change logic

src/black/lines.py

@@ -725,14 +725,15 @@ def _maybe_empty_lines_for_class_or_def(  # noqa: C901
             else:
                 newlines = 0
         else:
+            newlines = 1 if current_line.depth else 2
+            # If a user has left no space after a dummy implementation, don't insert
+            # new lines. This is useful for instance for @overload or Protocols.
             if (
                 Preview.dummy_implementations in self.mode
                 and self.previous_line.is_stub_def
-                and (current_line.is_stub_def or current_line.is_decorator)
+                and not bool(user_hint_before)
             ):
-                newlines = user_hint_before
-            else:
-                newlines = 1 if current_line.depth else 2
+                newlines = 0
         if comment_to_add_newlines is not None:
             previous_block = comment_to_add_newlines.previous_block
             if previous_block is not None:

tests/data/preview/dummy_implementations.py

@@ -34,6 +34,19 @@ def dummy_three():
 def dummy_four():
     ...
 
+@overload
+def b(arg: int) -> int: ...
+
+@overload
+def b(arg: str) -> str: ...
+@overload
+def b(arg: object) -> NoReturn: ...
+
+def b(arg: Union[int, str, object]) -> Union[int, str]:
+    if not isinstance(arg, (int, str)):
+        raise TypeError
+    return arg
+
 # output
 
 from typing import NoReturn, Protocol, Union, overload
@@ -49,8 +62,6 @@ def a(arg: int) -> int: ...
 def a(arg: str) -> str: ...
 @overload
 def a(arg: object) -> NoReturn: ...
-
-
 def a(arg: Union[int, str, object]) -> Union[int, str]:
     if not isinstance(arg, (int, str)):
         raise TypeError
@@ -68,4 +79,21 @@ def dummy_two(): ...
 @dummy
 def dummy_three(): ...
 
+
 def dummy_four(): ...
+
+
+@overload
+def b(arg: int) -> int: ...
+
+
+@overload
+def b(arg: str) -> str: ...
+@overload
+def b(arg: object) -> NoReturn: ...
+
+
+def b(arg: Union[int, str, object]) -> Union[int, str]:
+    if not isinstance(arg, (int, str)):
+        raise TypeError
+    return arg