checkout: split part of it to new command 'switch'

"git checkout" doing too many things is a source of confusion for many
users (and it even bites old timers sometimes). To remedy that, the
command will be split into two new ones: switch and restore. The good
old "git checkout" command is still here and will be until all (or most
of users) are sick of it.

See the new man page for the final design of switch. The actual
implementation though is still pretty much the same as "git checkout"
and not completely aligned with the man page. Following patches will
adjust their behavior to match the man page.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

14 files changed, 353 insertions, 27 deletions

diff --git a/.gitignore b/.gitignore
index 7374587f9d..c687b92b1c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -167,6 +167,7 @@
 /git-submodule
 /git-submodule--helper
 /git-svn
+/git-switch
 /git-symbolic-ref
 /git-tag
 /git-unpack-file
diff --git a/Documentation/config/advice.txt b/Documentation/config/advice.txt
index 88620429ea..239d479506 100644
--- a/Documentation/config/advice.txt
+++ b/Documentation/config/advice.txt
@@ -42,7 +42,8 @@ advice.*::
 		state in the output of linkgit:git-status[1], in
 		the template shown when writing commit messages in
 		linkgit:git-commit[1], and in the help message shown
-		by linkgit:git-checkout[1] when switching branch.
+		by linkgit:git-switch[1] or
+		linkgit:git-checkout[1] when switching branch.
 	statusUoption::
 		Advise to consider using the `-u` option to linkgit:git-status[1]
 		when the command takes more than 2 seconds to enumerate untracked
@@ -62,12 +63,14 @@ advice.*::
 		your information is guessed from the system username and
 		domain name.
 	detachedHead::
-		Advice shown when you used linkgit:git-checkout[1] to
-		move to the detach HEAD state, to instruct how to create
-		a local branch after the fact.
+		Advice shown when you used
+		linkgit:git-switch[1] or linkgit:git-checkout[1]
+		to move to the detach HEAD state, to instruct how to
+		create a local branch after the fact.
 	checkoutAmbiguousRemoteBranchName::
 		Advice shown when the argument to
-		linkgit:git-checkout[1] ambiguously resolves to a
+		linkgit:git-checkout[1] and linkgit:git-switch[1]
+		ambiguously resolves to a
 		remote tracking branch on more than one remote in
 		situations where an unambiguous argument would have
 		otherwise caused a remote-tracking branch to be
diff --git a/Documentation/config/branch.txt b/Documentation/config/branch.txt
index 019d60ede2..8050466159 100644
--- a/Documentation/config/branch.txt
+++ b/Documentation/config/branch.txt
@@ -1,5 +1,5 @@
 branch.autoSetupMerge::
-	Tells 'git branch' and 'git checkout' to set up new branches
+	Tells 'git branch', 'git switch' and 'git checkout' to set up new branches
 	so that linkgit:git-pull[1] will appropriately merge from the
 	starting point branch. Note that even if this option is not set,
 	this behavior can be chosen per-branch using the `--track`
@@ -11,7 +11,7 @@ branch.autoSetupMerge::
 	branch. This option defaults to true.
 
 branch.autoSetupRebase::
-	When a new branch is created with 'git branch' or 'git checkout'
+	When a new branch is created with 'git branch', 'git switch' or 'git checkout'
 	that tracks another branch, this variable tells Git to set
 	up pull to rebase instead of merge (see "branch.<name>.rebase").
 	When `never`, rebase is never automatically set to true.
diff --git a/Documentation/config/checkout.txt b/Documentation/config/checkout.txt
index c4118fa196..d6872ffa83 100644
--- a/Documentation/config/checkout.txt
+++ b/Documentation/config/checkout.txt
@@ -1,5 +1,6 @@
 checkout.defaultRemote::
-	When you run 'git checkout <something>' and only have one
+	When you run 'git checkout <something>'
+	or 'git switch <something>' and only have one
 	remote, it may implicitly fall back on checking out and
 	tracking e.g. 'origin/<something>'. This stops working as soon
 	as you have more than one remote with a '<something>'
@@ -8,8 +9,10 @@ checkout.defaultRemote::
 	disambiguation. The typical use-case is to set this to
 	`origin`.
