So we probably should have triggered the default behaviour under the
"--index" option, and triggered this "--working-tree-only" mode of
behaviour when "--index" option is not given.  From the same point
of view, "git checkout --cached <tree-ish> <pathspec>" would have
done the same as "git reset <tree-ish> <pathspec>" would do.  And
that may have made the command set a bit more consistent.

But that is merely a hindsight being 20/20, oh well.

Signed-off-by: Junio C Hamano <redacted>
---
 Documentation/git-checkout.txt | 22 +++++++++++++++-------
 builtin/checkout.c             | 10 +++++++++-
 t/t2022-checkout-paths.sh      | 21 +++++++++++++++++++++
 3 files changed, 45 insertions(+), 8 deletions(-)

diff --git a/Documentation/git-checkout.txt b/Documentation/git-checkout.txt
index 8e2c0662dd..201677752e 100644
--- a/Documentation/git-checkout.txt
+++ b/Documentation/git-checkout.txt

@@ -14,6 +14,7 @@ SYNOPSIS
 'git checkout' [-q] [-f] [-m] [[-b|-B|--orphan] <new_branch>] [<start_point>]
 'git checkout' [-f|--ours|--theirs|-m|--conflict=<style>] [<tree-ish>] [--] <paths>...
 'git checkout' [-p|--patch] [<tree-ish>] [--] [<paths>...]
+'git checkout' --working-tree-only <tree-ish> [--] [<paths>...]

 DESCRIPTION
 -----------

@@ -81,13 +82,14 @@ Omitting <branch> detaches HEAD at the tip of the current branch.
 'git checkout' [-p|--patch] [<tree-ish>] [--] <pathspec>...::

        When <paths> or `--patch` are given, 'git checkout' does *not*
-       switch branches.  It updates the named paths in the working tree
-       from the index file or from a named <tree-ish> (most often a
-       commit).  In this case, the `-b` and `--track` options are
-       meaningless and giving either of them results in an error.  The
-       <tree-ish> argument can be used to specify a specific tree-ish
-       (i.e.  commit, tag or tree) to update the index for the given
-       paths before updating the working tree.
+       switch branches.  In this case, the `-b` and `--track` options
+       are meaningless and giving either of them results in an error.
++
+The command checks out blobs for paths that match the given
+<pathspec> from the index to the working tree.  When an optional
+<tree-ish> is given, the blobs for paths that match the given
+<pathspec> are copied from the <tree-ish> to the index before
+they are checked out of the index.
 +
 'git checkout' with <paths> or `--patch` is used to restore modified or
 deleted paths to their original contents from the index or replace paths

@@ -101,6 +103,12 @@ specific side of the merge can be checked out of the index by
 using `--ours` or `--theirs`.  With `-m`, changes made to the working tree
 file can be discarded to re-create the original conflicted merge result.

+'git checkout' --working-tree-only <tree-ish> [--] <pathspec>...::
+       Similar to `git checkout <tree-ish> [--] <pathspec>`, but
+       the index file is left in the same state as it was before
+       running this command.

diff --git a/builtin/checkout.c b/builtin/checkout.c
index 9b2a5b31d4..d214e99521 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c

@@ -37,6 +37,7 @@ struct checkout_opts {
        int overwrite_ignore;
        int ignore_skipworktree;
        int ignore_other_worktrees;
+       int no_index;
        int show_progress;

        const char *new_branch;

@@ -268,6 +269,9 @@ static int checkout_paths(const struct checkout_opts *opts,
                die(_("Cannot update paths and switch to branch '%s' at the same time."),
                    opts->new_branch);

+       if (opts->no_index && !opts->source_tree)
+               die(_("'--working-tree-only' cannot be used without tree-ish"));

@@ -370,7 +374,9 @@ static int checkout_paths(const struct checkout_opts *opts,
                }
        }

-       if (write_locked_index(&the_index, lock_file, COMMIT_LOCK))
+       if (opts->no_index)
+               ; /* discard the in-core index */
+       else if (write_locked_index(&the_index, lock_file, COMMIT_LOCK))
                die(_("unable to write new index file"));

        read_ref_full("HEAD", 0, rev.hash, NULL);

@@ -1161,6 +1167,8 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
                OPT_BOOL(0, "ignore-other-worktrees", &opts.ignore_other_worktrees,
                         N_("do not check if another worktree is holding the given ref")),
                OPT_BOOL(0, "progress", &opts.show_progress, N_("force progress reporting")),
+               OPT_BOOL(0, "working-tree-only", &opts.no_index, N_("checkout to working tree only without touching the index")),
+