2 files changed, 21 insertions, 0 deletions
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-formals-miscompilation-b-261-regression.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-formals-miscompilation-b-261-regression.exp
new file mode 100644
index 000000000000..721a052bcc67
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-formals-miscompilation-b-261-regression.exp
@@ -0,0 +1 @@
+[ true null ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-formals-miscompilation-b-261-regression.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-formals-miscompilation-b-261-regression.nix
new file mode 100644
index 000000000000..43bef2938df5
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-formals-miscompilation-b-261-regression.nix
@@ -0,0 +1,20 @@
+# This is a regression test for https://b.tvl.fyi/261.
+#
+# The bug occurred when Tvix would unconditionally finalise the stack slot of
+# `finalise` (as its default expression needs a finaliser): Finalising an
+# manually provided, already forced thunk would cause the VM to crash.
+let
+  thunk = x: x;
+  bomb = thunk true;
+  f =
+    { finalise ? later == null
+    , later ? null
+    }:
+    [ finalise later ];
+in
+
+# Note that the crash did not occur if the offending expression was the rhs
+# argument to `builtins.seq`, hence we need to put the assert in between.
+assert builtins.seq bomb true;
+
+f { finalise = bomb; }