the revision
machinery already handles that for us.

Signed-off-by: brian m. carlson <sand...@crustytoothpaste.net>
---
I probably type "git push upstream HEAD" from five to thirty times a
day, many of those where I typo "HEAD", so I decided to implement the
shorter form.  This design handles @ as HEAD in both fetch and push,
whereas alternate solutions would not.

check_refname_format explicitly rejects "@"; I tried at first to simply
ignore that with a flag, but we end up calling that from several other
places in the codebase and rejecting it and all of those places would
have needed updating.

I thought about putting the if/else logic in a function, but since it's
just four lines, I decided not to.  However, if people think it would be
tidier, I can do so.

Note that the test portion of the patch is best read with git diff -w;
the current version is very noisy.

 refspec.c             |   6 ++-
 t/t5516-fetch-push.sh | 104 +++++++++++++++++++++---------------------
 2 files changed, 58 insertions(+), 52 deletions(-)

diff --git a/refspec.c b/refspec.c
index e8010dce0c..57c2f65104 100644
--- a/refspec.c
+++ b/refspec.c
@@ -62,8 +62,12 @@ static int parse_refspec(struct refspec_item *item, const 
char *refspec, int fet
                return 0;
        }
 
+       if (llen == 1 && lhs[0] == '@')
+               item->src = xstrdup("HEAD");
+       else
+               item->src = xstrndup(lhs, llen);
+
        item->pattern = is_glob;
-       item->src = xstrndup(lhs, llen);
        flags = REFNAME_ALLOW_ONELEVEL | (is_glob ? REFNAME_REFSPEC_PATTERN : 
0);
 
        if (fetch) {
diff --git a/t/t5516-fetch-push.sh b/t/t5516-fetch-push.sh
index a5077d8b7c..cbccbd2f8d 100755
--- a/t/t5516-fetch-push.sh
+++ b/t/t5516-fetch-push.sh
@@ -436,70 +436,72 @@ test_expect_success 'push ref expression with 
non-existent, incomplete dest' '
 
 '
 
-test_expect_success 'push with HEAD' '
+for ref in HEAD @
+do
+       test_expect_success "push with $ref" '
 
-       mk_test testrepo heads/master &&
-       git checkout master &&
-       git push testrepo HEAD &&
-       check_push_result testrepo $the_commit heads/master
+               mk_test testrepo heads/master &&
+               git checkout master &&
+               git push testrepo $ref &&
+               check_push_result testrepo $the_commit heads/master
 
-'
+       '
 
-test_expect_success 'push with HEAD nonexisting at remote' '
+       test_expect_success "push with $ref nonexisting at remote" '
 
-       mk_test testrepo heads/master &&
-       git checkout -b local master &&
-       git push testrepo HEAD &&
-       check_push_result testrepo $the_commit heads/local
-'
+               mk_test testrepo heads/master &&
+               git checkout -B local master &&
+               git push testrepo $ref &&
+               check_push_result testrepo $the_commit heads/local
+       '
 
-test_expect_success 'push with +HEAD' '
+       test_expect_success "push with +$ref" '
 
-       mk_test testrepo heads/master &&
-       git checkout master &&
-       git branch -D local &&
-       git checkout -b local &&
-       git push testrepo master local &&
-       check_push_result testrepo $the_commit heads/master &&
-       check_push_result testrepo $the_commit heads/local &&
+               mk_test testrepo heads/master &&
+               git checkout master &&
+               git branch -D local &&
+               git checkout -b local &&
+               git push testrepo master local &&
+               check_push_result testrepo $the_commit heads/master &&
+               check_push_result testrepo $the_commit heads/local &&
 
-       # Without force rewinding should fail
-       git reset --hard HEAD^ &&
-       test_must_fail git push testrepo HEAD &&
-       check_push_result testrepo $the_commit heads/local &&
+               # Without force rewinding should fail
+               git reset --hard HEAD^ &&
+               test_must_fail git push testrepo $ref &&
+               check_push_result testrepo $the_commit heads/local &&
 
-       # With force rewinding should succeed
-       git push testrepo +HEAD &&
-       check_push_result testrepo $the_first_commit heads/local
+               # With force rewinding should succeed
+               git push testrepo +$ref &&
+               check_push_result testrepo $the_first_commit heads/local
 
-'
+       '
 
-test_expect_success 'push HEAD with non-existent, incomplete dest' '
+       test_expect_success "push $ref with non-existent, incomplete dest" '
 
-       mk_test testrepo &&
-       git checkout master &&
-       git push testrepo HEAD:branch &&
-       check_push_result testrepo $the_commit heads/branch
+               mk_test testrepo &&
+               git checkout master &&
+               git push testrepo $ref:branch &&
+               check_push_result testrepo $the_commit heads/branch
 
-'
+       '
 
-test_expect_success 'push with config remote.*.push = HEAD' '
-
-       mk_test testrepo heads/local &&
-       git checkout master &&
-       git branch -f local $the_commit &&
-       (
-               cd testrepo &&
-               git checkout local &&
-               git reset --hard $the_first_commit
-       ) &&
-       test_config remote.there.url testrepo &&
-       test_config remote.there.push HEAD &&
-       test_config branch.master.remote there &&
-       git push &&
-       check_push_result testrepo $the_commit heads/master &&
-       check_push_result testrepo $the_first_commit heads/local
-'
+       test_expect_success "push with config remote.*.push = $ref" '
+               mk_test testrepo heads/local &&
+               git checkout master &&
+               git branch -f local $the_commit &&
+               (
+                       cd testrepo &&
+                       git checkout local &&
+                       git reset --hard $the_first_commit
+               ) &&
+               test_config remote.there.url testrepo &&
+               test_config remote.there.push $ref &&
+               test_config branch.master.remote there &&
+               git push &&
+               check_push_result testrepo $the_commit heads/master &&
+               check_push_result testrepo $the_first_commit heads/local
+       '
+done
 
 test_expect_success 'push with remote.pushdefault' '
        mk_test up_repo heads/master &&

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