Correcting my previous post (deleted):

You can get an assert failure without ASAN with this patch:

diff --git a/Parser/pegen.c b/Parser/pegen.c
index e29910bf86..65fa44921f 100644
--- a/Parser/pegen.c
+++ b/Parser/pegen.c
@@ -145,6 +145,9 @@ byte_offset_to_character_offset(PyObject *line, int col_offset)
     if (!str) {
         return 0;
     }
+    assert(col_offset <= PyUnicode_GET_LENGTH(line));
     PyObject *text = PyUnicode_DecodeUTF8(str, col_offset, "replace");
     if (!text) {
         return 0;