diff --git a/t/t2018-checkout-branch.sh b/t/t2018-checkout-branch.sh
new file mode 100755
index 0000000000..3c130659d6
--- /dev/null
+++ b/t/t2018-checkout-branch.sh
@@ -0,0 +1,121 @@
+#!/bin/sh
+
+test_description='checkout '
+
+. ./test-lib.sh
+
+# Arguments: <branch> <sha> [<checkout options>]
+#
+# Runs "git checkout" to switch to <branch>, testing that
+#
+#   1) we are on the specified branch, <branch>;
+#   2) HEAD is <sha>; if <sha> is not specified, the old HEAD is used.
+#
+# If <checkout options> is not specified, "git checkout" is run with -b.
+do_checkout() {
+	exp_branch=$1 &&
+	exp_ref="refs/heads/$exp_branch" &&
+
+	# if <sha> is not specified, use HEAD.
+	exp_sha=${2:-$(git rev-parse --verify HEAD)} &&
+
+	# default options for git checkout: -b
+	if [ -z "$3" ]; then
+		opts="-b"
+	else
+		opts="$3"
+	fi
+
+	git checkout $opts $exp_branch $exp_sha &&
+
+	test $exp_ref = $(git rev-parse --symbolic-full-name HEAD) &&
+	test $exp_sha = $(git rev-parse --verify HEAD)
+}
+
+test_dirty_unmergeable() {
+	! git diff --exit-code >/dev/null
+}
+
+setup_dirty_unmergeable() {
+	echo >>file1 change2
+}
+
+test_dirty_mergeable() {
+	! git diff --cached --exit-code >/dev/null
+}
+
+setup_dirty_mergeable() {
+	echo >file2 file2 &&
+	git add file2
+}
+
+test_expect_success 'setup' '
+	test_commit initial file1 &&
+	HEAD1=$(git rev-parse --verify HEAD) &&
+
+	test_commit change1 file1 &&
+	HEAD2=$(git rev-parse --verify HEAD) &&
+
+	git branch -m branch1
+'
+
+test_expect_success 'checkout -b to a new branch, set to HEAD' '
+	do_checkout branch2
+'
+
+test_expect_success 'checkout -b to a new branch, set to an explicit ref' '
+	git checkout branch1 &&
+	git branch -D branch2 &&
+
+	do_checkout branch2 $HEAD1
+'
+
+test_expect_success 'checkout -b to a new branch with unmergeable changes fails' '
+	git checkout branch1 &&
+
+	# clean up from previous test
+	git branch -D branch2 &&
+
+	setup_dirty_unmergeable &&
+	test_must_fail do_checkout branch2 $HEAD1 &&
+	test_dirty_unmergeable
+'
+
+test_expect_success 'checkout -f -b to a new branch with unmergeable changes discards changes' '
+	# still dirty and on branch1
+	do_checkout branch2 $HEAD1 "-f -b" &&
+	test_must_fail test_dirty_unmergeable
+'
+
+test_expect_success 'checkout -b to a new branch preserves mergeable changes' '
+	git checkout branch1 &&
+
+	# clean up from previous test
+	git branch -D branch2 &&
+
+	setup_dirty_mergeable &&
+	do_checkout branch2 $HEAD1 &&
+	test_dirty_mergeable
+'
+
+test_expect_success 'checkout -f -b to a new branch with mergeable changes discards changes' '
+	# clean up from previous test
+	git reset --hard &&
+
+	git checkout branch1 &&
+
+	# clean up from previous test
+	git branch -D branch2 &&
+
+	setup_dirty_mergeable &&
+	do_checkout branch2 $HEAD1 "-f -b" &&
+	test_must_fail test_dirty_mergeable
+'
+
+test_expect_success 'checkout -b to an existing branch fails' '
+	git reset --hard HEAD &&
+
+	test_must_fail do_checkout branch2 $HEAD2
+'
+
+test_done