use add_completion_class as a decorator

docs/shell-completion.rst

@@ -208,6 +208,9 @@ The example code is for a made up shell "My Shell" or "mysh" for short.
 
 .. code-block:: python
 
+    from click.shell_completion import add_completion_class
+    from click.shell_completion import ShellComplete
+
     _mysh_source = """\
     %(complete_func)s {
         response=$(%(complete_var)s=complete_mysh %(prog_name)s)
@@ -216,12 +219,11 @@ The example code is for a made up shell "My Shell" or "mysh" for short.
     call-on-complete %(prog_name)s %(complete_func)s
     """
 
+    @add_completion_class
     class MyshComplete(ShellComplete):
         name = "mysh"
         source_template = _mysh_source
 
-    add_completion_class("mysh", MyshComplete)
-
 Next, implement :meth:`~ShellComplete.get_completion_args`. This must
 get, parse, and return the complete args and incomplete value from the
 completion script. For example, for the Bash implementation the

src/click/shell_completion.py

@@ -149,6 +149,11 @@ class ShellComplete:
     .. versionadded:: 8.0
     """
 
+    name = None
+    """Name to register the shell as with :func:`add_completion_class`.
+    This is used in completion instructions (``source_{name}`` and
+    ``complete_{name}``).
+    """
     source_template = None
     """Completion script template formatted by :meth:`source`. This must
     be provided by subclasses.
@@ -234,6 +239,7 @@ def complete(self):
 class BashComplete(ShellComplete):
     """Shell completion for Bash."""
 
+    name = "bash"
     source_template = _SOURCE_BASH
 
     def _check_version(self):
@@ -279,6 +285,7 @@ def format_completion(self, item):
 class ZshComplete(ShellComplete):
     """Shell completion for Zsh."""
 
+    name = "zsh"
     source_template = _SOURCE_ZSH
 
     def get_completion_args(self):
@@ -301,6 +308,7 @@ def format_completion(self, item):
 class FishComplete(ShellComplete):
     """Shell completion for Fish."""
 
+    name = "fish"
     source_template = _SOURCE_FISH
 
     def get_completion_args(self):
@@ -331,16 +339,20 @@ def format_completion(self, item):
 }
 
 
-def add_completion_class(name, completion_class):
+def add_completion_class(cls, name=None):
     """Register a :class:`ShellComplete` subclass under the given name.
     The name will be provided by the completion instruction environment
     variable during completion.
 
-    :param name: Name to register the class under.
-    :param completion_class: The completion class that will handle
-        completion for the shell.
+    :param cls: The completion class that will handle completion for the
+        shell.
+    :param name: Name to register the class under. Defaults to the
+        class's ``name`` attribute.
     """
-    _available_shells[name] = completion_class
+    if name is None:
+        name = cls.name
+
+    _available_shells[name] = cls
 
 
 def get_completion_class(shell):