Fix feature detection for positional-only arguments in lambdas (#2532)

CHANGES.md

@@ -5,6 +5,7 @@
 ### _Black_
 
 - Add new `--workers` parameter (#2514)
+- Fixed feature detection for positional-only arguments in lambdas (#2532)
 - Bumped typed-ast version minimum to 1.4.3 for 3.10 compatiblity (#2519)
 
 ### _Blackd_

src/black/__init__.py

@@ -1123,7 +1123,11 @@ def get_features_used(node: Node) -> Set[Feature]:
                 features.add(Feature.NUMERIC_UNDERSCORES)
 
         elif n.type == token.SLASH:
-            if n.parent and n.parent.type in {syms.typedargslist, syms.arglist}:
+            if n.parent and n.parent.type in {
+                syms.typedargslist,
+                syms.arglist,
+                syms.varargslist,
+            }:
                 features.add(Feature.POS_ONLY_ARGUMENTS)
 
         elif n.type == token.COLONEQUAL:

tests/test_black.py

@@ -805,6 +805,10 @@ def test_get_features_used(self) -> None:
         self.assertEqual(black.get_features_used(node), set())
         node = black.lib2to3_parse(expected)
         self.assertEqual(black.get_features_used(node), set())
+        node = black.lib2to3_parse("lambda a, /, b: ...")
+        self.assertEqual(black.get_features_used(node), {Feature.POS_ONLY_ARGUMENTS})
+        node = black.lib2to3_parse("def fn(a, /, b): ...")
+        self.assertEqual(black.get_features_used(node), {Feature.POS_ONLY_ARGUMENTS})
 
     def test_get_future_imports(self) -> None:
         node = black.lib2to3_parse("\n")