 +
-Currently this is used by linkgit:git-checkout[1] when 'git checkout
-<something>' will checkout the '<something>' branch on another remote,
+Currently this is used by linkgit:git-switch[1] and
+linkgit:git-checkout[1] when 'git checkout <something>'
+or 'git switch <something>'
+will checkout the '<something>' branch on another remote,
 and by linkgit:git-worktree[1] when 'git worktree add' refers to a
 remote branch. This setting might be used for other checkout-like
 commands or functionality in the future.
diff --git a/Documentation/config/diff.txt b/Documentation/config/diff.txt
index e48bb987d7..b3b304ee12 100644
--- a/Documentation/config/diff.txt
+++ b/Documentation/config/diff.txt
@@ -78,7 +78,8 @@ diff.external::
 diff.ignoreSubmodules::
 	Sets the default value of --ignore-submodules. Note that this
 	affects only 'git diff' Porcelain, and not lower level 'diff'
-	commands such as 'git diff-files'. 'git checkout' also honors
+	commands such as 'git diff-files'. 'git checkout'
+	and 'git switch' also honor
 	this setting when reporting uncommitted changes. Setting it to
 	'all' disables the submodule summary normally shown by 'git commit'
 	and 'git status' when `status.submoduleSummary` is set unless it is
diff --git a/Documentation/git-checkout.txt b/Documentation/git-checkout.txt
index bf90966c95..c7192bdefe 100644
--- a/Documentation/git-checkout.txt
+++ b/Documentation/git-checkout.txt
@@ -560,6 +560,10 @@ $ edit frotz
 $ git add frotz
 ------------
 
+SEE ALSO
+--------
+linkgit:git-switch[1]
+
 GIT
 ---
 Part of the linkgit:git[1] suite
diff --git a/Documentation/git-switch.txt b/Documentation/git-switch.txt
new file mode 100644
index 0000000000..f15e5bdcf4
--- /dev/null
+++ b/Documentation/git-switch.txt
@@ -0,0 +1,271 @@
+git-switch(1)
+=============
+
+NAME
+----
+git-switch - Switch branches
+
+SYNOPSIS
+--------
+[verse]
+'git switch' [<options>] [--no-guess] <branch>
+'git switch' [<options>] --detach [<start-point>]
+'git switch' [<options>] (-c|-C) <new-branch> [<start-point>]
+'git switch' [<options>] --orphan <new-branch>
+
+DESCRIPTION
+-----------
+Switch to a specified branch. The working tree and the index are
+updated to match the branch. All new commits will be added to the tip
+of this branch.
+
+Optionally a new branch could be created with either `-c`, `-C`,
+automatically from a remote branch of same name (see `--guess`), or
+detach the working tree from any branch with `--detach`, along with
+switching.
+
+Switching branches does not require a clean index and working tree
+(i.e. no differences compared to `HEAD`). The operation is aborted
+however if the operation leads to loss of local changes, unless told
+otherwise with `--discard-changes` or `--merge`.
+
+OPTIONS
+-------
+<branch>::
+	Branch to switch to.
+
+<new-branch>::
+	Name for the new branch.
+
+<start-point>::
+	The starting point for the new branch. Specifying a
+	`<start-point>` allows you to create a branch based on some
+	other point in history than where HEAD currently points. (Or,
+	in the case of `--detach`, allows you to inspect and detach
+	from some other point.)
++
+You can use the `@{-N}` syntax to refer to the N-th last
+branch/commit switched to using "git switch" or "git checkout"
+operation. You may also specify `-` which is synonymous to `@{-1}`.
+This is often used to switch quickly between two branches, or to undo
+a branch switch by mistake.
++
+As a special case, you may use `A...B` as a shortcut for the merge
+base of `A` and `B` if there is exactly one merge base. You can leave
+out at most one of `A` and `B`, in which case it defaults to `HEAD`.
+
+-c <new-branch>::
+--create <new-branch>::
+	Create a new branch named `<new-branch>` starting at
+	`<start-point>` before switching to the branch. This is a
+	convenient shortcut for:
++
+------------
+$ git branch <new-branch>
+$ git switch <new-branch>
+------------
+
+-C <new-branch>::
+--force-create <new-branch>::
+	Similar to `--create` except that if `<new-branch>` already
+	exists, it will be reset to `<start-point>`. This is a
+	convenient shortcut for:
++
+------------
+$ git branch -f <new-branch>
+$ git switch <new-branch>
+------------
+
+-d::
+--detach::
+	Switch to a commit for inspection and discardable
+	experiments. See the "DETACHED HEAD" section in
+	linkgit:git-checkout[1] for details.
+
+--guess::
+--no-guess::
+	If `<branch>` is not found but there does exist a tracking
+	branch in exactly one remote (call it `<remote>`) with a
+	matching name, treat as equivalent to
++
+------------
+$ git switch -c <branch> --track <remote>/<branch>
+------------
++
+If the branch exists in multiple remotes and one of them is named by
+the `checkout.defaultRemote` configuration variable, we'll use that
+one for the purposes of disambiguation, even if the `<branch>` isn't
+unique across all remotes. Set it to e.g. `checkout.defaultRemote=origin`
+to always checkout remote branches from there if `<branch>` is
+ambiguous but exists on the 'origin' remote. See also
+`checkout.defaultRemote` in linkgit:git-config[1].
