New semantic analyzer: fix order=True in dataclass with deferral (#6987)

Fixes #6954.

Author: JukkaL

mypy/plugins/dataclasses.py

@@ -149,7 +149,7 @@ def transform(self) -> None:
                 ]
 
                 existing_method = info.get(method_name)
-                if existing_method is not None:
+                if existing_method is not None and not existing_method.plugin_generated:
                     assert existing_method.node
                     ctx.api.fail(
                         'You may not have a custom %s method when order=True' % method_name,

test-data/unit/check-dataclasses.test

@@ -624,3 +624,21 @@ class C:
     x: int = dataclasses.field(default=1)
     y: str = dataclasses.field(metadata={"doc": "foo"})  # E: Attributes without a default cannot follow attributes with one
 [builtins fixtures/dict.pyi]
+
+[case testDataclassOrderingDeferred]
+# flags: --python-version 3.6 --new-semantic-analyzer
+from dataclasses import dataclass
+
+defer: Yes
+
+@dataclass(order=True)
+class Application:
+  name: str
+  rating: int
+
+a = Application('', 0)
+b = Application('', 0)
+a < b
+
+class Yes: ...
+[builtins fixtures/list.pyi]