From 156505e798ef59aa953ab435b1009afe3e1afcc2 Mon Sep 17 00:00:00 2001
From: Alex Shinn <ashinn@users.noreply.github.com>
Date: Mon, 14 Oct 2013 08:10:34 +0900
Subject: [PATCH] The environment chain lookup should ignore undefined cells if
 there are defined cells available.  This avoids previously undefined values
 blocking an export-all import.

---
 eval.c | 31 ++++++++++++++++++++++++++-----
 1 file changed, 26 insertions(+), 5 deletions(-)

diff --git a/eval.c b/eval.c
index 462533f7..c8666573 100644
--- a/eval.c
+++ b/eval.c
@@ -78,25 +78,46 @@ sexp sexp_maybe_wrap_error (sexp ctx, sexp obj) {
 
 /********************** environment utilities ***************************/
 
+/* Look for the first defined instance of the variable.  If not found, */
+/* return the first undefined instance.  */
 static sexp sexp_env_cell_loc (sexp env, sexp key, int localp, sexp *varenv) {
-  sexp ls;
+  sexp ls, undefined = NULL, undefined_env = NULL;
 
   do {
 #if SEXP_USE_RENAME_BINDINGS
     for (ls=sexp_env_renames(env); sexp_pairp(ls); ls=sexp_env_next_cell(ls))
       if (sexp_car(ls) == key) {
-        if (varenv) *varenv = env;
-        return sexp_cdr(ls);
+        if (sexp_pairp(sexp_cdr(ls)) && sexp_cdr(sexp_cdr(ls)) == SEXP_UNDEF) {
+          if (!undefined) {
+            undefined_env = env;
+            undefined = sexp_cdr(ls);
+          }
+        } else {
+          if (varenv) *varenv = env;
+          return sexp_cdr(ls);
+        }
       }
 #endif
     for (ls=sexp_env_bindings(env); sexp_pairp(ls); ls=sexp_env_next_cell(ls))
       if (sexp_car(ls) == key) {
-        if (varenv) *varenv = env;
-        return ls;
+        if (sexp_cdr(ls) == SEXP_UNDEF) {
+          if (!undefined) {
+            undefined_env = env;
+            undefined = ls;
+          }
+        } else {
+          if (varenv) *varenv = env;
+          return ls;
+        }
       }
     env = (localp ? NULL : sexp_env_parent(env));
   } while (env && sexp_envp(env));
 
+  if (undefined) {
+    if (varenv) *varenv = undefined_env;
+    return undefined;
+  }
+
   return NULL;
 }