++
+`--guess` is the default behavior. Use `--no-guess` to disable it.
+
+-f::
+--force::
+	An alias for `--discard-changes`.
+
+--discard-changes::
+	Proceed even if the index or the working tree differs from
+	`HEAD`. Both the index and working tree are restored to match
+	the switching target. If `--recurse-submodules` is specified,
+	submodule content is also restored to match the switching
+	target. This is used to throw away local changes.
+
+-m::
+--merge::
+	If you have local modifications to one or more files that are
+	different between the current branch and the branch to which
+	you are switching, the command refuses to switch branches in
+	order to preserve your modifications in context.  However,
+	with this option, a three-way merge between the current
+	branch, your working tree contents, and the new branch is
+	done, and you will be on the new branch.
++
+When a merge conflict happens, the index entries for conflicting
+paths are left unmerged, and you need to resolve the conflicts
+and mark the resolved paths with `git add` (or `git rm` if the merge
+should result in deletion of the path).
+
+--conflict=<style>::
+	The same as `--merge` option above, but changes the way the
+	conflicting hunks are presented, overriding the
+	`merge.conflictStyle` configuration variable.  Possible values are
+	"merge" (default) and "diff3" (in addition to what is shown by
+	"merge" style, shows the original contents).
+
+-q::
+--quiet::
+	Quiet, suppress feedback messages.
+
+--progress::
+--no-progress::
+	Progress status is reported on the standard error stream
+	by default when it is attached to a terminal, unless `--quiet`
+	is specified. This flag enables progress reporting even if not
+	attached to a terminal, regardless of `--quiet`.
+
+-t::
+--track::
+	When creating a new branch, set up "upstream" configuration.
+	`-c` is implied. See `--track` in linkgit:git-branch[1] for
+	details.
++
+If no `-c` option is given, the name of the new branch will be derived
+from the remote-tracking branch, by looking at the local part of the
+refspec configured for the corresponding remote, and then stripping
+the initial part up to the "*".  This would tell us to use `hack` as
+the local branch when branching off of `origin/hack` (or
+`remotes/origin/hack`, or even `refs/remotes/origin/hack`).  If the
+given name has no slash, or the above guessing results in an empty
+name, the guessing is aborted.  You can explicitly give a name with
+`-c` in such a case.
+
+--no-track::
+	Do not set up "upstream" configuration, even if the
+	`branch.autoSetupMerge` configuration variable is true.
+
+--orphan <new-branch>::
+	Create a new 'orphan' branch, named `<new-branch>`. All
+	tracked files are removed.
+
+--ignore-other-worktrees::
+	`git switch` refuses when the wanted ref is already
+	checked out by another worktree. This option makes it check
+	the ref out anyway. In other words, the ref can be held by
+	more than one worktree.
+
+--recurse-submodules::
+--no-recurse-submodules::
+	Using `--recurse-submodules` will update the content of all
+	initialized submodules according to the commit recorded in the
+	superproject. If nothing (or `--no-recurse-submodules`) is
+	used, the work trees of submodules will not be updated. Just
+	like linkgit:git-submodule[1], this will detach `HEAD` of the
+	submodules.
+
+EXAMPLES
+--------
+
+The following command switches to the "master" branch:
+
+------------
+$ git switch master
+------------
+
+After working in the wrong branch, switching to the correct branch
+would be done using:
+
+------------
+$ git switch mytopic
+------------
+
+However, your "wrong" branch and correct "mytopic" branch may differ
+in files that you have modified locally, in which case the above
+switch would fail like this:
+
+------------
+$ git switch mytopic
+error: You have local changes to 'frotz'; not switching branches.
+------------
+
+You can give the `-m` flag to the command, which would try a three-way
+merge:
+
+------------
+$ git switch -m mytopic
+Auto-merging frotz
+------------
+
+After this three-way merge, the local modifications are _not_
+registered in your index file, so `git diff` would show you what
+changes you made since the tip of the new branch.
