propagate value to all subclasses

src/python/pants/engine/target.py

@@ -55,8 +55,10 @@
 _DefaultBase = TypeVar("_DefaultBase", bound=Optional[ImmutableValue])
 
 
+# This @dataclass declaration is necessary for the `value` field to be propagated to the __hash__ of
+# all subclasses, but MyPy doesn't recognize generic abstract dataclasses yet.
+@dataclass(unsafe_hash=True)    # type: ignore[misc]
 class Field(Generic[_DefaultBase], metaclass=ABCMeta):
-    # This is defined in PrimitiveField and AsyncField.
     value: _DefaultBase
     # Subclasses must define these.
     alias: ClassVar[str]
@@ -65,12 +67,9 @@ class Field(Generic[_DefaultBase], metaclass=ABCMeta):
     required: ClassVar[bool] = False
 
     # This is a little weird to have an abstract __init__(). We do this to ensure that all
-    # subclasses have this exact type signature for their constructor.
-    #
-    # Normally, with dataclasses, each constructor parameter would instead be specified via a
-    # dataclass field declaration. But, we don't want to declare either `address` or `raw_value` as
-    # attributes because we make no assumptions whether the subclasses actually store those values
-    # on each instance. All that we care about is a common constructor interface.
+    # subclasses have this exact type signature for their constructor. Normally we would rely on the
+    # generated __init__ by declaring `address` as a field, but we don't want that on *every* Field
+    # subclass.
     @abstractmethod
     def __init__(self, raw_value: Optional[Any], *, address: Address) -> None:
         pass