Accept dict subclasses as warning module globals (#9062)

Preserve overridable loader lookup while reading module names from stored dictionary data.

Assisted-by: OpenAI Codex:model identifier unavailable

crates/vm/src/stdlib/_warnings.rs

@@ -21,7 +21,7 @@ pub fn warn(
 mod _warnings {
     use crate::{
         AsObject, PyObject, PyObjectRef, PyResult, VirtualMachine,
-        builtins::{PyDictRef, PyListRef, PyStrRef, PyTupleRef, PyTypeRef},
+        builtins::{PyDict, PyDictRef, PyListRef, PyStrRef, PyTupleRef, PyTypeRef},
         convert::TryFromObject,
         function::OptionalArg,
     };
@@ -180,7 +180,7 @@ mod _warnings {
         let source_line = if let Some(mg) = args.module_globals.into_option() {
             if vm.is_none(&mg) {
                 None
-            } else if !mg.class().is(vm.ctx.types.dict_type) {
+            } else if mg.downcast_ref::<PyDict>().is_none() {
                 return Err(vm.new_type_error(format!(
                     "module_globals must be a dict, not '{}'",
                     mg.class().name()

crates/vm/src/warn.rs

@@ -201,7 +201,7 @@ pub(crate) fn get_source_line(
     };
 
     let module_name = if let Some(dict) = module_globals.downcast_ref::<PyDict>() {
-        match dict.get_item_opt(identifier!(vm, __name__), vm)? {
+        match dict.inner_getitem_opt(identifier!(vm, __name__), vm)? {
             Some(name) => name,
             None => return Ok(None),
         }

extra_tests/snippets/stdlib_warnings.py

@@ -18,3 +18,50 @@ for category in (None, DeprecationWarning, int, 42):
     assert len(caught) == 1
     assert caught[0].message is message
     assert caught[0].category is CustomWarning
+
+
+from importlib.machinery import ModuleSpec
+
+events = []
+
+
+class WarningLoader:
+    def get_source(self, name):
+        events.append(("source", name))
+        return "source line\n"
+
+
+class WarningGlobals(dict):
+    def get(self, key, default=None):
+        events.append(("get", key))
+        return super().get(key, default)
+
+    def __getitem__(self, key):
+        raise AssertionError("module name must use stored dict lookup")
+
+    __missing__ = __getitem__
+
+
+loader = WarningLoader()
+module_globals = WarningGlobals(
+    __name__="stored_name",
+    __loader__=loader,
+    __spec__=ModuleSpec("stored_name", loader),
+)
+for has_name in (True, False):
+    events.clear()
+    with warnings.catch_warnings(record=True) as caught:
+        warnings.simplefilter("always")
+        warnings.warn_explicit(
+            "subclass globals",
+            UserWarning,
+            "fixed_ascii.py",
+            1,
+            module_globals=module_globals,
+        )
+    assert len(caught) == 1
+    expected = [("get", "__loader__"), ("get", "__spec__")]
+    if has_name:
+        expected.append(("source", "stored_name"))
+        del module_globals["__name__"]
+    assert events == expected