Document new bytecode and make it fail gracefully if mis-compiled.

Doc/library/dis.rst

@@ -1247,6 +1247,14 @@ All of the following opcodes use their arguments.
 
    .. versionadded:: 3.10
 
+.. opcode:: GEN_START (kind)
+
+    Pops TOS. If TOS was not ``None``, raises an exception. The ``kind``
+    operand corresponds to the type of generator or coroutine and determines
+    the error message. The legal kinds are 0 for generator, 1 for coroutine,
+    and 2 for async generator.
+
+   .. versionadded:: 3.10
 
 .. opcode:: HAVE_ARGUMENT
 

Python/ceval.c

@@ -2652,25 +2652,29 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, PyFrameObject *f, int throwflag)
         }
 
         case TARGET(GEN_START): {
-            assert(oparg < 3);
             PyObject *none = POP();
             Py_DECREF(none);
             if (none != Py_None) {
-                static const char *gen_kind[3] = {
-                    "generator",
-                    "coroutine",
-                    "async generator"
-                };
-                _PyErr_Format(tstate, PyExc_TypeError,
-                    "can't send non-None value to a "
-                              "just-started %s",
-                              gen_kind[oparg]);
+                if (oparg > 2) {
+                    _PyErr_SetString(tstate, PyExc_SystemError,
+                        "Illegal kind for GEN_START");
+                }
+                else {
+                    static const char *gen_kind[3] = {
+                        "generator",
+                        "coroutine",
+                        "async generator"
+                    };
+                    _PyErr_Format(tstate, PyExc_TypeError,
+                        "can't send non-None value to a "
+                                "just-started %s",
+                                gen_kind[oparg]);
+                }
                 goto error;
             }
             DISPATCH();
         }
 
-
         case TARGET(POP_EXCEPT): {
             PyObject *type, *value, *traceback;
             _PyErr_StackItem *exc_info;