fix inconsistent choice of init list constructor

On some compilers value{x} invokes initializer_list constructor, on
others it is equivalent to value(x). This is very problematic, but this
isn't something we can fix. On the other hand, we CAN make init list
construction to be equivalent to value(x), if the size of init list is
one. This commit does exactly that.

include/boost/json/impl/value.ipp

@@ -231,13 +231,23 @@ value(
     storage_ptr sp)
 {
     if(value_ref::maybe_object(init))
+    {
         ::new(&obj_) object(
             value_ref::make_object(
                 init, std::move(sp)));
-    else
+    }
+    else if( init.size() != 1 )
+    {
         ::new(&arr_) array(
             value_ref::make_array(
                 init, std::move(sp)));
+    }
+    else
+    {
+        ::new(&sca_) scalar();
+        value temp = init.begin()->make_value( std::move(sp) );
+        swap(temp);
+    }
 }
 
 //----------------------------------------------------------

include/boost/json/value.hpp

@@ -1014,11 +1014,16 @@ class value
 
     /** Construct from an initializer-list
 
-        If the initializer list consists of key/value
-        pairs, an @ref object is created. Otherwise
-        an @ref array is created. The contents of the
-        initializer list are copied to the newly constructed
-        value using the specified memory resource.
+        @li If the initializer list consists of key/value
+        pairs, an @ref object is created; otherwise,
+
+        @li if the size of the initializer list is exactly 1, the object is
+        constructed directly from that sole element; otherwise,
+
+        @li an @ref array is created.
+
+        The contents of the initializer list are copied to the newly
+        constructed value using the specified memory resource.
 
         @par Complexity
         Linear in `init.size()`.

test/value.cpp

@@ -2131,6 +2131,15 @@ class value_test
     void
     testInitList()
     {
+        {
+            value jv{};
+            BOOST_TEST( jv.is_null() );
+        }
+        {
+            value jv{0};
+            BOOST_TEST( jv == 0 );
+        }
+
         check_array(value{0,0,0},               0, 0, 0);
         check_array(value{false,false,false},   false, false, false);
         check_array(value{false,2,false},       false, 2, false);