[PEP 695] Fix covariance of frozen dataclasses

mypy/subtypes.py

@@ -2004,7 +2004,8 @@ def infer_variance(info: TypeInfo, i: int) -> bool:
         tvar = info.defn.type_vars[i]
         self_type = fill_typevars(info)
         for member in all_non_object_members(info):
-            if member in ("__init__", "__new__"):
+            # __mypy-replace is an implementation detail of the dataclass plugin
+            if member in ("__init__", "__new__", "__mypy-replace"):
                 continue
             node = info[member].node
             if isinstance(node, Var) and node.type is None:

test-data/unit/check-python312.test

@@ -249,6 +249,25 @@ if int():
 if int():
     f = e
 
+[case testPEP695InferVarianceInFrozenDataclass]
+# flags: --enable-incomplete-feature=NewGenericSyntax
+from dataclasses import dataclass
+
+@dataclass(frozen=True)
+class Covariant[T]:
+    x: T
+
+cov1: Covariant[float] = Covariant[int](1)
+cov2: Covariant[int] = Covariant[float](1)  # E: Incompatible types in assignment (expression has type "Covariant[float]", variable has type "Covariant[int]")
+
+@dataclass(frozen=True)
+class Invariant[T]:
+    x: list[T]
+
+inv1: Invariant[float] = Invariant[int]([1])  # E: Incompatible types in assignment (expression has type "Invariant[int]", variable has type "Invariant[float]")
+inv2: Invariant[int] = Invariant[float]([1])  # E: Incompatible types in assignment (expression has type "Invariant[float]", variable has type "Invariant[int]")
+[builtins fixtures/tuple.pyi]
+
 [case testPEP695InferVarianceCalculateOnDemand]
 # flags: --enable-incomplete-feature=NewGenericSyntax