From 69c8fac029d18d161af06fa7edc73de238b367d6 Mon Sep 17 00:00:00 2001
From: Ryan Heard <ryanwheard@gmail.com>
Date: Tue, 6 Oct 2026 23:20:09 -0400
Subject: [PATCH] [mypyc] Fix segfault from annotations naming classes that
 don't exist yet

---
 mypyc/irbuild/classdef.py            | 31 ++++++++++++++++--
 mypyc/test-data/run-classes.test     | 18 ++++++++++
 mypyc/test-data/run-multimodule.test | 49 ++++++++++++++++++++++++++++
 3 files changed, 96 insertions(+), 2 deletions(-)

diff --git a/mypyc/irbuild/classdef.py b/mypyc/irbuild/classdef.py
index 3eb617cf86550..aa62c84e0014d 100644
--- a/mypyc/irbuild/classdef.py
+++ b/mypyc/irbuild/classdef.py
@@ -685,7 +685,7 @@ def add_non_ext_class_attr_ann(
             if builder.current_module == type_info.module_name and stmt.line < type_info.line:
                 typ = builder.load_str(type_info.fullname)
             else:
-                typ = load_type(builder, type_info, stmt.unanalyzed_type, stmt.line)
+                typ = load_type_for_annotation(builder, type_info, stmt)
 
     if typ is None:
         # FIXME: if get_type_info is not provided, don't fall back to stmt.type?
@@ -701,7 +701,7 @@ def add_non_ext_class_attr_ann(
             # actually a forward reference due to the __annotations__ future?
             typ = builder.load_str(stmt.unanalyzed_type.original_str_expr)
         elif isinstance(ann_type, Instance):
-            typ = load_type(builder, ann_type.type, stmt.unanalyzed_type, stmt.line)
+            typ = load_type_for_annotation(builder, ann_type.type, stmt)
         else:
             typ = builder.add(LoadAddress(type_object_op.type, type_object_op.src, stmt.line))
 
@@ -709,6 +709,33 @@ def add_non_ext_class_attr_ann(
     builder.call_c(exact_dict_set_item_op, [non_ext.anns, key, typ], stmt.line)
 
 
+def load_type_for_annotation(
+    builder: IRBuilder, type_info: TypeInfo, stmt: AssignmentStmt
+) -> Value:
+    """Load the class object of a class named in an annotation in a class body.
+
+    A native class doesn't exist until its class statement has run, which may not have
+    happened yet: its module may not have been imported (for example if it's only imported
+    under TYPE_CHECKING), or it may be defined later in this module, or be the non-extension
+    class being defined. Python doesn't evaluate annotations like these with
+    "from __future__ import annotations", so use the name of the class instead in that case.
+    """
+    typ = load_type(builder, type_info, stmt.unanalyzed_type, stmt.line)
+    if type_info not in builder.mapper.type_to_ir:
+        return typ
+    # The class object is NULL if the class doesn't exist yet
+    result = Register(object_rprimitive)
+    missing, present, done = BasicBlock(), BasicBlock(), BasicBlock()
+    builder.add(Branch(typ, missing, present, Branch.IS_ERROR))
+    builder.activate_block(present)
+    builder.assign(result, typ, stmt.line)
+    builder.goto(done)
+    builder.activate_block(missing)
+    builder.assign(result, builder.load_str(type_info.fullname), stmt.line)
+    builder.goto_and_activate(done)
+    return result
+
+
 def add_non_ext_class_attr(
     builder: IRBuilder,
     non_ext: NonExtClassInfo,
diff --git a/mypyc/test-data/run-classes.test b/mypyc/test-data/run-classes.test
index 77f029b6b80b5..a329dc5b2c8c9 100644
--- a/mypyc/test-data/run-classes.test
+++ b/mypyc/test-data/run-classes.test
@@ -125,6 +125,24 @@ print(C.__annotations__["spam"] is JSONDecoder)
 [out]
 True
 
+[case testNonExtClassAnnotationOfClassNotCreatedYet]
+from __future__ import annotations
+
+from mypy_extensions import mypyc_attr
+
+@mypyc_attr(native_class=False)
+class Node:
+    parent: Node
+    later: Later
+
+class Later:
+    pass
+
+[file driver.py]
+from native import Node
+
+assert Node.__annotations__ == {"parent": "native.Node", "later": "native.Later"}
+
 [case testClassWithDeletableAttributes]
 from typing import Any, cast
 from testutil import assertRaises
diff --git a/mypyc/test-data/run-multimodule.test b/mypyc/test-data/run-multimodule.test
index baab2f52ce08f..89c7da82e77cb 100644
--- a/mypyc/test-data/run-multimodule.test
+++ b/mypyc/test-data/run-multimodule.test
@@ -1042,6 +1042,55 @@ except TypeError:
 else:
     assert False
 
+[case testMultiModuleAnnotationOfClassNotCreatedYet]
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import TYPE_CHECKING
+
+from mypy_extensions import mypyc_attr
+
+from other_a import A
+
+if TYPE_CHECKING:
+    from other_b import B
+
+@dataclass(frozen=True)
+class Data:
+    a: A
+    b: B
+    quoted: "B"
+
+@mypyc_attr(native_class=False)
+class NonNative:
+    a: A
+    b: B
+
+[file other_a.py]
+class A:
+    pass
+
+[file other_b.py]
+class B:
+    pass
+
+[file driver.py]
+from dataclasses import fields
+
+# Importing native doesn't import other_b, so class B doesn't exist yet when the
+# classes in native are created
+import native
+from other_a import A
+
+assert native.Data.__annotations__ == {"a": A, "b": "other_b.B", "quoted": "other_b.B"}
+assert [f.type for f in fields(native.Data)] == [A, "other_b.B", "other_b.B"]
+assert native.NonNative.__annotations__ == {"a": A, "b": "other_b.B"}
+
+from other_b import B
+
+d = native.Data(A(), B(), B())
+assert isinstance(d.b, B)
+
 [case testMultiModuleRelative]
 from package.a import f
 [file package/__init__.py]