checkout/restore: refuse unmerging paths unless checking out of the index

Recreating unmerged index entries using resolve-undo data,
recreating conflicted working tree files using unmerged index
entries, and writing data out of unmerged index entries, make
sense only when we are checking paths out of the index and not when
we are checking paths out of a tree-ish.

Add an extra check to make sure "--merge" and "--ours/--theirs"
options are rejected when checking out from a tree-ish, update the
document (especially the SYNOPSIS section) to highlight that they
are incompatible, and add a few tests to make sure the combination
fails.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-checkout.txt

@@ -12,8 +12,10 @@ SYNOPSIS
 'git checkout' [-q] [-f] [-m] --detach [<branch>]
 'git checkout' [-q] [-f] [-m] [--detach] <commit>
 'git checkout' [-q] [-f] [-m] [[-b|-B|--orphan] <new-branch>] [<start-point>]
-'git checkout' [-f|--ours|--theirs|-m|--conflict=<style>] [<tree-ish>] [--] <pathspec>...
-'git checkout' [-f|--ours|--theirs|-m|--conflict=<style>] [<tree-ish>] --pathspec-from-file=<file> [--pathspec-file-nul]
+'git checkout' [-f] <tree-ish> [--] <pathspec>...
+'git checkout' [-f] <tree-ish> --pathspec-from-file=<file> [--pathspec-file-nul]
+'git checkout' [-f|--ours|--theirs|-m|--conflict=<style>] [--] <pathspec>...
+'git checkout' [-f|--ours|--theirs|-m|--conflict=<style>] --pathspec-from-file=<file> [--pathspec-file-nul]
 'git checkout' (-p|--patch) [<tree-ish>] [--] [<pathspec>...]
 
 DESCRIPTION
@@ -260,7 +262,8 @@ and mark the resolved paths with `git add` (or `git rm` if the merge
 should result in deletion of the path).
 +
 When checking out paths from the index, this option lets you recreate
-the conflicted merge in the specified paths.
+the conflicted merge in the specified paths.  This option cannot be
+used when checking out paths from a tree-ish.
 +
 When switching branches with `--merge`, staged changes may be lost.
 

Documentation/git-restore.txt

@@ -78,6 +78,8 @@ all modified paths.
 --theirs::
 	When restoring files in the working tree from the index, use
 	stage #2 ('ours') or #3 ('theirs') for unmerged paths.
+	This option cannot be used when checking out paths from a
+	tree-ish (i.e. with the `--source` option).
 +
 Note that during `git rebase` and `git pull --rebase`, 'ours' and
 'theirs' may appear swapped. See the explanation of the same options
@@ -87,6 +89,8 @@ in linkgit:git-checkout[1] for details.
 --merge::
 	When restoring files on the working tree from the index,
 	recreate the conflicted merge in the unmerged paths.
+	This option cannot be used when checking out paths from a
+	tree-ish (i.e. with the `--source` option).
 
 --conflict=<style>::
 	The same as `--merge` option above, but changes the way the

builtin/checkout.c

@@ -520,6 +520,15 @@ static int checkout_paths(const struct checkout_opts *opts,
 			    "--merge", "--conflict", "--staged");
 	}
 
+	/*
+	 * recreating unmerged index entries and writing out data from
+	 * unmerged index entries would make no sense when checking out
+	 * of a tree-ish.
+	 */
+	if ((opts->merge || opts->writeout_stage) && opts->source_tree)
+		die(_("'%s', '%s', or '%s' cannot be used when checking out of a tree"),
+		    "--merge", "--ours", "--theirs");
+
 	if (opts->patch_mode) {
 		enum add_p_mode patch_mode;
 		const char *rev = new_branch_info->name;

t/t2070-restore.sh

@@ -137,11 +137,14 @@ test_expect_success 'restore --staged invalidates cache tree for deletions' '
 	test_must_fail git rev-parse HEAD:new1
 '
 
-test_expect_success 'restore with merge options rejects --staged' '
+test_expect_success 'restore with merge options are incompatible with certain options' '
 	for opts in \
 		"--staged --ours" \
 		"--staged --theirs" \
 		"--staged --merge" \
+		"--source=HEAD --ours" \
+		"--source=HEAD --theirs" \
+		"--source=HEAD --merge" \
 		"--staged --conflict=diff3" \
 		"--staged --worktree --ours" \
 		"--staged --worktree --theirs" \
@@ -149,7 +152,7 @@ test_expect_success 'restore with merge options rejects --staged' '
 		"--staged --worktree --conflict=zdiff3"
 	do
 		test_must_fail git restore $opts . 2>err &&
-		grep "cannot be used with --staged" err || return
+		grep "cannot be used" err || return
 	done
 '
 

t/t7201-co.sh

@@ -497,6 +497,11 @@ test_expect_success 'checkout unmerged stage' '
 	test ztheirside = "z$(cat file)"
 '
 
+test_expect_success 'checkout path with --merge from tree-ish is a no-no' '
+	setup_conflicting_index &&
+	test_must_fail git checkout -m HEAD -- file
+'
+
 test_expect_success 'checkout with --merge' '
 	setup_conflicting_index &&
 	echo "none of the above" >sample &&