From 9b88fdf4f09b77d866a3591c9e64b4e8a5b25920 Mon Sep 17 00:00:00 2001
From: "Eric V. Smith" <eric@trueblade.com>
Date: Mon, 7 Nov 2016 17:54:01 -0500
Subject: [PATCH] Fixed issue #28633: segfault when concatenating bytes literal
 and f-string.

---
 Lib/test/test_fstring.py | 7 +++++++
 Python/ast.c             | 9 +++++----
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/Lib/test/test_fstring.py b/Lib/test/test_fstring.py
index 086bf673f9d..82050835f6a 100644
--- a/Lib/test/test_fstring.py
+++ b/Lib/test/test_fstring.py
@@ -92,6 +92,13 @@ f'{a * x()}'"""
         exec(c)
         self.assertEqual(x[0], 'foo3')
 
+    def test_compile_time_concat_errors(self):
+        self.assertAllRaise(SyntaxError,
+                            'cannot mix bytes and nonbytes literals',
+                            [r"""f'' b''""",
+                             r"""b'' f''""",
+                             ])
+
     def test_literal(self):
         self.assertEqual(f'', '')
         self.assertEqual(f'a', 'a')
diff --git a/Python/ast.c b/Python/ast.c
index 91e7d0129f4..fcd1563a69a 100644
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -5147,7 +5147,8 @@ parsestrplus(struct compiling *c, const node *n)
         /* Check that we're not mixing bytes with unicode. */
         if (i != 0 && bytesmode != this_bytesmode) {
             ast_error(c, n, "cannot mix bytes and nonbytes literals");
-            Py_DECREF(s);
+            /* s is NULL if the current string part is an f-string. */
+            Py_XDECREF(s);
             goto error;
         }
         bytesmode = this_bytesmode;
@@ -5161,11 +5162,12 @@ parsestrplus(struct compiling *c, const node *n)
             if (result < 0)
                 goto error;
         } else {
+            /* A string or byte string. */
+            assert(s != NULL && fstr == NULL);
+
             assert(bytesmode ? PyBytes_CheckExact(s) :
                    PyUnicode_CheckExact(s));
 
-            /* A string or byte string. */
-            assert(s != NULL && fstr == NULL);
             if (bytesmode) {
                 /* For bytes, concat as we go. */
                 if (i == 0) {
@@ -5177,7 +5179,6 @@ parsestrplus(struct compiling *c, const node *n)
                         goto error;
                 }
             } else {
-                assert(s != NULL && fstr == NULL);
                 /* This is a regular string. Concatenate it. */
                 if (FstringParser_ConcatAndDel(&state, s) < 0)
                     goto error;