From e72a16a339092359e58ac7626ea89d4182b26642 Mon Sep 17 00:00:00 2001
From: Ben Burdette <bburdette@gmail.com>
Date: Tue, 30 Jun 2020 11:00:51 -0600
Subject: [PATCH 1/2] check for a null symbol

---
 src/libutil/error.hh | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index 1e6102ce1..6982e30aa 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -67,7 +67,11 @@ struct ErrPos {
     {
         line = pos.line;
         column = pos.column;
-        file = pos.file;
+        // is file symbol null?
+        if (pos.file.set())
+            file = pos.file;
+        else
+            file = "";
         return *this;
     }
 

From a0705e0dd152f2a19d096d02024723a5614c7726 Mon Sep 17 00:00:00 2001
From: Ben Burdette <bburdette@gmail.com>
Date: Tue, 30 Jun 2020 11:01:46 -0600
Subject: [PATCH 2/2] invalid pos check

---
 src/libutil/tests/logging.cc | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/src/libutil/tests/logging.cc b/src/libutil/tests/logging.cc
index 6a6fb4ac3..6a58b9425 100644
--- a/src/libutil/tests/logging.cc
+++ b/src/libutil/tests/logging.cc
@@ -289,4 +289,22 @@ namespace nix {
             "what about this " ANSI_YELLOW "%3%" ANSI_NORMAL " " ANSI_YELLOW "one" ANSI_NORMAL);
 
     }
+
+    /* ----------------------------------------------------------------------------
+     * ErrPos
+     * --------------------------------------------------------------------------*/
+
+    TEST(errpos, invalidPos) {
+
+      // contains an invalid symbol, which we should not dereference!
+      Pos invalid;
+
+      // constructing without access violation.
+      ErrPos ep(invalid);
+    
+      // assignment without access violation.
+      ep = invalid;
+
+    }
+
 }