Fix _PyGen_yf()

Issue #28782: Fix a bug in the implementation ``yield from`` when checking
if the next instruction is YIELD_FROM. Regression introduced by WORDCODE
(issue #26647).

Reviewed by Serhiy Storchaka and Yury Selivanov.

Misc/NEWS

@@ -10,6 +10,10 @@ What's New in Python 3.6.0 release candidate 1
 Core and Builtins
 -----------------
 
+- Issue #28782: Fix a bug in the implementation ``yield from`` when checking
+  if the next instruction is YIELD_FROM. Regression introduced by WORDCODE
+  (issue #26647).
+
 Library
 -------
 

Objects/genobject.c

@@ -355,6 +355,14 @@ _PyGen_yf(PyGenObject *gen)
         PyObject *bytecode = f->f_code->co_code;
         unsigned char *code = (unsigned char *)PyBytes_AS_STRING(bytecode);
 
+        if (f->f_lasti < 0) {
+            /* Return immediately if the frame didn't start yet. YIELD_FROM
+               always come after LOAD_CONST: a code object should not start
+               with YIELD_FROM */
+            assert(code[0] != YIELD_FROM);
+            return NULL;
+        }
+
         if (code[f->f_lasti + sizeof(_Py_CODEUNIT)] != YIELD_FROM)
             return NULL;
         yf = f->f_stacktop[-1];
@@ -463,6 +471,7 @@ _gen_throw(PyGenObject *gen, int close_on_genexit,
             assert(ret == yf);
             Py_DECREF(ret);
             /* Termination repetition of YIELD_FROM */
+            assert(gen->gi_frame->f_lasti >= 0);
             gen->gi_frame->f_lasti += sizeof(_Py_CODEUNIT);
             if (_PyGen_FetchStopIterationValue(&val) == 0) {
                 ret = gen_send_ex(gen, val, 0, 0);

Python/ceval.c

@@ -2043,6 +2043,7 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
             f->f_stacktop = stack_pointer;
             why = WHY_YIELD;
             /* and repeat... */
+            assert(f->f_lasti >= (int)sizeof(_Py_CODEUNIT));
             f->f_lasti -= sizeof(_Py_CODEUNIT);
             goto fast_yield;
         }