Fix frozenset subclass keyword arguments (#8287)

Assisted-by: Codex:gpt-5.6

Lib/test/test_set.py

@@ -828,7 +828,6 @@ class TestFrozenSetSubclass(TestFrozenSet):
     thetype = FrozenSetSubclass
     basetype = frozenset
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON
     def test_keywords_in_subclass(self):
         class subclass(frozenset):
             pass

crates/vm/src/builtins/set.rs

@@ -958,31 +958,54 @@ impl Constructor for PyFrozenSet {
     type Args = Vec<PyObjectRef>;
 
     fn slot_new(cls: PyTypeRef, args: FuncArgs, vm: &VirtualMachine) -> PyResult {
-        let iterable: OptionalArg<PyObjectRef> = args.bind(vm)?;
+        let is_exact_frozenset = cls.is(vm.ctx.types.frozenset_type);
+        let is_frozenset_init = {
+            let cls_init = cls.slots.init.load().map(|init| init as usize);
+            let frozenset_init = vm
+                .ctx
+                .types
+                .frozenset_type
+                .slots
+                .init
+                .load()
+                .map(|init| init as usize);
+            cls_init == frozenset_init
+        };
 
         // Optimizations for exact frozenset type
-        if cls.is(vm.ctx.types.frozenset_type) {
+        let iterable_opt = if is_exact_frozenset || is_frozenset_init {
+            let iterable: OptionalArg<PyObjectRef> = args.bind(vm)?;
+
             // Return exact frozenset as-is
-            if let OptionalArg::Present(ref input) = iterable
-                && let Ok(fs) = input.clone().downcast_exact::<Self>(vm)
+            if is_exact_frozenset
+                && let OptionalArg::Present(input) = &iterable
+                && input.class().is(vm.ctx.types.frozenset_type)
             {
-                return Ok(fs.into_pyref().into());
+                return Ok(input.clone());
             }
 
-            // Return empty frozenset singleton
-            if iterable.is_missing() {
-                return Ok(vm.ctx.empty_frozenset.clone().into());
+            iterable.into_option()
+        } else {
+            match &args.args[..] {
+                [] => None,
+                [iterable] => Some(iterable.clone()),
+                slice => {
+                    return Err(vm.new_type_error(format!(
+                        "frozenset expected at most 1 argument, got {}",
+                        slice.len()
+                    )));
+                }
             }
-        }
+        };
 
-        let elements: Vec<PyObjectRef> = if let OptionalArg::Present(iterable) = iterable {
+        let elements = if let Some(iterable) = iterable_opt {
             iterable.try_to_value(vm)?
         } else {
             vec![]
         };
 
-        // Return empty frozenset singleton for exact frozenset types (when iterable was empty)
-        if elements.is_empty() && cls.is(vm.ctx.types.frozenset_type) {
+        // Return empty frozenset singleton
+        if is_exact_frozenset && elements.is_empty() {
             return Ok(vm.ctx.empty_frozenset.clone().into());
         }