fix(libexpr/eval-inline): get rid of references to nullptr env

src/libexpr/eval-inline.hh

@@ -87,11 +87,22 @@ void EvalState::forceValue(Value & v, const PosIdx pos)
 {
     if (v.isThunk()) {
         Env * env = v.payload.thunk.env;
+        assert(env || v.isBlackhole());
         Expr * expr = v.payload.thunk.expr;
         try {
             v.mkBlackhole();
             //checkInterrupt();
-            expr->eval(*this, *env, v);
+            if (env) [[likely]] {
+                expr->eval(*this, *env, v);
+            } else {
+                // NOTE: This is necessary in order to avoid forming references to nullptr
+                // when the value is already a blackhole. This should always throw an
+                // InfiniteRecursionError.
+                // TBD: Maybe this case be handled directly without an idirection and a dummy
+                // environment (which isn't accesses anyway)?
+                Env dummyEnv;
+                expr->eval(*this, dummyEnv, v);
+            }
         } catch (...) {
             v.mkThunk(env, expr);
             tryFixupBlackHolePos(v, pos);