From e79278b4fcbc5da5a0213b784741c1202ff899e0 Mon Sep 17 00:00:00 2001
From: skye <lix@skye-c.at>
Date: Fri, 3 Apr 2026 18:24:27 -0400
Subject: [PATCH] Check for throw while evaluating throw message
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This is a small tweak to the logic added in cl/1511 to detect explicit
throws when printing stack traces. Now when deciding whether to print
"caused by explicit throw", it checks not only that the error is a
ThrowError and that we are in a throw, but also that the ThrowError was
thrown by *this* throw, and not by another while in the process of
evaluating this throw's operand.

It turns this:

```
let
  set = {
    inner = throw "nested throw";
  };
in
throw set.inner

error:
       … caused by explicit throw
         at /pwd/in.nix:6:1:
            5| in
            6| throw set.inner
             | ^
            7|

       … while evaluating the attribute 'inner'
         at /pwd/in.nix:3:5:
            2|   set = {
            3|     inner = throw "nested throw";
             |     ^
            4|   };

       … caused by explicit throw
         at /pwd/in.nix:3:13:
            2|   set = {
            3|     inner = throw "nested throw";
             |             ^
            4|   };

       error: nested throw
```

into this:

```
error:
       … while calling the 'throw' builtin
         at /pwd/in.nix:6:1:
            5| in
            6| throw set.inner
             | ^
            7|

       … while evaluating the attribute 'inner'
         at /pwd/in.nix:3:5:
            2|   set = {
            3|     inner = throw "nested throw";
             |     ^
            4|   };

       … caused by explicit throw
         at /pwd/in.nix:3:13:
            2|   set = {
            3|     inner = throw "nested throw";
             |             ^
            4|   };

       error: nested throw
```

Notice the difference in the top context frame. Before it incorrectly
attributed the throw error to the both throws instead of just the one
that actually threw.

Change-Id: If3b0b8311f1ae2ff1471e260fe59d9166a6a6964
---
 lix/libexpr/eval.cc                           |  2 +-
 .../lang/select-err/eval-fail-2.err.exp       | 23 +++++++++++++++++++
 tests/functional2/lang/select-err/in-2.nix    |  6 +++++
 3 files changed, 30 insertions(+), 1 deletion(-)
 create mode 100644 tests/functional2/lang/select-err/eval-fail-2.err.exp
 create mode 100644 tests/functional2/lang/select-err/in-2.nix

diff --git a/lix/libexpr/eval.cc b/lix/libexpr/eval.cc
index d3f203748..4882199f4 100644
--- a/lix/libexpr/eval.cc
+++ b/lix/libexpr/eval.cc
@@ -1206,7 +1206,7 @@ Value EvalState::callFunction(Value & fun, std::span<Value> args, const PosIdx p
                 } catch (ThrownError & e) {
                     // Distinguish between an error that simply happened while "throw"
                     // was being evaluated and an explicit thrown error.
-                    if (fn->name == "throw") {
+                    if (fn->name == "throw" && !e.hasTrace()) {
                         e.addTrace(ctx.positions[pos], "caused by explicit %s", "throw");
                     } else {
                         e.addTrace(ctx.positions[pos], "while calling the '%s' builtin", fn->name);
diff --git a/tests/functional2/lang/select-err/eval-fail-2.err.exp b/tests/functional2/lang/select-err/eval-fail-2.err.exp
new file mode 100644
index 000000000..6ed4d7594
--- /dev/null
+++ b/tests/functional2/lang/select-err/eval-fail-2.err.exp
@@ -0,0 +1,23 @@
+error:
+       … while calling the 'throw' builtin
+         at /pwd/in.nix:6:1:
+            5| in
+            6| throw set.inner
+             | ^
+            7|
+
+       … while evaluating the attribute 'inner'
+         at /pwd/in.nix:3:5:
+            2|   set = {
+            3|     inner = throw "nested throw";
+             |     ^
+            4|   };
+
+       … caused by explicit throw
+         at /pwd/in.nix:3:13:
+            2|   set = {
+            3|     inner = throw "nested throw";
+             |             ^
+            4|   };
+
+       error: nested throw
diff --git a/tests/functional2/lang/select-err/in-2.nix b/tests/functional2/lang/select-err/in-2.nix
new file mode 100644
index 000000000..2d4319ed2
--- /dev/null
+++ b/tests/functional2/lang/select-err/in-2.nix
@@ -0,0 +1,6 @@
+let
+  set = {
+    inner = throw "nested throw";
+  };
+in
+throw set.inner