Assume that dataclasses have no dynamic attributes (#456)

docs/changelog.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- Assume that dataclasses have no dynamic attributes (#456)
 - Treat Thrift enums as compatible with `int` (#455)
 - Fix treatment of `TypeVar` with bounds or constraints
   as callables (#454)

pyanalyze/name_check_visitor.py

@@ -85,7 +85,13 @@
 from .patma import PatmaVisitor
 from .shared_options import Paths, ImportPaths, EnforceNoUnused
 from .reexport import ImplicitReexportTracker
-from .safe import safe_getattr, is_hashable, all_of_type, safe_issubclass
+from .safe import (
+    safe_getattr,
+    is_hashable,
+    all_of_type,
+    safe_issubclass,
+    is_dataclass_type,
+)
 from .stacked_scopes import (
     EMPTY_ORIGIN,
     AbstractConstraint,
@@ -4114,7 +4120,7 @@ def _has_only_known_attributes(self, typ: object) -> bool:
             return True
         ts_finder = self.checker.ts_finder
         if (
-            ts_finder.has_stubs(typ)
+            (ts_finder.has_stubs(typ) or is_dataclass_type(typ))
             and not ts_finder.has_attribute(typ, "__getattr__")
             and not ts_finder.has_attribute(typ, "__getattribute__")
             and not attributes.may_have_dynamic_attributes(typ)

pyanalyze/safe.py

@@ -169,3 +169,12 @@ def get_fully_qualified_name(obj: object) -> Optional[str]:
     if hasattr(obj, "__module__") and hasattr(obj, "__qualname__"):
         return f"{obj.__module__}.{obj.__qualname__}"
     return None
+
+
+def is_dataclass_type(cls: type) -> bool:
+    """Like dataclasses.is_dataclass(), but works correctly for a
+    non-dataclass subclass of a dataclass."""
+    try:
+        return "__dataclass_fields__" in cls.__dict__
+    except Exception:
+        return False