From f14660d5e20df07d7b32d7658c83a76c713bc915 Mon Sep 17 00:00:00 2001
From: Taeer Bar-Yam <taeer@bar-yam.me>
Date: Wed, 29 Sep 2021 19:37:51 -0400
Subject: reset yylloc when yyless(0) is called

---
 src/libexpr/lexer.l | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/libexpr/lexer.l b/src/libexpr/lexer.l
index 8ad6a1957..51593eccd 100644
--- a/src/libexpr/lexer.l
+++ b/src/libexpr/lexer.l
@@ -28,6 +28,8 @@ using namespace nix;
 
 namespace nix {
 
+// backup to recover from yyless(0)
+YYLTYPE prev_yylloc;
 
 static void initLoc(YYLTYPE * loc)
 {
@@ -38,6 +40,8 @@ static void initLoc(YYLTYPE * loc)
 
 static void adjustLoc(YYLTYPE * loc, const char * s, size_t len)
 {
+    prev_yylloc = *loc;
+
     loc->first_line = loc->last_line;
     loc->first_column = loc->last_column;
 
@@ -210,6 +214,7 @@ or          { return OR_KW; }
 {HPATH_START}\$\{ {
   PUSH_STATE(PATH_START);
   yyless(0);
+  *yylloc = prev_yylloc;
 }
 
 <PATH_START>{PATH_SEG} {
@@ -265,6 +270,7 @@ or          { return OR_KW; }
      context (it may be ')', ';', or something of that sort) */
   POP_STATE();
   yyless(0);
+  *yylloc = prev_yylloc;
   return PATH_END;
 }
 
-- 
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