Fix types of inherited attributes in generic dataclasses

Fixes #12633.

Author: JukkaL

mypy/plugins/dataclasses.py

@@ -101,10 +101,8 @@ def deserialize(
     def expand_typevar_from_subtype(self, sub_type: TypeInfo) -> None:
         """Expands type vars in the context of a subtype when an attribute is inherited
         from a generic super type."""
-        if not isinstance(self.type, TypeVarType):
-            return
-
-        self.type = map_type_from_supertype(self.type, sub_type, self.info)
+        if self.type is not None:
+            self.type = map_type_from_supertype(self.type, sub_type, self.info)
 
 
 class DataclassTransformer:

test-data/unit/check-dataclasses.test

@@ -1568,3 +1568,35 @@ class B:
 class Derived(A, B):
     pass
 [builtins fixtures/dataclasses.pyi]
+
+[case testDataclassGenericInheritance2]
+# flags: --python-version 3.7
+from dataclasses import dataclass
+from typing import Any, Callable, Generic, TypeVar, List
+
+T = TypeVar("T")
+S = TypeVar("S")
+
+@dataclass
+class Parent(Generic[T]):
+    f: Callable[[T], Any]
+
+@dataclass
+class Child(Parent[T]): ...
+
+class A: ...
+def func(obj: A) -> bool: ...
+
+reveal_type(Child[A](func).f)  # N: Revealed type is "def (__main__.A) -> Any"
+
+@dataclass
+class Parent2(Generic[T]):
+    a: List[T]
+
+@dataclass
+class Child2(Generic[T, S], Parent2[S]):
+    b: List[T]
+
+reveal_type(Child2([A()], [1]).a)  # N: Revealed type is "builtins.list[__main__.A]"
+reveal_type(Child2[int, A]([A()], [1]).b)  # N: Revealed type is "builtins.list[builtins.int]"
+[builtins fixtures/dataclasses.pyi]