In stub files, enforce a blank line between a function and a decorated class definition

[AlexWaygood]

Description

In .pyi stub files, Black already enforces a blank line before a (decorated or undecorated) class definition in many situations. However, it fails to do so if the decorated class definition is immediately preceded by a function.

For this example file, saved as foo.pyi:

def a(): ...
class A: ...

@deco
class B: ...
@deco
class C: ...

def d(): ...

@deco
class D: ...

def e(): ...
@deco
class E: ...

Black applies this formatting on main with --preview

--- foo.pyi	2026-04-11 13:21:51.522132+00:00
+++ foo.pyi	2026-04-11 13:22:54.144533+00:00
@@ -1,15 +1,16 @@
 def a(): ...
+
 class A: ...
 
 @deco
 class B: ...
+
 @deco
 class C: ...
 
 def d(): ...
-
 @deco
 class D: ...
 
 def e(): ...
 @deco

On this PR branch, the formatting changes to:

--- foo.pyi	2026-04-11 13:21:51.522132+00:00
+++ foo.pyi	2026-04-11 13:23:18.497329+00:00
@@ -1,16 +1,19 @@
 def a(): ...
+
 class A: ...
 
 @deco
 class B: ...
+
 @deco
 class C: ...
 
 def d(): ...
 
 @deco
 class D: ...
 
 def e(): ...
+
 @deco
 class E: ...

which feels much more consistent.

The fix adds a _decorator_decorates_class() helper that walks the AST to check whether a decorator line decorates a classdef node, and uses it to insert a blank line in _maybe_empty_lines_for_class_or_def when appropriate.

Fixes #4256. The same bug was fixed in the Ruff formatter in June, in astral-sh/ruff#18888

Checklist - did you ...

• Implement any code style changes under the --preview style, following the stability policy?
• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

[github-actions]

diff-shades results comparing this PR (49f7333) to main (0eddb3d):

--preview style (View full diff):

╭─────────────────────── Summary ────────────────────────╮
│ 2 projects & 36 files changed / 60 changes [+60/-0]    │
│                                                        │
│ ... out of 2 911 008 lines, 13 690 files & 22 projects │
╰────────────────────────────────────────────────────────╯
Differences found.

--stable style: no changes

---

What is this? | Workflow run | diff-shades documentation

[AlexWaygood]

The diff-shades preview changes all look like clear improvements to me