PRs look good to me. Here is a test that fails for me with the master branch and works with your fixes. It has a slightly odd place in the subinterpreter tests, but I would argue that it's not entirely misplaced there. You would want to know when subinterpreters start rejecting valid code, wouldn't you? :)


diff --git a/Lib/test/test_capi.py b/Lib/test/test_capi.py
index 73e167a0b0..9bed8255e1 100644
--- a/Lib/test/test_capi.py
+++ b/Lib/test/test_capi.py
@@ -627,6 +627,24 @@ class SubinterpreterTest(unittest.TestCase):
             self.assertNotEqual(pickle.load(f), id(sys.modules))
             self.assertNotEqual(pickle.load(f), id(builtins))
 
+    def test_subinterps_recent_language_features(self):
+        r, w = os.pipe()
+        code = """if 1:
+            import pickle
+            with open({:d}, "wb") as f:
+
+                @(lambda x:x)  # Py 3.9
+                def noop(x): return x
+
+                a = (b := f'1{{2}}3') + noop('x')  # Py3.8 (:=) / 3.6 (f'')
+                pickle.dump(dict(a=a, b=b), f)
+            """.format(w)
+
+        with open(r, "rb") as f:
+            ret = support.run_in_subinterp(code)
+            self.assertEqual(ret, 0)
+            self.assertEqual(pickle.load(f), {'a': '123x', 'b': '123'})
+
     def test_mutate_exception(self):
         """
         Exceptions saved in global module state get shared between
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 808483ebd7..468e25ff9e 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -3468,6 +3468,8 @@ run_in_subinterp(PyObject *self, PyObject *args)
     const char *code;
     int r;
     PyThreadState *substate, *mainstate;
+    /* only initialise 'cflags.cf_flags' to test backwards compatibility */
+    PyCompilerFlags cflags = {0};
 
     if (!PyArg_ParseTuple(args, "s:run_in_subinterp",
                           &code))
@@ -3486,7 +3488,7 @@ run_in_subinterp(PyObject *self, PyObject *args)
         PyErr_SetString(PyExc_RuntimeError, "sub-interpreter creation failed");
         return NULL;
     }
-    r = PyRun_SimpleString(code);
+    r = PyRun_SimpleStringFlags(code, &cflags);
     Py_EndInterpreter(substate);
 
     PyThreadState_Swap(mainstate);