adds subclassing test case

To account for the reordering of keyword only parameters `all_attrs` is sorted
based on the attr.kw_only value

Author: tgallant

mypy/plugins/dataclasses.py

@@ -72,6 +72,7 @@ def serialize(self) -> JsonDict:
             'line': self.line,
             'column': self.column,
             'type': self.type.serialize(),
+            'kw_only': self.kw_only,
         }
 
     @classmethod
@@ -335,6 +336,7 @@ def collect_attributes(self) -> Optional[List[DataclassAttribute]]:
                             super_attrs.append(attr)
                             break
             all_attrs = super_attrs + all_attrs
+            all_attrs.sort(key=lambda a: a.kw_only)
 
         # Ensure that arguments without a default don't follow
         # arguments that have a default.

test-data/unit/check-dataclasses.test

@@ -368,6 +368,24 @@ class Application:
 
 [builtins fixtures/list.pyi]
 
+[case testDataclassesOrderingKwOnlyWithSentinelAndSubclass]
+# flags: --python-version 3.10
+from dataclasses import dataclass, field, KW_ONLY
+
+@dataclass
+class Base:
+    x: str
+    _: KW_ONLY
+    y: int = 0
+    w: int = 1
+
+@dataclass
+class D(Base):
+    z: str
+    a: str = "a"
+
+[builtins fixtures/list.pyi]
+
 [case testDataclassesClassmethods]
 # flags: --python-version 3.7
 from dataclasses import dataclass