The "--" notation disambiguates files and branches, but as a side-effect
of the previous implementation, also disabled the branch auto-creation
when $branch does not exist.

A possible scenario is then:

git checkout $branch
=> fails if $branch is both a ref and a file, and suggests --

git checkout $branch --
=> refuses to create the $branch

This patch allows the second form to create $branch, and since the -- is
provided, it does not look for file named $branch.

Signed-off-by: Matthieu Moy <matthieu....@imag.fr>
---
Since v1: added a paragraph in the block comment.

 builtin/checkout.c       | 38 ++++++++++++++++++++++++++++++--------
 t/t2024-checkout-dwim.sh | 22 ++++++++++++++++++++++
 2 files changed, 52 insertions(+), 8 deletions(-)

diff --git a/builtin/checkout.c b/builtin/checkout.c
index 0f57397..a5a12f6 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -863,6 +863,14 @@ static const char *unique_tracking_name(const char *name, 
unsigned char *sha1)
        return NULL;
 }
 
+static int error_invalid_ref(const char *arg, int has_dash_dash, int argcount)
+{
+       if (has_dash_dash)
+               die(_("invalid reference: %s"), arg);
+       else
+               return argcount;
+}
+
 static int parse_branchname_arg(int argc, const char **argv,
                                int dwim_new_local_branch_ok,
                                struct branch_info *new,
@@ -881,6 +889,12 @@ static int parse_branchname_arg(int argc, const char 
**argv,
         *   <ref> must be a valid tree, everything after the '--' must be
         *   a path.
         *
+        *   A sub-case of (1) is "git checkout <ref> --". In this
+        *   case, checkout behaves like case (3), except that it does
+        *   not attempt to understand <ref> as a file (hence, the
+        *   short-hand to create branch <ref> works even if <ref>
+        *   exists as a filename).
+        *
         * case 2: git checkout -- [<paths>]
         *
         *   everything after the '--' must be paths.
@@ -916,20 +930,28 @@ static int parse_branchname_arg(int argc, const char 
**argv,
        if (!strcmp(arg, "-"))
                arg = "@{-1}";
 
-       if (get_sha1_mb(arg, rev)) {
-               if (has_dash_dash)          /* case (1) */
-                       die(_("invalid reference: %s"), arg);
-               if (dwim_new_local_branch_ok &&
-                   !check_filename(NULL, arg) &&
-                   argc == 1) {
+       if (get_sha1_mb(arg, rev)) { /* case (1)? */
+               int try_dwim = dwim_new_local_branch_ok;
+
+               if (check_filename(NULL, arg) && !has_dash_dash)
+                       try_dwim = 0;
+               /*
+                * Accept "git checkout foo" and "git checkout foo --"
+                * as candidates for dwim.
+                */
+               if (!(argc == 1 && !has_dash_dash) &&
+                   !(argc == 2 && has_dash_dash))
+                       try_dwim = 0;
+
+               if (try_dwim) {
                        const char *remote = unique_tracking_name(arg, rev);
                        if (!remote)
-                               return argcount;
+                               return error_invalid_ref(arg, has_dash_dash, 
argcount);
                        *new_branch = arg;
                        arg = remote;
                        /* DWIMmed to create local branch */
                } else {
-                       return argcount;
+                       return error_invalid_ref(arg, has_dash_dash, argcount);
                }
        }
 
diff --git a/t/t2024-checkout-dwim.sh b/t/t2024-checkout-dwim.sh
index 094b92e..d9afdb2 100755
--- a/t/t2024-checkout-dwim.sh
+++ b/t/t2024-checkout-dwim.sh
@@ -164,4 +164,26 @@ test_expect_success 'checkout of branch from a single 
remote succeeds #4' '
        test_branch_upstream eggs repo_d eggs
 '
 
+test_expect_success 'checkout of branch with a file having the same name 
fails' '
+       git checkout -B master &&
+       test_might_fail git branch -D spam &&
+
+       >spam &&
+       test_must_fail git checkout spam &&
+       test_must_fail git checkout spam &&
+       test_must_fail git rev-parse --verify refs/heads/spam &&
+       test_branch master
+'
+
+test_expect_success 'checkout <branch> -- succeeds, even if a file with the 
same name exists' '
+       git checkout -B master &&
+       test_might_fail git branch -D spam &&
+
+       >spam &&
+       git checkout spam -- &&
+       test_branch spam &&
+       test_cmp_rev refs/remotes/extra_dir/repo_c/extra_dir/spam HEAD &&
+       test_branch_upstream spam repo_c spam
+'
+
 test_done
-- 
1.8.4.474.g128a96c

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