relax iterator requirements

Author: grisumbras

include/boost/json/array.hpp

@@ -282,7 +282,7 @@ class array
         @par Constraints
 
         @code
-        std::is_constructible_v<value, std::iterator_traits<InputIt>::value_type>
+        std::is_constructible_v<value, std::iterator_traits<InputIt>::reference>
         @endcode
 
         @par Complexity
@@ -312,7 +312,7 @@ class array
         ,class = typename std::enable_if<
             std::is_constructible<value,
                 typename std::iterator_traits<
-                    InputIt>::value_type>::value>::type
+                    InputIt>::reference>::value>::type
     #endif
     >
     array(
@@ -1239,7 +1239,7 @@ class array
 
         @par Mandates
         @code
-        std::is_constructible_v<value, std::iterator_traits<InputIt>::value_type>
+        std::is_constructible_v<value, std::iterator_traits<InputIt>::reference>
         @endcode
 
         @par Complexity
@@ -1270,7 +1270,7 @@ class array
         ,class = typename std::enable_if<
             std::is_constructible<value,
                 typename std::iterator_traits<
-                    InputIt>::value_type>::value>::type
+                    InputIt>::reference>::value>::type
     #endif
     >
     iterator

include/boost/json/object.hpp

@@ -86,7 +86,7 @@ class object
     template<class T>
     using is_inputit = typename std::enable_if<
         std::is_constructible<key_value_pair,
-        typename std::iterator_traits<T>::value_type
+        typename std::iterator_traits<T>::reference
             >::value>::type;
 
     BOOST_JSON_DECL
@@ -249,7 +249,7 @@ class object
         @code
         std::is_constructible_v<
             key_value_pair,
-            std::iterator_traits<InputIt>::value_type>
+            std::iterator_traits<InputIt>::reference>
         @endcode
 
         @par Complexity
@@ -988,7 +988,7 @@ class object
 
         @par Constraints
         @code
-        std::is_constructible_v<value_type, std::iterator_traits<InputIt>::value_type>
+        std::is_constructible_v<value_type, std::iterator_traits<InputIt>::reference>
         @endcode
 
         @par Complexity

include/boost/json/string.hpp

@@ -138,7 +138,7 @@ class string
     template<class T>
     using is_inputit = typename std::enable_if<
         std::is_convertible<typename
-            std::iterator_traits<T>::value_type,
+            std::iterator_traits<T>::reference,
             char>::value>::type;
 
     storage_ptr sp_; // must come first