7 files changed, 36 insertions, 24 deletions

diff --git a/Lib/test/test_ast.py b/Lib/test/test_ast.py
index a8853c7..dc24126 100644
--- a/Lib/test/test_ast.py
+++ b/Lib/test/test_ast.py
@@ -399,6 +399,14 @@ class AST_Tests(unittest.TestCase):
             compile(m, "<test>", "exec")
         self.assertIn("string must be of type str", str(cm.exception))
 
+    def test_empty_yield_from(self):
+        # Issue 16546: yield from value is not optional.
+        empty_yield_from = ast.parse("def f():\n yield from g()")
+        empty_yield_from.body[0].body[0].value.value = None
+        with self.assertRaises(ValueError) as cm:
+            compile(empty_yield_from, "<test>", "exec")
+        self.assertIn("field value is required", str(cm.exception))
+
 
 class ASTHelpers_Test(unittest.TestCase):
 
diff --git a/Misc/NEWS b/Misc/NEWS
index f17cfea..b2e41dd 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,8 @@ What's New in Python 3.3.1?
 Core and Builtins
 -----------------
 
+- Issue #16546: Fix: ast.YieldFrom argument is now mandatory.
+
 - Issue #16514: Fix regression causing a traceback when sys.path[0] is None
   (actually, any non-string or non-bytes type).
 
diff --git a/Parser/Python.asdl b/Parser/Python.asdl
index 6b06dec..c24d840 100644
--- a/Parser/Python.asdl
+++ b/Parser/Python.asdl
@@ -60,7 +60,7 @@ module Python
          | GeneratorExp(expr elt, comprehension* generators)
          -- the grammar constrains where yield expressions can occur
          | Yield(expr? value)
-             | YieldFrom(expr? value)
+         | YieldFrom(expr value)
          -- need sequences for compare to distinguish between
          -- x < 4 < 3 and (x < 4) < 3
          | Compare(expr left, cmpop* ops, expr* comparators)
diff --git a/Python/Python-ast.c b/Python/Python-ast.c
index 805f2b8..e6f1e58 100644
--- a/Python/Python-ast.c
+++ b/Python/Python-ast.c
@@ -1802,6 +1802,11 @@ expr_ty
 YieldFrom(expr_ty value, int lineno, int col_offset, PyArena *arena)
 {
         expr_ty p;
+        if (!value) {
+                PyErr_SetString(PyExc_ValueError,
+                                "field value is required for YieldFrom");
+                return NULL;
+        }
         p = (expr_ty)PyArena_Malloc(arena, sizeof(*p));
         if (!p)
                 return NULL;
@@ -5431,7 +5436,8 @@ obj2ast_expr(PyObject* obj, expr_ty* out, PyArena* arena)
                         Py_XDECREF(tmp);
                         tmp = NULL;
                 } else {
-                        value = NULL;
+                        PyErr_SetString(PyExc_TypeError, "required field \"value\" missing from YieldFrom");
+                        return 1;
                 }
                 *out = YieldFrom(value, lineno, col_offset, arena);
                 if (*out == NULL) goto failed;
diff --git a/Python/ast.c b/Python/ast.c
index 0c0c1a6..7657b22 100644
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -224,8 +224,7 @@ validate_expr(expr_ty exp, expr_context_ty ctx)
     case Yield_kind:
         return !exp->v.Yield.value || validate_expr(exp->v.Yield.value, Load);
     case YieldFrom_kind:
-        return !exp->v.YieldFrom.value ||
-            validate_expr(exp->v.YieldFrom.value, Load);
+        return validate_expr(exp->v.YieldFrom.value, Load);
     case Compare_kind:
         if (!asdl_seq_LEN(exp->v.Compare.comparators)) {
             PyErr_SetString(PyExc_ValueError, "Compare with no comparators");
diff --git a/Python/compile.c b/Python/compile.c
index 5016f99..3cf71ef 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -3341,27 +3341,24 @@ compiler_visit_expr(struct compiler *c, expr_ty e)
     case DictComp_kind:
         return compiler_dictcomp(c, e);
     case Yield_kind:
-    case YieldFrom_kind: {
-        expr_ty value;
         if (c->u->u_ste->ste_type != FunctionBlock)
             return compiler_error(c, "'yield' outside function");
-        value = (e->kind == YieldFrom_kind) ? e->v.YieldFrom.value : e->v.Yield.value;
-        if (value) {
-            VISIT(c, expr, value);
+        if (e->v.Yield.value) {
+            VISIT(c, expr, e->v.Yield.value);
         }
         else {
             ADDOP_O(c, LOAD_CONST, Py_None, consts);
         }
-        if (e->kind == YieldFrom_kind) {
-            ADDOP(c, GET_ITER);
-            ADDOP_O(c, LOAD_CONST, Py_None, consts);
-            ADDOP(c, YIELD_FROM);
-        }
-        else {
-            ADDOP(c, YIELD_VALUE);
-        }
+        ADDOP(c, YIELD_VALUE);
+        break;
+    case YieldFrom_kind:
+        if (c->u->u_ste->ste_type != FunctionBlock)
+            return compiler_error(c, "'yield' outside function");
+        VISIT(c, expr, e->v.YieldFrom.value);
+        ADDOP(c, GET_ITER);
+        ADDOP_O(c, LOAD_CONST, Py_None, consts);
+        ADDOP(c, YIELD_FROM);
         break;
-    }
     case Compare_kind:
         return compiler_compare(c, e);
     case Call_kind:
diff --git a/Python/symtable.c b/Python/symtable.c
index 9dde908..55898f9 100644
--- a/Python/symtable.c
+++ b/Python/symtable.c
@@ -1371,14 +1371,14 @@ symtable_visit_expr(struct symtable *st, expr_ty e)
             VISIT_QUIT(st, 0);
         break;
     case Yield_kind:
-    case YieldFrom_kind: {
-        expr_ty value;
-        value = (e->kind == YieldFrom_kind) ? e->v.YieldFrom.value : e->v.Yield.value;
-        if (value)
-            VISIT(st, expr, value);
+        if (e->v.Yield.value)
+            VISIT(st, expr, e->v.Yield.value);
+        st->st_cur->ste_generator = 1;
+        break;
+    case YieldFrom_kind:
+        VISIT(st, expr, e->v.YieldFrom.value);
         st->st_cur->ste_generator = 1;
         break;
-    }
     case Compare_kind:
         VISIT(st, expr, e->v.Compare.left);
         VISIT_SEQ(st, expr, e->v.Compare.comparators);