From a812b6c6e628f878c485a1b78e3fc7ee90352c2c Mon Sep 17 00:00:00 2001
From: Robert Hensing <robert@roberthensing.nl>
Date: Fri, 21 Nov 2025 23:51:07 +0100
Subject: [PATCH] libexpr: add list index to deepSeq error traces

When deepSeq encounters an error while evaluating a list element, the
error trace now includes the list index, making it easier to locate
the problematic element.
---
 src/libexpr/eval.cc                           |  9 ++++++-
 .../lang/eval-fail-deepseq-list-attr.err.exp  | 25 +++++++++++++++++++
 .../lang/eval-fail-deepseq-list-attr.nix      | 10 ++++++++
 3 files changed, 43 insertions(+), 1 deletion(-)
 create mode 100644 tests/functional/lang/eval-fail-deepseq-list-attr.err.exp
 create mode 100644 tests/functional/lang/eval-fail-deepseq-list-attr.nix

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 837e674cb..0a6b199bf 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -2216,8 +2216,15 @@ void EvalState::forceValueDeep(Value & v)
         }
 
         else if (v.isList()) {
+            size_t index = 0;
             for (auto v2 : v.listView())
-                recurse(*v2);
+                try {
+                    recurse(*v2);
+                    index++;
+                } catch (Error & e) {
+                    state.addErrorTrace(e, "while evaluating list element at index %1%", index);
+                    throw;
+                }
         }
     }(v);
 }
diff --git a/tests/functional/lang/eval-fail-deepseq-list-attr.err.exp b/tests/functional/lang/eval-fail-deepseq-list-attr.err.exp
new file mode 100644
index 000000000..9abd937ba
--- /dev/null
+++ b/tests/functional/lang/eval-fail-deepseq-list-attr.err.exp
@@ -0,0 +1,25 @@
+error:
+       … while calling the 'deepSeq' builtin
+         at /pwd/lang/eval-fail-deepseq-list-attr.nix:3:1:
+            2|
+            3| builtins.deepSeq [
+             | ^
+            4|   1
+
+       … while evaluating list element at index 1
+
+       … while evaluating the attribute 'b'
+         at /pwd/lang/eval-fail-deepseq-list-attr.nix:7:5:
+            6|     a = 2;
+            7|     b = throw "error in attr in list element";
+             |     ^
+            8|   }
+
+       … while calling the 'throw' builtin
+         at /pwd/lang/eval-fail-deepseq-list-attr.nix:7:9:
+            6|     a = 2;
+            7|     b = throw "error in attr in list element";
+             |         ^
+            8|   }
+
+       error: error in attr in list element
diff --git a/tests/functional/lang/eval-fail-deepseq-list-attr.nix b/tests/functional/lang/eval-fail-deepseq-list-attr.nix
new file mode 100644
index 000000000..5ffd8c196
--- /dev/null
+++ b/tests/functional/lang/eval-fail-deepseq-list-attr.nix
@@ -0,0 +1,10 @@
+# Test that deepSeq reports list index and attribute name in error traces.
+
+builtins.deepSeq [
+  1
+  {
+    a = 2;
+    b = throw "error in attr in list element";
+  }
+  3
+] "unexpected success"