branch: error copying or renaming a detached HEAD

In c847f53 (Detached HEAD (experimental), 2007-01-01) an error
condition was introduced in rename_branch() to prevent renaming, later
also copying, a detached HEAD.

The condition used was checking for NULL in oldname, the source branch
to rename/copy.  That condition cannot be satisfied because if no source
branch is specified, HEAD is going to be used in the call.

The error issued instead is:

	fatal: Invalid branch name: 'HEAD'

Let's remove the condition in copy_or_rename_branch() (the current
function name) and check for HEAD before calling it, dying with the
original intended error if we're in a detached HEAD.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/branch.c

@@ -520,13 +520,6 @@ static void copy_or_rename_branch(const char *oldname, const char *newname, int
 	const char *interpreted_newname = NULL;
 	int recovery = 0;
 
-	if (!oldname) {
-		if (copy)
-			die(_("cannot copy the current branch while not on any."));
-		else
-			die(_("cannot rename the current branch while not on any."));
-	}
-
 	if (strbuf_check_branch_ref(&oldref, oldname)) {
 		/*
 		 * Bad name --- this could be an attempt to rename a
@@ -827,24 +820,19 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 		strbuf_release(&buf);
 
 		return ret;
-	} else if (copy) {
-		if (!argc)
-			die(_("branch name required"));
-		else if (argc == 1)
-			copy_or_rename_branch(head, argv[0], 1, copy > 1);
-		else if (argc == 2)
-			copy_or_rename_branch(argv[0], argv[1], 1, copy > 1);
-		else
-			die(_("too many branches for a copy operation"));
-	} else if (rename) {
+	} else if (copy || rename) {
 		if (!argc)
 			die(_("branch name required"));
+		else if ((argc == 1) && filter.detached)
+			die(copy? _("cannot copy the current branch while not on any.")
+				: _("cannot rename the current branch while not on any."));
 		else if (argc == 1)
-			copy_or_rename_branch(head, argv[0], 0, rename > 1);
+			copy_or_rename_branch(head, argv[0], copy, copy + rename > 1);
 		else if (argc == 2)
-			copy_or_rename_branch(argv[0], argv[1], 0, rename > 1);
+			copy_or_rename_branch(argv[0], argv[1], copy, copy + rename > 1);
 		else
-			die(_("too many arguments for a rename operation"));
+			die(copy? _("too many branches for a copy operation")
+				: _("too many arguments for a rename operation"));
 	} else if (new_upstream) {
 		struct branch *branch;
 		struct strbuf buf = STRBUF_INIT;

t/t3200-branch.sh

@@ -268,6 +268,17 @@ test_expect_success 'git branch -M topic topic should work when main is checked
 	git branch -M topic topic
 '
 
+test_expect_success 'git branch -M and -C fail on detached HEAD' '
+	git checkout HEAD^{} &&
+	test_when_finished git checkout - &&
+	echo "fatal: cannot rename the current branch while not on any." >expect &&
+	test_must_fail git branch -M must-fail 2>err &&
+	test_cmp expect err &&
+	echo "fatal: cannot copy the current branch while not on any." >expect &&
+	test_must_fail git branch -C must-fail 2>err &&
+	test_cmp expect err
+'
+
 test_expect_success 'git branch -v -d t should work' '
 	git branch t &&
 	git rev-parse --verify refs/heads/t &&