attrs, dataclasses: don't enforce slots when base doesn't (#15976)

Doing the same thing we do for regular classes.

Fixes #15975

mypy/plugins/attrs.py

@@ -893,6 +893,11 @@ def _add_attrs_magic_attribute(
 
 
 def _add_slots(ctx: mypy.plugin.ClassDefContext, attributes: list[Attribute]) -> None:
+    if any(p.slots is None for p in ctx.cls.info.mro[1:-1]):
+        # At least one type in mro (excluding `self` and `object`)
+        # does not have concrete `__slots__` defined. Ignoring.
+        return
+
     # Unlike `@dataclasses.dataclass`, `__slots__` is rewritten here.
     ctx.cls.info.slots = {attr.name for attr in attributes}
 

mypy/plugins/dataclasses.py

@@ -443,6 +443,12 @@ def add_slots(
                 self._cls,
             )
             return
+
+        if any(p.slots is None for p in info.mro[1:-1]):
+            # At least one type in mro (excluding `self` and `object`)
+            # does not have concrete `__slots__` defined. Ignoring.
+            return
+
         info.slots = generated_slots
 
         # Now, insert `.__slots__` attribute to class namespace:

test-data/unit/check-dataclasses.test

@@ -1519,6 +1519,22 @@ class Some:
         self.y = 1  # E: Trying to assign name "y" that is not in "__slots__" of type "__main__.Some"
 [builtins fixtures/dataclasses.pyi]
 
+[case testDataclassWithSlotsDerivedFromNonSlot]
+# flags: --python-version 3.10
+from dataclasses import dataclass
+
+class A:
+    pass
+
+@dataclass(slots=True)
+class B(A):
+    x: int
+
+    def __post_init__(self) -> None:
+        self.y = 42
+
+[builtins fixtures/dataclasses.pyi]
+
 [case testDataclassWithSlotsConflict]
 # flags: --python-version 3.10
 from dataclasses import dataclass

test-data/unit/check-plugin-attrs.test

@@ -1677,6 +1677,21 @@ class C:
         self.c = 2  # E: Trying to assign name "c" that is not in "__slots__" of type "__main__.C"
 [builtins fixtures/plugin_attrs.pyi]
 
+[case testAttrsClassWithSlotsDerivedFromNonSlots]
+import attrs
+
+class A:
+    pass
+
+@attrs.define(slots=True)
+class B(A):
+    x: int
+
+    def __attrs_post_init__(self) -> None:
+        self.y = 42
+
+[builtins fixtures/plugin_attrs.pyi]
+
 [case testRuntimeSlotsAttr]
 from attr import dataclass