Re: Inconsistent behavior of the path disambiguator

-- 8< --
Subject: checkout: apply Dscho's dwim even with "--" present

git reset and similar commands use -- to disambiguate between
revisions and paths on the command line.  The same syntax is not
necessary to specify a revision to git checkout (for convenience
and historical reasons, revisions are preferred over paths), but
for consistency it is accepted:

	git checkout master --; # check out master branch, not "master" file.

The autovivification of branches introduced by 70c9ac2f1 (DWIM "git
checkout frotz" to "git checkout -b frotz origin/frotz", 2009-10-18)
is currently disabled by that syntax, for no good reason.  Paranoid
scripts can still use

	git checkout --no-guess master

or even better,

	old=$(git rev-parse --verify HEAD)
	new=$(git rev-parse --verify refs/heads/master^0)
	git read-tree -m -u --exclude-standard $old $new
	git symbolic-ref -m "$me: switching branches" HEAD refs/heads/master

Requested-by: Dun Peal [off-list ref]
Signed-off-by: Jonathan Nieder <redacted>
---

diff --git a/builtin/checkout.c b/builtin/checkout.c
index 9240faf..1dc3640 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c

@@ -771,6 +771,12 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 	 *   <ref> must be a valid tree, everything after the '--' must be
 	 *   a path.
 	 *
+	 *   Except: with no paths, if <something> does not resolve as
+	 *   an object, no -t nor -b was given, and there is a tracking
+	 *   branch whose name is <something> in one and only one remote,
+	 *   then this is a short-hand to fork local <something> from
+	 *   that remote-tracking branch.
+	 *
 	 * case 2: git checkout -- [<paths>]
 	 *
 	 *   everything after the '--' must be paths.

@@ -808,13 +814,11 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 			arg = "@{-1}";
 
 		if (get_sha1_mb(arg, rev)) {
-			if (has_dash_dash)          /* case (1) */
-				die("invalid reference: %s", arg);
 			if (!patch_mode &&
 			    dwim_new_local_branch &&
 			    opts.track == BRANCH_TRACK_UNSPECIFIED &&
 			    !opts.new_branch &&
-			    !check_filename(NULL, arg) &&
+			    (has_dash_dash || !check_filename(NULL, arg)) &&
 			    argc == 1) {
 				const char *remote = unique_tracking_name(arg);
 				if (!remote || get_sha1(remote, rev))

@@ -822,9 +826,11 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 				opts.new_branch = arg;
 				arg = remote;
 				/* DWIMmed to create local branch */
-			}
-			else
+			} else if (has_dash_dash) {	/* case (1) */
+				die("invalid reference: %s", arg);
+			} else {
 				goto no_reference;
+			}
 		}
 
 		/* we can't end up being in (2) anymore, eat the argument */