Unpack Never iterables

Author: A5rocks

mypy/checker.py

@@ -4251,7 +4251,7 @@ def check_multi_assignment_from_iterable(
     ) -> None:
         rvalue_type = get_proper_type(rvalue_type)
         if self.type_is_iterable(rvalue_type) and isinstance(
-            rvalue_type, (Instance, CallableType, TypeType, Overloaded)
+            rvalue_type, (Instance, CallableType, TypeType, Overloaded, UninhabitedType)
         ):
             item_type = self.iterable_item_type(rvalue_type, context)
             for lv in lvalues:
@@ -7556,7 +7556,9 @@ def note(
         self.msg.note(msg, context, offset=offset, code=code)
 
     def iterable_item_type(
-        self, it: Instance | CallableType | TypeType | Overloaded, context: Context
+        self,
+        it: Instance | CallableType | TypeType | Overloaded | UninhabitedType,
+        context: Context,
     ) -> Type:
         if isinstance(it, Instance):
             iterable = map_instance_to_supertype(it, self.lookup_typeinfo("typing.Iterable"))

test-data/unit/check-unreachable-code.test

@@ -1535,3 +1535,16 @@ x = 0  # not unreachable
 
 f2: Callable[[], NoReturn] = lambda: foo()
 x = 0  # not unreachable
+
+[case testUnpackNever]
+x: str
+if isinstance(x, int):
+    # mypy wants a type annotation for y because it's `Never`.
+    # This doesn't make much sense in this case, but it's a
+    # requirement for composable behavior.
+    # (and `ys` should also get flagged.)
+
+    y, *ys = x  # E: Need type annotation for "y"
+    reveal_type(y)  # N: Revealed type is "Never"
+    reveal_type(ys)  # N: Revealed type is "builtins.list[Never]"
+[builtins fixtures/isinstancelist.pyi]