prim_foldlStrict: call forceValue() before value is copied

forceValue() were called after a value is copied effectively forcing only one of the copies keeping another copy not evaluated.
This resulted in its evaluation of the same lazy value more than once (the number of hits is not big though)

1 files changed, 7 insertions, 6 deletions

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index e71e3a6d46..311a32b0b3 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -1508,19 +1508,20 @@ static void prim_foldlStrict(EvalState & state, const Pos & pos, Value * * args,
     state.forceFunction(*args[0], pos);
     state.forceList(*args[2], pos);
 
-    Value * vCur = args[1];
+    if (args[2]->listSize()) {
+        Value * vCur = args[1];
 
-    if (args[2]->listSize())
         for (unsigned int n = 0; n < args[2]->listSize(); ++n) {
             Value vTmp;
             state.callFunction(*args[0], *vCur, vTmp, pos);
             vCur = n == args[2]->listSize() - 1 ? &v : state.allocValue();
             state.callFunction(vTmp, *args[2]->listElems()[n], *vCur, pos);
         }
-    else
-        v = *vCur;
-
-    state.forceValue(v);
+        state.forceValue(v);
+    } else {
+        state.forceValue(*args[1]);
+        v = *args[1];
+    }
 }