Update another affected test, add explicit example of None -> method override.

Author: sterliakov

test-data/unit/check-dataclasses.test

@@ -260,15 +260,21 @@ class FrozenBase:
     pass
 
 @dataclass
-class BadNormalDerived(FrozenBase):  # E: Non-frozen dataclass cannot inherit from a frozen dataclass
+class BadNormalDerived(FrozenBase):  # E: Incompatible override of '__hash__': dataclasses without 'frozen' or 'unsafe_hash' have '__hash__' set to None \
+                                     # E: Non-frozen dataclass cannot inherit from a frozen dataclass
     pass
 
 @dataclass
 class NormalBase:
     pass
 
 @dataclass(frozen=True)
-class BadFrozenDerived(NormalBase):  # E: Frozen dataclass cannot inherit from a non-frozen dataclass
+class BadFrozenDerived(NormalBase):  # E: Signature of "__hash__" incompatible with supertype "NormalBase" \
+                                     # N:      Superclass: \
+                                     # N:          None \
+                                     # N:      Subclass: \
+                                     # N:          def __hash__() -> int \
+                                     # E: Frozen dataclass cannot inherit from a non-frozen dataclass
     pass
 
 [builtins fixtures/dataclasses.pyi]
@@ -2636,3 +2642,33 @@ Good(4)
 AlsoGood(4)
 Bad(4)
 [builtins fixtures/tuple.pyi]
+
+[case testDataclassHash2]
+import dataclasses
+
+# This
+@dataclasses.dataclass()
+class Parent:
+    a: int
+
+# is (at runtime, ignoring other methods) the same as
+class ExplicitParent:
+    a: int
+    __hash__ = None
+
+class Child(Parent):
+    def __hash__(self) -> int:  # E: Signature of "__hash__" incompatible with supertype "Parent" \
+                                # N:      Superclass: \
+                                # N:          None \
+                                # N:      Subclass: \
+                                # N:          def __hash__(self) -> int
+        return 0
+
+class ExplicitChild(ExplicitParent):
+    def __hash__(self) -> int:  # E: Signature of "__hash__" incompatible with supertype "ExplicitParent" \
+                                # N:      Superclass: \
+                                # N:          None \
+                                # N:      Subclass: \
+                                # N:          def __hash__(self) -> int
+        return 0
+[builtins fixtures/tuple.pyi]