+
+To switch back to the previous branch before we switched to mytopic
+(i.e. "master" branch):
+
+------------
+$ git switch -
+------------
+
+You can grow a new branch from any commit. For example, switch to
+"HEAD~3" and create branch "fixup":
+
+------------
+$ git switch -c fixup HEAD~3
+Switched to a new branch 'fixup'
+------------
+
+If you want to start a new branch from a remote branch of the same
+name:
+
+------------
+$ git switch new-topic
+Branch 'new-topic' set up to track remote branch 'new-topic' from 'origin'
+Switched to a new branch 'new-topic'
+------------
+
+To check out commit `HEAD~3` for temporary inspection or experiment
+without creating a new branch:
+
+------------
+$ git switch --detach HEAD~3
+HEAD is now at 9fc9555312 Merge branch 'cc/shared-index-permbits'
+------------
+
+If it turns out whatever you have done is worth keeping, you can
+always create a new name for it (without switching away):
+
+------------
+$ git switch -c good-surprises
+------------
+
+SEE ALSO
+--------
+linkgit:git-checkout[1],
+linkgit:git-branch[1]
+
+GIT
+---
+Part of the linkgit:git[1] suite
diff --git a/Documentation/gitattributes.txt b/Documentation/gitattributes.txt
index bdd11a2ddd..af16a35a9a 100644
--- a/Documentation/gitattributes.txt
+++ b/Documentation/gitattributes.txt
@@ -112,7 +112,8 @@ Checking-out and checking-in
 
 These attributes affect how the contents stored in the
 repository are copied to the working tree files when commands
-such as 'git checkout' and 'git merge' run.  They also affect how
+such as 'git switch', 'git checkout'  and 'git merge' run.
+They also affect how
 Git stores the contents you prepare in the working tree in the
 repository upon 'git add' and 'git commit'.
 
diff --git a/Documentation/githooks.txt b/Documentation/githooks.txt
index 5bf653c111..8ff72f0613 100644
--- a/Documentation/githooks.txt
+++ b/Documentation/githooks.txt
@@ -165,12 +165,13 @@ rebased, and is not set when rebasing the current branch.
 post-checkout
 ~~~~~~~~~~~~~
 
-This hook is invoked when a linkgit:git-checkout[1] is run after having updated the
+This hook is invoked when a linkgit:git-checkout[1] or
+linkgit:git-switch[1] is run after having updated the
 worktree.  The hook is given three parameters: the ref of the previous HEAD,
 the ref of the new HEAD (which may or may not have changed), and a flag
 indicating whether the checkout was a branch checkout (changing branches,
 flag=1) or a file checkout (retrieving a file from the index, flag=0).
-This hook cannot affect the outcome of `git checkout`.
+This hook cannot affect the outcome of `git switch` or `git checkout`.
 
 It is also run after linkgit:git-clone[1], unless the `--no-checkout` (`-n`) option is
 used. The first parameter given to the hook is the null-ref, the second the
@@ -406,7 +407,8 @@ exit with a zero status.
 For example, the hook can simply run `git read-tree -u -m HEAD "$1"`
 in order to emulate `git fetch` that is run in the reverse direction
 with `git push`, as the two-tree form of `git read-tree -u -m` is
-essentially the same as `git checkout` that switches branches while
+essentially the same as `git switch` or `git checkout`
+that switches branches while
 keeping the local changes in the working tree that do not interfere
 with the difference between the branches.
 
diff --git a/Makefile b/Makefile
index 537493822b..f85f364029 100644
--- a/Makefile
+++ b/Makefile
@@ -802,6 +802,7 @@ BUILT_INS += git-merge-subtree$X
 BUILT_INS += git-show$X
 BUILT_INS += git-stage$X
 BUILT_INS += git-status$X
+BUILT_INS += git-switch$X
 BUILT_INS += git-whatchanged$X
 
 # what 'all' will build and 'install' will install in gitexecdir,
diff --git a/builtin.h b/builtin.h
index 6538932e99..c64e44450e 100644
--- a/builtin.h
+++ b/builtin.h
@@ -227,6 +227,7 @@ extern int cmd_show_index(int argc, const char **argv, const char *prefix);
 extern int cmd_status(int argc, const char **argv, const char *prefix);
 extern int cmd_stripspace(int argc, const char **argv, const char *prefix);
 extern int cmd_submodule__helper(int argc, const char **argv, const char *prefix);
+extern int cmd_switch(int argc, const char **argv, const char *prefix);
 extern int cmd_symbolic_ref(int argc, const char **argv, const char *prefix);
