refactor: apply suggestions from code review

Co-authored-by: Denis Laxalde <denis@laxalde.org>

src/click/core.py

@@ -1843,7 +1843,9 @@ def command(
 
         func: t.Optional[t.Callable] = None
 
-        if len(args) == 1 and not kwargs and callable(args[0]):
+        if callable(args[0]):
+            assert len(args) == 1, "Please use command(name, ...)(callable) instead"
+            assert not kwargs, "Please use command(name, ...)(callable) instead"
             (func,) = args
             args = ()
 
@@ -1891,7 +1893,7 @@ def group(
         if args and callable(args[0]):
             assert len(args) == 1, "Please use group(...)(callable) instead"
             assert not kwargs, "Please use group(...)(callable) instead"
-            func = args[0]
+            (func,) = args
             args = ()
 
         if self.group_class is not None and kwargs.get("cls") is None:

src/click/decorators.py

@@ -126,7 +126,7 @@ def new_func(*args, **kwargs):  # type: ignore
 
 @t.overload
 def command(
-    __name: t.Callable[..., t.Any],
+    __func: t.Callable[..., t.Any],
 ) -> Command:
     ...
 
@@ -228,7 +228,7 @@ def decorator(f: t.Callable[..., t.Any]) -> Command:
 
 @t.overload
 def group(
-    __name: t.Callable,
+    __func: t.Callable,
 ) -> Group:
     ...