checkout: fix interaction between --conflict and --merge

When using "git checkout" to recreate merge conflicts or merge
uncommitted changes when switching branch "--conflict" sensibly implies
"--merge". Unfortunately the way this is implemented means that "git
checkout --conflict=diff3 --no-merge" implies "--merge" violating the
usual last-one-wins rule. Fix this by only overriding the value of
opts->merge if "--conflicts" comes after "--no-merge" or "-[-no]-merge"
is not given on the command line.

The behavior of "git checkout --merge --no-conflict" is unchanged and
will still merge on the basis that the "-[-no]-conflict" options are
primarily intended to affect the conflict style and so "--no-conflict"
should cancel a previous "--conflict" but not override "--merge".

Of the four new tests the second one tests the behavior change
introduced by this commit, the other three check that this commit does
not regress the existing behavior.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -100,7 +100,7 @@ struct checkout_opts {
 	struct tree *source_tree;
 };
 
-#define CHECKOUT_OPTS_INIT { .conflict_style = -1 }
+#define CHECKOUT_OPTS_INIT { .conflict_style = -1, .merge = -1 }
 
 struct branch_info {
 	char *name; /* The short name used */
@@ -1633,6 +1633,9 @@ static int parse_opt_conflict(const struct option *o, const char *arg, int unset
 	opts->conflict_style = parse_conflict_style_name(arg);
 	if (opts->conflict_style < 0)
 		return error(_("unknown conflict style '%s'"), arg);
+	/* --conflict overrides a previous --no-merge */
+	if (!opts->merge)
+		opts->merge = -1;
 
 	return 0;
 }
@@ -1740,8 +1743,9 @@ static int checkout_main(int argc, const char **argv, const char *prefix,
 			opts->show_progress = isatty(2);
 	}
 
-	if (opts->conflict_style >= 0)
+	/* --conflicts implies --merge */
-		opts->merge = 1; /* implied */
+	if (opts->merge == -1)
+		opts->merge = opts->conflict_style >= 0;
 
 	if (opts->force) {
 		opts->discard_changes = 1;

t/t7201-co.sh

@@ -631,6 +631,66 @@ test_expect_success 'checkout --conflict=diff3' '
 	test_cmp merged file
 '
 
+test_expect_success 'checkout --conflict=diff3 --no-conflict does not merge' '
+	setup_conflicting_index &&
+	echo "none of the above" >expect &&
+	cat expect >fild &&
+	cat expect >file &&
+	test_must_fail git checkout --conflict=diff3 --no-conflict -- fild file 2>err &&
+	test_cmp expect file &&
+	test_cmp expect fild &&
+	echo "error: path ${SQ}file${SQ} is unmerged" >expect &&
+	test_cmp expect err
+'
+
+test_expect_success 'checkout --conflict=diff3 --no-merge does not merge' '
+	setup_conflicting_index &&
+	echo "none of the above" >expect &&
+	cat expect >fild &&
+	cat expect >file &&
+	test_must_fail git checkout --conflict=diff3 --no-merge -- fild file 2>err &&
+	test_cmp expect file &&
+	test_cmp expect fild &&
+	echo "error: path ${SQ}file${SQ} is unmerged" >expect &&
+	test_cmp expect err
+'
+
+test_expect_success 'checkout --no-merge --conflict=diff3 does merge' '
+	setup_conflicting_index &&
+	echo "none of the above" >fild &&
+	echo "none of the above" >file &&
+	git checkout --no-merge --conflict=diff3 -- fild file &&
+	echo "ourside" >expect &&
+	test_cmp expect fild &&
+	cat >expect <<-\EOF &&
+	<<<<<<< ours
+	ourside
+	||||||| base
+	original
+	=======
+	theirside
+	>>>>>>> theirs
+	EOF
+	test_cmp expect file
+'
+
+test_expect_success 'checkout --merge --conflict=diff3 --no-conflict does merge' '
+	setup_conflicting_index &&
+	echo "none of the above" >fild &&
+	echo "none of the above" >file &&
+	git checkout --merge --conflict=diff3 --no-conflict -- fild file &&
+	echo "ourside" >expect &&
+	test_cmp expect fild &&
+	cat >expect <<-\EOF &&
+	<<<<<<< ours
+	ourside
+	=======
+	theirside
+	>>>>>>> theirs
+	EOF
+	test_cmp expect file
+'
+
 test_expect_success 'checkout with invalid conflict style' '
 	test_must_fail git checkout --conflict=bad 2>actual -- file &&
 	echo "error: unknown conflict style ${SQ}bad${SQ}" >expect &&