Fix issue with json null representation in containers

3 files changed, 20 insertions, 5 deletions

diff --git a/libbuild2/variable.cxx b/libbuild2/variable.cxx
index ab65237..078c13a 100644
--- a/libbuild2/variable.cxx
+++ b/libbuild2/variable.cxx
@@ -1799,9 +1799,16 @@ namespace build2
     {
     case json_type::null:
       {
-        // Return empty to be consistent with other places.
+        // Note that here we cannot return empty (e.g., to be consistent with
+        // other places) because we treat empty name (as opposed to empty
+        // names) as string, not null (see to_json_value() above).
         //
-#if 0
+        // Thankfully this version of reverse() is only used when json_value
+        // representation is needed as part of a container. Which means in
+        // "consumption" contexts (e.g., result of subscript) null will still
+        // decay to empty.
+        //
+#if 1
         return name ("null");
 #else
         return name ();
diff --git a/libbuild2/variable.txx b/libbuild2/variable.txx
index 12a2667..0b831e9 100644
--- a/libbuild2/variable.txx
+++ b/libbuild2/variable.txx
@@ -1347,6 +1347,10 @@ namespace build2
   // value), but we don't yet have pair<> as value type so we let the generic
   // implementation return an untyped pair.
   //
+  // BTW, one negative consequence of returning untyped pair is that
+  // $first()/$second() don't return types values either, which is quite
+  // unfortunate for something like json_map.
+  //
   template <typename K, typename V>
   const map_value_type<K, V>
   value_traits<map<K, V>>::value_type = build2::value_type // VC14 wants =
diff --git a/tests/type/json/testscript b/tests/type/json/testscript
index 6beaf5d..36287b7 100644
--- a/tests/type/json/testscript
+++ b/tests/type/json/testscript
@@ -468,15 +468,19 @@
   print ($m[1])
   print $type($m[1])
   print ($m[2][b])
+  print ($m[0])
+  print ($m[-1])
   EOI
-  -1@"" 0@"" 1@[1,2] 2@{"a":1,"b":2}
-  -1 ""
-  0 ""
+  -1@null 0@null 1@[1,2] 2@{"a":1,"b":2}
+  -1 null
+  0 null
   1 [1,2]
   2 {"a":1,"b":2}
   [1,2]
   json
   2
+
+
   EOO
 }