Fix line ranges decorator edge case

CHANGES.md

@@ -22,6 +22,8 @@
 - Remove support for pre-python 3.7 `await/async` as soft keywords/variable names
   (#4676)
 - Fix crash on parenthesized expression inside a type parameter bound (#4684)
+- Fix crash when using line ranges excluding indented single line decorated items
+  (#4670)
 
 ### Preview style
 

src/black/ranges.py

@@ -330,6 +330,19 @@ def _convert_node_to_standalone_comment(node: LN) -> None:
     first.prefix = ""
     index = node.remove()
     if index is not None:
+        # Because of the special handling of multiple decorators, if the decorated
+        # item is a single line then there will be a missing newline between the
+        # decorator and item, so add it back. This doesn't affect any other case
+        # since a decorated item with a newline would hit the earlier suite case
+        # in _convert_unchanged_line_by_line that correctly handles the newlines.
+        if node.type == syms.decorated:
+            # A leaf of type decorated wouldn't make sense, since it should always
+            # have at least the decorator + the decorated item, so if this assert
+            # hits that means there's a problem in the parser.
+            assert isinstance(node, Node)
+            # 1 will always be the correct index since before this function is
+            # called all the decorators are collapsed into a single leaf
+            node.insert_child(1, Leaf(NEWLINE, "\n"))
         # Remove the '\n', as STANDALONE_COMMENT will have '\n' appended when
         # generating the formatted code.
         value = str(node)[:-1]

tests/data/cases/line_ranges_decorator_edge_case.py

@@ -0,0 +1,8 @@
+# flags: --line-ranges=6-7
+class Foo:
+
+    @overload
+    def foo(): ...
+
+    def fox(self):
+        print()