Treat __dataclass_fields__ as ClassVar (#13721)

Author: hauntsaninja

mypy/plugins/dataclasses.py

@@ -593,6 +593,7 @@ def _add_dataclass_fields_magic_attribute(self) -> None:
         var = Var(name=attr_name, type=attr_type)
         var.info = self._ctx.cls.info
         var._fullname = self._ctx.cls.info.fullname + "." + attr_name
+        var.is_classvar = True
         self._ctx.cls.info.names[attr_name] = SymbolTableNode(
             kind=MDEF, node=var, plugin_generated=True
         )

test-data/unit/check-dataclasses.test

@@ -1819,10 +1819,10 @@ c.x  # E: "C" has no attribute "x"
 [case testDataclassCheckTypeVarBounds]
 # flags: --python-version 3.7
 from dataclasses import dataclass
-from typing import Protocol, Dict, TypeVar, Generic
+from typing import ClassVar, Protocol, Dict, TypeVar, Generic
 
 class DataclassProtocol(Protocol):
-    __dataclass_fields__: Dict
+    __dataclass_fields__: ClassVar[Dict]
 
 T = TypeVar("T", bound=DataclassProtocol)
 
@@ -1896,3 +1896,20 @@ FirstClass().FIRST_CONST = 42  # E: Cannot assign to final attribute "FIRST_CONS
 reveal_type(SecondClass().SECOND_CONST)  # N: Revealed type is "Literal[3]?"
 SecondClass().SECOND_CONST = 42  # E: Cannot assign to final attribute "SECOND_CONST"
 [builtins fixtures/dataclasses.pyi]
+
+[case testDataclassFieldsProtocol]
+# flags: --python-version 3.9
+from dataclasses import dataclass
+from typing import Any, Protocol
+
+class ConfigProtocol(Protocol):
+    __dataclass_fields__: dict[str, Any]
+
+def takes_cp(cp: ConfigProtocol): ...
+
+@dataclass
+class MyDataclass:
+    x: int = 3
+
+takes_cp(MyDataclass)
+[builtins fixtures/dataclasses.pyi]

test-data/unit/fine-grained.test

@@ -9795,11 +9795,11 @@ class ExampleClass(Generic[T]):
 [case testDataclassCheckTypeVarBoundsInReprocess]
 # flags: --python-version 3.7
 from dataclasses import dataclass
-from typing import Protocol, Dict, TypeVar, Generic
+from typing import ClassVar, Protocol, Dict, TypeVar, Generic
 from m import x
 
 class DataclassProtocol(Protocol):
-    __dataclass_fields__: Dict
+    __dataclass_fields__: ClassVar[Dict]
 
 T = TypeVar("T", bound=DataclassProtocol)