Re: [RFC PATCH v2 1/4] Prevent diff machinery from examining assume-unchanged entries on worktree

diff --git a/diff-lib.c b/diff-lib.c
index b7813af..f5787f6 100644
--- a/diff-lib.c
+++ b/diff-lib.c

@@ -337,6 +337,8 @@ static void do_oneway_diff(struct unpack_trees_options *o,
 	struct rev_info *revs = o->unpack_data;
 	int match_missing, cached;
 
+	/* if the entry is not checked out, don't examine work tree */
+	cached = o->index_only || (idx && (idx->ce_flags & CE_VALID));
 	/*
 	 * Backward compatibility wart - "diff-index -m" does
 	 * not mean "do not ignore merges", but "match_missing".

@@ -344,7 +346,6 @@ static void do_oneway_diff(struct unpack_trees_options *o,
 	 * But with the revision flag parsing, that's found in
 	 * "!revs->ignore_merges".
 	 */
-	cached = o->index_only;
 	match_missing = !revs->ignore_merges;
 
 	if (cached && idx && ce_stage(idx)) {

diff --git a/t/t4039-diff-assume-unchanged.sh b/t/t4039-diff-assume-unchanged.sh
new file mode 100755
index 0000000..d0e46a7
--- /dev/null
+++ b/t/t4039-diff-assume-unchanged.sh

@@ -0,0 +1,31 @@
+#!/bin/sh
+
+test_description='diff with assume-unchanged entries'
+
+. ./test-lib.sh
+
+# external diff has been tested in t4020-diff-external.sh
+
+test_expect_success 'setup' '
+	echo zero > zero &&
+	git add zero &&
+	git commit -m zero &&
+	echo one > one &&
+	echo two > two &&
+	git add one two &&
+	git commit -m onetwo &&
+	git update-index --assume-unchanged one &&
+	echo borked >> one &&
+	test "$(git ls-files -v one)" = "h one"
+'