diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index 6d968b9012d..2825783049c 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -49,10 +49,9 @@ ifdef::git-log[]
 --diff-merges=m:::
 -m:::
 	This option makes diff output for merge commits to be shown in
-	the default format. `-m` will produce the output only if `-p`
-	is given as well. The default format could be changed using
+	the default format. The default format could be changed using
 	`log.diffMerges` configuration parameter, which default value
-	is `separate`.
+	is `separate`. `-m` implies `-p`.
 +
 --diff-merges=first-parent:::
 --diff-merges=1:::
@@ -62,7 +61,8 @@ ifdef::git-log[]
 --diff-merges=separate:::
 	This makes merge commits show the full diff with respect to
 	each of the parents. Separate log entry and diff is generated
-	for each parent.
+	for each parent. This is the format that `-m` produced
+	historically.
 +
 --diff-merges=combined:::
 --diff-merges=c:::
diff --git a/diff-merges.c b/diff-merges.c
index d897fd8a293..0dfcaa1b11b 100644
--- a/diff-merges.c
+++ b/diff-merges.c
@@ -107,6 +107,7 @@ int diff_merges_parse_opts(struct rev_info *revs, const char **argv)
 
 	if (!strcmp(arg, "-m")) {
 		set_to_default(revs);
+		revs->merges_imply_patch = 1;
 	} else if (!strcmp(arg, "-c")) {
 		set_combined(revs);
 		revs->merges_imply_patch = 1;
diff --git a/t/t4013-diff-various.sh b/t/t4013-diff-various.sh
index e561a8e4852..7fadc985ccc 100755
--- a/t/t4013-diff-various.sh
+++ b/t/t4013-diff-various.sh
@@ -455,8 +455,8 @@ diff-tree --stat --compact-summary initial mode
 diff-tree -R --stat --compact-summary initial mode
 EOF
 
-test_expect_success 'log -m matches pure log' '
-	git log master >result &&
+test_expect_success 'log -m matches log -m -p' '
+	git log -m -p master >result &&
 	process_diffs result >expected &&
 	git log -m >result &&
 	process_diffs result >actual &&