[PATCH v3] builtin/merges: clarify --squash behavior with --commit

[Vishal Verma <vishal@kernel.org>]

From: Vishal Verma <vishal@stellar.sh>

Convert option_commit to tristate, representing the states of
'default/untouched', 'enabled-by-cli', 'disabled-by-cli'. With this in
place, check whether option_commit was enabled by cli when squashing a
merge. If so, error out, as this is not supported.

Previously, when --squash was supplied, 'option_commit' was silently
dropped. This could have been surprising to a user who tried to override
the no-commit behavior of squash using --commit explicitly.

Add a note to the --squash option for git-merge to clarify the
incompatibility, and add a test case to t7600-merge.sh

Cc: Junio C Hamano <gitster@pobox.com>
Cc: Rafael Ascensão <rafa.almas@gmail.com>
Signed-off-by: Vishal Verma <vishal@stellar.sh>
---

v3: Add a test case for this behavior in t7600-merge.sh

 Documentation/merge-options.txt |  2 ++
 builtin/merge.c                 | 18 +++++++++++++++++-
 t/t7600-merge.sh                |  6 ++++++
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/Documentation/merge-options.txt b/Documentation/merge-options.txt
index 61876dbc33..79a00d2a4a 100644
--- a/Documentation/merge-options.txt
+++ b/Documentation/merge-options.txt
@@ -102,6 +102,8 @@ merge.
 +
 With --no-squash perform the merge and commit the result. This
 option can be used to override --squash.
++
+With --squash, --commit is not allowed, and will fail.
 
 -s <strategy>::
 --strategy=<strategy>::
diff --git a/builtin/merge.c b/builtin/merge.c
index e96f72af80..4730b075c1 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -58,7 +58,7 @@ static const char * const builtin_merge_usage[] = {
 };
 
 static int show_diffstat = 1, shortlog_len = -1, squash;
-static int option_commit = 1;
+static int option_commit = -1;
 static int option_edit = -1;
 static int allow_trivial = 1, have_message, verify_signatures;
 static int overwrite_ignore = 1;
@@ -1336,9 +1336,25 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	if (verbosity < 0)
 		show_diffstat = 0;
 
+	/*
+	 * This indicates option_commit was influenced by the command line.
+	 * Check and error out for the squash case.
+	 */
+	if ((option_commit > 0) && squash)
+		die(_("You cannot combine --squash with --commit."));
+
+	/* If option_commit is the default '-1', we can 'enable' it */
+	if (option_commit < 0)
+		option_commit = 1;
+
 	if (squash) {
 		if (fast_forward == FF_NO)
 			die(_("You cannot combine --squash with --no-ff."));
+		/*
+		 * squash can now silently disable option_commit - this is not
+		 * a problem as it is only overriding the default, not a user
+		 * supplied option.
+		 */
 		option_commit = 0;
 	}
 
diff --git a/t/t7600-merge.sh b/t/t7600-merge.sh
index 7f9c68cbe7..4ec5d9ec79 100755
--- a/t/t7600-merge.sh
+++ b/t/t7600-merge.sh
@@ -570,6 +570,12 @@ test_expect_success 'combining --squash and --no-ff is refused' '
 	test_must_fail git merge --no-ff --squash c1
 '
 
+test_expect_success 'combining --squash and --commit is refused' '
+	git reset --hard c0 &&
+	test_must_fail git merge --squash --commit c1 &&
+	test_must_fail git merge --commit --squash c1
+'
+
 test_expect_success 'option --ff-only overwrites --no-ff' '
 	git merge --no-ff --ff-only c1 &&
 	test_must_fail git merge --no-ff --ff-only c2
-- 
2.21.0