[mypyc] Don't load forward ref targets while setting up non-ext __ann…

…otations__

Take this example:

    from typing import NamedTuple

    class VTableMethod(NamedTuple):
        cls: "ClassIR"

    class ClassIR: pass

In irbuild::classdef::add_non_ext_class_attr_ann(), mypyc tries to
assign the ClassIR type object to VTableMethod's __annotations__. This
causes a segfault as ClassIR won't be initialized and allocated until
*after* the NamedTuple is set up.

Fortunately, AssignmentStmt preserves the unanalyzed type (UnboundType).
If `stmt.unanalyzed_type.orginal_str_expr` is not None, then we know
we're dealing with a forward ref and should just load the string
instead.

Unfortunately, it seems difficult (or impossible?) to infer whether
an annotation is a forward reference when the annotations future is
enabled and the annotation isn't a string.

mypyc/irbuild/classdef.py

@@ -556,6 +556,7 @@ def add_non_ext_class_attr_ann(
     get_type_info: Callable[[AssignmentStmt], TypeInfo | None] | None = None,
 ) -> None:
     """Add a class attribute to __annotations__ of a non-extension class."""
+    # FIXME: try to better preserve the special forms and type parameters of generics.
     typ: Value | None = None
     if get_type_info is not None:
         type_info = get_type_info(stmt)
@@ -565,7 +566,14 @@ def add_non_ext_class_attr_ann(
     if typ is None:
         # FIXME: if get_type_info is not provided, don't fall back to stmt.type?
         ann_type = get_proper_type(stmt.type)
-        if isinstance(ann_type, Instance):
+        if stmt.unanalyzed_type.original_str_expr is not None:
+            # Annotation is a forward reference, so don't attempt to load the actual
+            # type and load the string instead.
+            #
+            # TODO: is it possible to determine whether a non-string annotation is
+            # 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.line)
         else:
             typ = builder.add(LoadAddress(type_object_op.type, type_object_op.src, stmt.line))

mypyc/test-data/run-tuples.test

@@ -99,8 +99,6 @@ assert f(Sub(3, 2)) == 3
 [case testNamedTupleClassSyntax]
 from typing import Dict, List, NamedTuple, Optional, Tuple, Union
 
-class ClassIR: pass
-
 class FuncIR: pass
 
 StealsDescription = Union[bool, List[bool]]
@@ -119,8 +117,12 @@ class Record(NamedTuple):
     ordering: Optional[List[int]]
     extra_int_constants: List[Tuple[int]]
 
+# Make sure mypyc loads the annotation string for this forward reference.
+# Ref: https://github.com/mypyc/mypyc/issues/938
+class ClassIR: pass
+
 [file driver.py]
-from typing import Optional
+from typing import ForwardRef, Optional
 from native import ClassIR, FuncIR, Record
 
 assert Record.__annotations__ == {
@@ -129,7 +131,7 @@ assert Record.__annotations__ == {
     'is_borrowed': bool,
     'hash': str,
     'python_path': tuple,
-    'type': ClassIR,
+    'type': ForwardRef('ClassIR'),
     'method': FuncIR,
     'shadow_method': type,
     'classes': dict,