 extern int cmd_tag(int argc, const char **argv, const char *prefix);
 extern int cmd_tar_tree(int argc, const char **argv, const char *prefix);
diff --git a/builtin/checkout.c b/builtin/checkout.c
index c15dc91e16..4989ca73a3 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -34,6 +34,11 @@ static const char * const checkout_usage[] = {
 	NULL,
 };
 
+static const char * const switch_branch_usage[] = {
+	N_("git switch [<options>] [<branch>]"),
+	NULL,
+};
+
 struct checkout_opts {
 	int patch_mode;
 	int quiet;
@@ -1411,33 +1416,25 @@ static struct option *add_checkout_path_options(struct checkout_opts *opts,
 	return newopts;
 }
 
-int cmd_checkout(int argc, const char **argv, const char *prefix)
+static int checkout_main(int argc, const char **argv, const char *prefix,
+			 struct checkout_opts *opts, struct option *options,
+			 const char * const usagestr[])
 {
-	struct checkout_opts real_opts;
-	struct checkout_opts *opts = &real_opts;
 	struct branch_info new_branch_info;
 	int dwim_remotes_matched = 0;
 	int dwim_new_local_branch;
-	struct option *options = NULL;
 
-	memset(opts, 0, sizeof(*opts));
 	memset(&new_branch_info, 0, sizeof(new_branch_info));
 	opts->overwrite_ignore = 1;
 	opts->prefix = prefix;
 	opts->show_progress = -1;
 	opts->overlay_mode = -1;
-	opts->no_dwim_new_local_branch = 0;
 
 	git_config(git_checkout_config, opts);
 
 	opts->track = BRANCH_TRACK_UNSPECIFIED;
 
-	options = parse_options_dup(options);
-	options = add_common_options(opts, options);
-	options = add_switch_branch_options(opts, options);
-	options = add_checkout_path_options(opts, options);
-
-	argc = parse_options(argc, argv, prefix, options, checkout_usage,
+	argc = parse_options(argc, argv, prefix, options, usagestr,
 			     PARSE_OPT_KEEP_DASHDASH);
 
 	dwim_new_local_branch = !opts->no_dwim_new_local_branch;
@@ -1570,3 +1567,42 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 		return checkout_branch(opts, &new_branch_info);
 	}
 }
+
+int cmd_checkout(int argc, const char **argv, const char *prefix)
+{
+	struct checkout_opts opts;
+	struct option *options = NULL;
+	int ret;
+
+	memset(&opts, 0, sizeof(opts));
+	opts.no_dwim_new_local_branch = 0;
+
+	options = parse_options_dup(options);
+	options = add_common_options(&opts, options);
+	options = add_switch_branch_options(&opts, options);
+	options = add_checkout_path_options(&opts, options);
+
+	ret = checkout_main(argc, argv, prefix, &opts,
+			    options, checkout_usage);
+	FREE_AND_NULL(options);
+	return ret;
+}
+
+int cmd_switch(int argc, const char **argv, const char *prefix)
+{
+	struct checkout_opts opts;
+	struct option *options = NULL;
+	int ret;
+
+	memset(&opts, 0, sizeof(opts));
+	opts.no_dwim_new_local_branch = 0;
+
+	options = parse_options_dup(options);
+	options = add_common_options(&opts, options);
+	options = add_switch_branch_options(&opts, options);
+
+	ret = checkout_main(argc, argv, prefix, &opts,
+			    options, switch_branch_usage);
+	FREE_AND_NULL(options);
+	return ret;
+}
diff --git a/command-list.txt b/command-list.txt
index 3a9af104b5..13317f47d4 100644
--- a/command-list.txt
+++ b/command-list.txt
@@ -171,6 +171,7 @@ git-status                              mainporcelain           info
 git-stripspace                          purehelpers
 git-submodule                           mainporcelain
 git-svn                                 foreignscminterface
+git-switch                              mainporcelain           history
 git-symbolic-ref                        plumbingmanipulators
 git-tag                                 mainporcelain           history
 git-unpack-file                         plumbinginterrogators
diff --git a/git.c b/git.c
index 2014aab6b8..39582cf511 100644
--- a/git.c
+++ b/git.c
@@ -573,6 +573,7 @@ static struct cmd_struct commands[] = {
 	{ "status", cmd_status, RUN_SETUP | NEED_WORK_TREE },
 	{ "stripspace", cmd_stripspace },
 	{ "submodule--helper", cmd_submodule__helper, RUN_SETUP | SUPPORT_SUPER_PREFIX | NO_PARSEOPT },
+	{ "switch", cmd_switch, RUN_SETUP | NEED_WORK_TREE },
 	{ "symbolic-ref", cmd_symbolic_ref, RUN_SETUP },
 	{ "tag", cmd_tag, RUN_SETUP | DELAY_PAGER_CONFIG },
 	{ "unpack-file", cmd_unpack_file, RUN_SETUP | NO_PARSEOPT },