thodg/got

diff --git a/got/got.1 b/got/got.1
index d79284e..107ba1a 100644
--- a/got/got.1
+++ b/got/got.1
@@ -332,8 +332,9 @@ If a
 .Ar path
 is specified, only show changes within this path.
 .Pp
-If two arguments are provided, treat each argument as a reference,
-or an object ID SHA1 hash, and display differences between these objects.
+If two arguments are provided, treat each argument as a reference, a tag
+name, or an object ID SHA1 hash, and display differences between the
+corresponding objects.
 Both objects must be of the same type (blobs, trees, or commits).
 An abbreviated hash argument will be expanded to a full SHA1 hash
 automatically, provided the abbreviation is unique.
diff --git a/got/got.c b/got/got.c
index 6303aab..02fe06d 100644
--- a/got/got.c
+++ b/got/got.c
@@ -1858,11 +1858,25 @@ match_object_id(struct got_object_id **id, char **label,
     const char *id_str, int obj_type, struct got_repository *repo)
 {
 	const struct got_error *err;
+	struct got_tag_object *tag;
 	struct got_reference *ref = NULL;
 
 	*id = NULL;
 	*label = NULL;
 
+	err = got_repo_object_match_tag(&tag, id_str, GOT_OBJ_TYPE_ANY, repo);
+	if (err == NULL) {
+		*id = got_object_id_dup(got_object_tag_get_object_id(tag));
+		if (*id == NULL)
+			err = got_error_from_errno("got_object_id_dup");
+		if (asprintf(label, "refs/tags/%s",
+		    strdup(got_object_tag_get_name(tag))) == -1)
+			err = got_error_from_errno("asprintf");
+		got_object_tag_close(tag);
+		return err;
+	} else if (err->code != GOT_ERR_NO_OBJ)
+		return err;
+
 	err = got_repo_match_object_id_prefix(id, id_str, obj_type, repo);
 	if (err) {
 		if (err->code != GOT_ERR_BAD_OBJ_ID_STR)
diff --git a/include/got_object.h b/include/got_object.h
index 5e46d49..505fd64 100644
--- a/include/got_object.h
+++ b/include/got_object.h
@@ -221,6 +221,9 @@ const struct got_error *got_object_open_as_tag(struct got_tag_object **,
 /* Dispose of a tag object. */
 void got_object_tag_close(struct got_tag_object *);
 
+/* Get the name of a tag. */
+const char *got_object_tag_get_name(struct got_tag_object *);
+
 /* Get type of the object a tag points to. */
 int got_object_tag_get_object_type(struct got_tag_object *);
 
diff --git a/lib/object.c b/lib/object.c
index d247b09..42e0035 100644
--- a/lib/object.c
+++ b/lib/object.c
@@ -1356,6 +1356,12 @@ got_object_tag_open(struct got_tag_object **tag,
 	return open_tag(tag, repo, got_object_get_id(obj), 1);
 }
 
+const char *
+got_object_tag_get_name(struct got_tag_object *tag)
+{
+	return tag->tag;
+}
+
 int
 got_object_tag_get_object_type(struct got_tag_object *tag)
 {
diff --git a/lib/repository.c b/lib/repository.c
index 60e8e27..2937089 100644
--- a/lib/repository.c
+++ b/lib/repository.c
@@ -1147,7 +1147,8 @@ got_repo_object_match_tag(struct got_tag_object **tag, const char *name,
 		free(tag_id);
 		if (err)
 			break;
-		if (got_object_tag_get_object_type(*tag) == obj_type)
+		if (obj_type == GOT_OBJ_TYPE_ANY ||
+		    got_object_tag_get_object_type(*tag) == obj_type)
 			break;
 		got_object_tag_close(*tag);
 		*tag = NULL;
diff --git a/regress/cmdline/diff.sh b/regress/cmdline/diff.sh
index 9193624..e7cea61 100755
--- a/regress/cmdline/diff.sh
+++ b/regress/cmdline/diff.sh
@@ -136,5 +136,66 @@ function test_diff_shows_conflict {
 	test_done "$testroot" "$ret"
 }
 
+function test_diff_tag {
+	local testroot=`test_init diff_tag`
+	local commit_id0=`git_show_head $testroot/repo`
+	local tag1=1.0.0
+	local tag2=2.0.0
+
+	echo "modified alpha" > $testroot/repo/alpha
+	git_commit $testroot/repo -m "changed alpha"
+	local commit_id1=`git_show_head $testroot/repo`
+
+	(cd $testroot/repo && git tag -m "test" $tag1)
+
+	echo "new file" > $testroot/repo/new
+	(cd $testroot/repo && git add new)
+	git_commit $testroot/repo -m "new file"
+	local commit_id2=`git_show_head $testroot/repo`
+
+	(cd $testroot/repo && git tag -m "test" $tag2)
+
+	echo "diff $commit_id0 refs/tags/$tag1" > $testroot/stdout.expected
+	echo -n 'blob - ' >> $testroot/stdout.expected
+	got tree -r $testroot/repo -c $commit_id0 -i | grep 'alpha$' | \
+		cut -d' ' -f 1 >> $testroot/stdout.expected
+	echo -n 'blob + ' >> $testroot/stdout.expected
+	got tree -r $testroot/repo -i | grep 'alpha$' | cut -d' ' -f 1 \
+		>> $testroot/stdout.expected
+	echo '--- alpha' >> $testroot/stdout.expected
+	echo '+++ alpha' >> $testroot/stdout.expected
+	echo '@@ -1 +1 @@' >> $testroot/stdout.expected
+	echo '-alpha' >> $testroot/stdout.expected
+	echo '+modified alpha' >> $testroot/stdout.expected
+
+	got diff -r $testroot/repo $commit_id0 $tag1 > $testroot/stdout
+	cmp -s $testroot/stdout.expected $testroot/stdout
+	ret="$?"
+	if [ "$ret" != "0" ]; then
+		diff -u $testroot/stdout.expected $testroot/stdout
+		test_done "$testroot" "$ret"
+		return 1
+	fi
+
+	echo "diff refs/tags/$tag1 refs/tags/$tag2" > $testroot/stdout.expected
+	echo "blob - /dev/null" >> $testroot/stdout.expected
+	echo -n 'blob + ' >> $testroot/stdout.expected
+	got tree -r $testroot/repo -i -c $commit_id2 | grep 'new$' | \
+		cut -d' ' -f 1 >> $testroot/stdout.expected
+	echo '--- /dev/null' >> $testroot/stdout.expected
+	echo '+++ new' >> $testroot/stdout.expected
+	echo '@@ -0,0 +1 @@' >> $testroot/stdout.expected
+	echo '+new file' >> $testroot/stdout.expected
+
+	got diff -r $testroot/repo $tag1 $tag2 > $testroot/stdout
+	cmp -s $testroot/stdout.expected $testroot/stdout
+	ret="$?"
+	if [ "$ret" != "0" ]; then
+		diff -u $testroot/stdout.expected $testroot/stdout
+	fi
+	test_done "$testroot" "$ret"
+}
+
 run_test test_diff_basic
 run_test test_diff_shows_conflict
+run_test test_diff_tag