py/runtime: Fix PEP479 behaviour throwing StopIteration into yield from.

Commit 3f6ffe059f implemented PEP479 but did
not catch the case fixed in this commit.  Found by coverage analysis, that
the VM had uncovered code.

py/runtime.c

@@ -1319,7 +1319,12 @@ mp_vm_return_kind_t mp_resume(mp_obj_t self_in, mp_obj_t send_value, mp_obj_t th
         // will be propagated up. This behavior is approved by test_pep380.py
         // test_delegation_of_close_to_non_generator(),
         //  test_delegating_throw_to_non_generator()
-        *ret_val = mp_make_raise_obj(throw_value);
+        if (mp_obj_exception_match(throw_value, MP_OBJ_FROM_PTR(&mp_type_StopIteration))) {
+            // PEP479: if StopIteration is raised inside a generator it is replaced with RuntimeError
+            *ret_val = mp_obj_new_exception_msg(&mp_type_RuntimeError, "generator raised StopIteration");
+        } else {
+            *ret_val = mp_make_raise_obj(throw_value);
+        }
         return MP_VM_RETURN_EXCEPTION;
     }
 }

py/vm.c

@@ -1266,17 +1266,10 @@ yield:
                         DISPATCH();
                     } else {
                         assert(ret_kind == MP_VM_RETURN_EXCEPTION);
+                        assert(!EXC_MATCH(ret_value, MP_OBJ_FROM_PTR(&mp_type_StopIteration)));
                         // Pop exhausted gen
                         sp--;
-                        if (EXC_MATCH(ret_value, MP_OBJ_FROM_PTR(&mp_type_StopIteration))) {
-                            PUSH(mp_obj_exception_get_value(ret_value));
-                            // If we injected GeneratorExit downstream, then even
-                            // if it was swallowed, we re-raise GeneratorExit
-                            GENERATOR_EXIT_IF_NEEDED(t_exc);
-                            DISPATCH();
-                        } else {
-                            RAISE(ret_value);
-                        }
+                        RAISE(ret_value);
                     }
                 }
 

tests/basics/generator_pep479.py

@@ -27,3 +27,14 @@ try:
     g.throw(StopIteration)
 except RuntimeError:
     print('RuntimeError')
+
+# throwing a StopIteration through yield from, will be converted to a RuntimeError
+def gen():
+    yield from range(2)
+    print('should not get here')
+g = gen()
+print(next(g))
+try:
+    g.throw(StopIteration)
+except RuntimeError:
+    print('RuntimeError')

tests/basics/generator_pep479.py.exp

@@ -3,3 +3,5 @@ RuntimeError
 StopIteration
 1
 RuntimeError
+0
+RuntimeError