pythongh-119666: fix multiple class-scope comprehensions referencing __class__ (python#120295)

Author: carljm

Lib/test/test_listcomps.py

@@ -168,6 +168,31 @@ def test_references___class__(self):
         """
         self._check_in_scopes(code, raises=NameError)
 
+    def test_references___class___defined(self):
+        code = """
+            __class__ = 2
+            res = [__class__ for x in [1]]
+        """
+        self._check_in_scopes(
+                code, outputs={"res": [2]}, scopes=["module", "function"])
+        self._check_in_scopes(code, raises=NameError, scopes=["class"])
+
+    def test_references___class___enclosing(self):
+        code = """
+            __class__ = 2
+            class C:
+                res = [__class__ for x in [1]]
+            res = C.res
+        """
+        self._check_in_scopes(code, raises=NameError)
+
+    def test_super_and_class_cell_in_sibling_comps(self):
+        code = """
+            [super for _ in [1]]
+            [__class__ for _ in [1]]
+        """
+        self._check_in_scopes(code, raises=NameError)
+
     def test_inner_cell_shadows_outer(self):
         code = """
             items = [(lambda: i) for i in range(5)]

Misc/NEWS.d/next/Core and Builtins/2024-06-09-19-13-38.gh-issue-119666.S0G4rZ.rst

@@ -0,0 +1 @@
+Fix a compiler crash in the case where two comprehensions in class scope both reference ``__class__``.

Python/symtable.c

@@ -781,22 +781,19 @@ inline_comprehension(PySTEntryObject *ste, PySTEntryObject *comp,
         if (existing == NULL && PyErr_Occurred()) {
             return 0;
         }
+        // __class__ is never allowed to be free through a class scope (see
+        // drop_class_free)
+        if (scope == FREE && ste->ste_type == ClassBlock &&
+                _PyUnicode_EqualToASCIIString(k, "__class__")) {
+            scope = GLOBAL_IMPLICIT;
+            if (PySet_Discard(comp_free, k) < 0) {
+                return 0;
+            }
+            remove_dunder_class = 1;
+        }
         if (!existing) {
             // name does not exist in scope, copy from comprehension
             assert(scope != FREE || PySet_Contains(comp_free, k) == 1);
-            if (scope == FREE && ste->ste_type == ClassBlock &&
-                _PyUnicode_EqualToASCIIString(k, "__class__")) {
-                // if __class__ is unbound in the enclosing class scope and free
-                // in the comprehension scope, it needs special handling; just
-                // letting it be marked as free in class scope will break due to
-                // drop_class_free
-                scope = GLOBAL_IMPLICIT;
-                only_flags &= ~DEF_FREE;
-                if (PySet_Discard(comp_free, k) < 0) {
-                    return 0;
-                }
-                remove_dunder_class = 1;
-            }
             PyObject *v_flags = PyLong_FromLong(only_flags);
             if (v_flags == NULL) {
                 return 0;