Subject: [PATCH v3] diff: document -U without <n> as using default context

From: Tian Yuchen <[email-masked]>
Date: Tue, 10 Mar 2026 17:50:17 +0800

The documentation for '-U<n>' implies that the numeric value '<n>' is
mandatory. However, the command line parser has historically accepted
'-U' without a number.

Strictly requiring a number for '-U' would break existing tests
(e.g., in 't4013') and likely disrupt user scripts relying on this
undocumented behavior.

Hence we retain this fallback behavior for backward compatibility, but
document it as such.

Signed-off-by: Tian Yuchen <[email-masked]>
---
 Documentation/diff-context-options.adoc | 6 ++++--
 Documentation/diff-options.adoc         | 6 ++++--
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/Documentation/diff-context-options.adoc b/Documentation/diff-context-options.adoc
index e161260358..306abfd9ab 100644
--- a/Documentation/diff-context-options.adoc
+++ b/Documentation/diff-context-options.adoc
@@ -1,7 +1,9 @@
 `-U<n>`::
 `--unified=<n>`::
-	Generate diffs with _<n>_ lines of context. Defaults to `diff.context`
-	or 3 if the config option is unset.
+	Generate diffs with _<n>_ lines of context. The number of context
+	lines defaults to `diff.context` or 3 if the config option is unset.
+	(`-U` without `<n>` is silently accepted as a synonym for `-p` due
+	to a historical accident).
 
 `--inter-hunk-context=<n>`::
 	Show the context between diff hunks, up to the specified _<number>_
diff --git a/Documentation/diff-options.adoc b/Documentation/diff-options.adoc
index 9cdad6f72a..e1f20bedf6 100644
--- a/Documentation/diff-options.adoc
+++ b/Documentation/diff-options.adoc
@@ -127,8 +127,10 @@ endif::git-log[]
 
 `-U<n>`::
 `--unified=<n>`::
-	Generate diffs with _<n>_ lines of context instead of
-	the usual three.
+	Generate diffs with _<n>_ lines of context. The number of context
+	lines defaults to `diff.context` or 3 if the config option is unset.
+	(`-U` without `<n>` is silently accepted as a synonym for `-p` due
+	to a historical accident).	
 ifndef::git-format-patch[]
 	Implies `--patch`.
 endif::git-format-patch[]
-- 
2.43.0