bpo-40904: Fix segfault in the new parser with f-string containing yield statements with no value (GH-20701)

(cherry picked from commit 972ab0327675e695373fc6272d5ac24e187579ad)

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>

3 files changed, 7 insertions, 0 deletions

diff --git a/Lib/test/test_fstring.py b/Lib/test/test_fstring.py
index 3237b58..f6be64b 100644
--- a/Lib/test/test_fstring.py
+++ b/Lib/test/test_fstring.py
@@ -725,9 +725,11 @@ non-important content
         #  a function into a generator
         def fn(y):
             f'y:{yield y*2}'
+            f'{yield}'
 
         g = fn(4)
         self.assertEqual(next(g), 8)
+        self.assertEqual(next(g), None)
 
     def test_yield_send(self):
         def fn(x):
diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-06-08-01-08-57.bpo-40904.76qQzo.rst b/Misc/NEWS.d/next/Core and Builtins/2020-06-08-01-08-57.bpo-40904.76qQzo.rst
new file mode 100644
index 0000000..09009b1
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2020-06-08-01-08-57.bpo-40904.76qQzo.rst
@@ -0,0 +1,2 @@
+Fix possible segfault in the new PEG parser when parsing f-string containing
+yield statements with no value (:code:`f"{yield}"`). Patch by Pablo Galindo
diff --git a/Parser/pegen/parse_string.c b/Parser/pegen/parse_string.c
index efe82df..94241e1 100644
--- a/Parser/pegen/parse_string.c
+++ b/Parser/pegen/parse_string.c
@@ -278,6 +278,9 @@ static void fstring_shift_argument(expr_ty parent, arg_ty args, int lineno, int
 
 
 static inline void shift_expr(expr_ty parent, expr_ty n, int line, int col) {
+    if (n == NULL) {
+        return;
+    }
     if (parent->lineno < n->lineno) {
         col = 0;
     }