* Fix blackholing. If evaluation fails due to an assertion failure,

  then the blackhole has to be removed to ensure that repeated
  evaluation of the same value gives an assertion failure again rather
  than an "infinite recursion" error.

1 files changed, 8 insertions, 2 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 98b6b7bdd1..e72538e608 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -681,8 +681,14 @@ bool EvalState::evalBool(Env & env, Expr e)
 void EvalState::forceValue(Value & v)
 {
     if (v.type == tThunk) {
-        //v.type = tBlackhole;
-        eval(*v.thunk.env, v.thunk.expr, v);
+        ValueType saved = v.type;
+        try {
+            v.type = tBlackhole;
+            eval(*v.thunk.env, v.thunk.expr, v);
+        } catch (Error & e) {
+            v.type = saved;
+            throw;
+        }
     }
     else if (v.type == tCopy) {
         forceValue(*v.val);