Merge branch 'nd/checkout-dwim-fix'

"git checkout frotz" (without any double-dash) avoids ambiguity by
making sure 'frotz' cannot be interpreted as a revision and as a
path at the same time.  This safety has been updated to check also
a unique remote-tracking branch 'frotz' in a remote, when dwimming
to create a local branch 'frotz' out of a remote-tracking branch
'frotz' from a remote.

* nd/checkout-dwim-fix:
  checkout: disambiguate dwim tracking branches and local files

Documentation/git-checkout.txt

@@ -276,6 +276,10 @@ section of linkgit:git-add[1] to learn how to operate the `--patch` mode.
 	Just like linkgit:git-submodule[1], this will detach the
 	submodules HEAD.
 
+--no-guess::
+	Do not attempt to create a branch if a remote tracking branch
+	of the same name exists.
+
 <branch>::
 	Branch to checkout; if it refers to a branch (i.e., a name that,
 	when prepended with "refs/heads/", is a valid ref), then that

builtin/checkout.c

@@ -1080,9 +1080,12 @@ static int parse_branchname_arg(int argc, const char **argv,
 		 */
 		int recover_with_dwim = dwim_new_local_branch_ok;
 
-		if (!has_dash_dash &&
-		    (check_filename(opts->prefix, arg) || !no_wildcard(arg)))
+		int could_be_checkout_paths = !has_dash_dash &&
+			check_filename(opts->prefix, arg);
+
+		if (!has_dash_dash && !no_wildcard(arg))
 			recover_with_dwim = 0;
+
 		/*
 		 * Accept "git checkout foo" and "git checkout foo --"
 		 * as candidates for dwim.
@@ -1095,6 +1098,10 @@ static int parse_branchname_arg(int argc, const char **argv,
 			const char *remote = unique_tracking_name(arg, rev,
 								  dwim_remotes_matched);
 			if (remote) {
+				if (could_be_checkout_paths)
+					die(_("'%s' could be both a local file and a tracking branch.\n"
+					      "Please use -- (and optionally --no-guess) to disambiguate"),
+					    arg);
 				*new_branch = arg;
 				arg = remote;
 				/* DWIMmed to create local branch, case (3).(b) */
@@ -1229,7 +1236,7 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 	struct checkout_opts opts;
 	struct branch_info new_branch_info;
 	char *conflict_style = NULL;
-	int dwim_new_local_branch = 1;
+	int dwim_new_local_branch, no_dwim_new_local_branch = 0;
 	int dwim_remotes_matched = 0;
 	struct option options[] = {
 		OPT__QUIET(&opts.quiet, N_("suppress progress reporting")),
@@ -1259,8 +1266,8 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 		OPT_BOOL('p', "patch", &opts.patch_mode, N_("select hunks interactively")),
 		OPT_BOOL(0, "ignore-skip-worktree-bits", &opts.ignore_skipworktree,
 			 N_("do not limit pathspecs to sparse entries only")),
-		OPT_HIDDEN_BOOL(0, "guess", &dwim_new_local_branch,
-				N_("second guess 'git checkout <no-such-branch>'")),
+		OPT_BOOL(0, "no-guess", &no_dwim_new_local_branch,
+			 N_("do not second guess 'git checkout <no-such-branch>'")),
 		OPT_BOOL(0, "ignore-other-worktrees", &opts.ignore_other_worktrees,
 			 N_("do not check if another worktree is holding the given ref")),
 		{ OPTION_CALLBACK, 0, "recurse-submodules", NULL,
@@ -1283,6 +1290,7 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 	argc = parse_options(argc, argv, prefix, options, checkout_usage,
 			     PARSE_OPT_KEEP_DASHDASH);
 
+	dwim_new_local_branch = !no_dwim_new_local_branch;
 	if (opts.show_progress < 0) {
 		if (opts.quiet)
 			opts.show_progress = 0;

t/t2024-checkout-dwim.sh

@@ -278,4 +278,35 @@ test_expect_success 'loosely defined local base branch is reported correctly' '
 	test_cmp expect actual
 '
 
+test_expect_success 'reject when arg could be part of dwim branch' '
+	git remote add foo file://non-existent-place &&
+	git update-ref refs/remotes/foo/dwim-arg HEAD &&
+	echo foo >dwim-arg &&
+	git add dwim-arg &&
+	echo bar >dwim-arg &&
+	test_must_fail git checkout dwim-arg &&
+	test_must_fail git rev-parse refs/heads/dwim-arg -- &&
+	grep bar dwim-arg
+'
+
+test_expect_success 'disambiguate dwim branch and checkout path (1)' '
+	git update-ref refs/remotes/foo/dwim-arg1 HEAD &&
+	echo foo >dwim-arg1 &&
+	git add dwim-arg1 &&
+	echo bar >dwim-arg1 &&
+	git checkout -- dwim-arg1 &&
+	test_must_fail git rev-parse refs/heads/dwim-arg1 -- &&
+	grep foo dwim-arg1
+'
+
+test_expect_success 'disambiguate dwim branch and checkout path (2)' '
+	git update-ref refs/remotes/foo/dwim-arg2 HEAD &&
+	echo foo >dwim-arg2 &&
+	git add dwim-arg2 &&
+	echo bar >dwim-arg2 &&
+	git checkout dwim-arg2 -- &&
+	git rev-parse refs/heads/dwim-arg2 -- &&
+	grep bar dwim-arg2
+'
+
 test_done

t/t9902-completion.sh

@@ -1434,7 +1434,8 @@ test_expect_success 'double dash "git checkout"' '
 	--ignore-other-worktrees Z
 	--recurse-submodules Z
 	--progress Z
-	--no-quiet Z
+	--guess Z
+	--no-guess Z
 	--no-... Z
 	EOF
 '