Add json_map and json_set buildfile value types

These expose the std::map<json_value,json_value> and std::set<json_value>
types to buildfiles.

New functions:

  $size(<json-set>)
  $size(<json-map>)
  $keys(<json-map>)

Note that the $keys() function returns the list of map key as a json array.

For example:

m = [json_map] 2@([json] a@1 b@2) 1@([json] 1 2)
s = [json_set] ([json] x@1 y@2) ([json] a@1 b@2)

print ($m[2][b])              # 2
print ($s[([json] y@2 x@1)])  # true

1 files changed, 21 insertions, 1 deletions

diff --git a/libbuild2/variable.hxx b/libbuild2/variable.hxx
index d754edf..aed3350 100644
--- a/libbuild2/variable.hxx
+++ b/libbuild2/variable.hxx
@@ -100,6 +100,10 @@ namespace build2
 
     // If NULL, then the value is never empty.
     //
+    // Note that this is "semantically empty", not necessarily
+    // "representationally empty". For example, an empty JSON array is
+    // semantically empty but its representation (`[]`) is not.
+    //
     bool (*const empty) (const value&);
 
     // Custom subscript function. If NULL, then the generic implementation is
@@ -347,6 +351,10 @@ namespace build2
     // Check in a type-independent way if the value is empty. The value must
     // not be NULL.
     //
+    // Note that this is "semantically empty", not necessarily
+    // "representationally empty". For example, an empty JSON array is
+    // semantically empty but its representation (`[]`) is not.
+    //
     bool
     empty () const;
 
@@ -691,7 +699,7 @@ namespace build2
   // case (container) if invalid_argument is thrown, the names are not
   // guaranteed to be unchanged.
   //
-  //template <typename T> T convert (names&&); (declaration causes ambiguity)
+  template <typename T> T convert (names&&);
 
   // Convert value to T. If value is already of type T, then simply cast it.
   // Otherwise call convert(names) above. If value is NULL, then throw
@@ -1254,6 +1262,14 @@ namespace build2
     static void prepend (value&, json_value&&);
     static bool empty (const json_value&); // null or empty array/object
 
+    // These are provided to make it possible to use json_value as a container
+    // element.
+    //
+    static json_value convert (name&&, name*);
+    static name reverse (const json_value&);
+    static int compare (const json_value& x, const json_value& y) {
+      return x.compare (y);}
+
     static const json_value empty_instance; // null
     static const char* const type_name;
     static const build2::value_type value_type;
@@ -1348,11 +1364,15 @@ namespace build2
   value_traits<vector<pair<string, optional<bool>>>>;
 
   extern template struct LIBBUILD2_DECEXPORT value_traits<set<string>>;
+  extern template struct LIBBUILD2_DECEXPORT value_traits<set<json_value>>;
 
   extern template struct LIBBUILD2_DECEXPORT
   value_traits<map<string, string>>;
 
   extern template struct LIBBUILD2_DECEXPORT
+  value_traits<map<json_value, json_value>>;
+
+  extern template struct LIBBUILD2_DECEXPORT
   value_traits<map<string, optional<string>>>;
 
   extern template struct LIBBUILD2_DECEXPORT