1 files changed, 9 insertions, 6 deletions

diff --git a/src/libexpr/attr-path.cc b/src/libexpr/attr-path.cc
index 06b472d8b..4545bfd72 100644
--- a/src/libexpr/attr-path.cc
+++ b/src/libexpr/attr-path.cc
@@ -32,7 +32,7 @@ static Strings parseAttrPath(const string & s)
 }
 
 
-Value * findAlongAttrPath(EvalState & state, const string & attrPath,
+std::pair<Value *, Pos> findAlongAttrPath(EvalState & state, const string & attrPath,
     Bindings & autoArgs, Value & vIn)
 {
     Strings tokens = parseAttrPath(attrPath);
@@ -41,6 +41,7 @@ Value * findAlongAttrPath(EvalState & state, const string & attrPath,
         Error(format("attribute selection path '%1%' does not match expression") % attrPath);
 
     Value * v = &vIn;
+    Pos pos = noPos;
 
     for (auto & attr : tokens) {
 
@@ -70,8 +71,9 @@ Value * findAlongAttrPath(EvalState & state, const string & attrPath,
 
             Bindings::iterator a = v->attrs->find(state.symbols.create(attr));
             if (a == v->attrs->end())
-                throw Error(format("attribute '%1%' in selection path '%2%' not found") % attr % attrPath);
+                throw AttrPathNotFound("attribute '%1%' in selection path '%2%' not found", attr, attrPath);
             v = &*a->value;
+            pos = *a->pos;
         }
 
         else if (apType == apIndex) {
@@ -82,14 +84,15 @@ Value * findAlongAttrPath(EvalState & state, const string & attrPath,
                     % attrPath % showType(*v));
 
             if (attrIndex >= v->listSize())
-                throw Error(format("list index %1% in selection path '%2%' is out of range") % attrIndex % attrPath);
+                throw AttrPathNotFound("list index %1% in selection path '%2%' is out of range", attrIndex, attrPath);
 
             v = v->listElems()[attrIndex];
+            pos = noPos;
         }
 
     }
 
-    return v;
+    return {v, pos};
 }
 
 
@@ -98,9 +101,9 @@ Pos findDerivationFilename(EvalState & state, Value & v, std::string what)
     Value * v2;
     try {
         auto dummyArgs = state.allocBindings(0);
-        v2 = findAlongAttrPath(state, "meta.position", *dummyArgs, v);
+        v2 = findAlongAttrPath(state, "meta.position", *dummyArgs, v).first;
     } catch (Error &) {
-        throw Error("package '%s' has no source location information", what);
+        throw NoPositionInfo("package '%s' has no source location information", what);
     }
 
     // FIXME: is it possible to extract the Pos object instead of doing this