Detect a void-returning erase() through a named trait

erase_from_object() distinguished its two overloads with a decltype of a
member call written inline in a default template argument. Every other
detection in the library goes through the detector machinery in detected.hpp
instead -- has_erase_with_key_type is the same question about the same member
function -- and the inline form is the one shape older compilers are least
reliable about.

Express it the same way: detect_erase_with_iterator plus is_detected_exact,
both of which the library already relies on elsewhere. No behaviour changes.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/meta/type_traits.hpp

@@ -779,6 +779,15 @@ using has_erase_with_key_type = typename std::conditional <
                                 std::true_type,
                                 std::false_type >::type;
 
+template<typename ObjectType, typename IteratorType>
+using detect_erase_with_iterator = decltype(std::declval<ObjectType&>().erase(std::declval<IteratorType>()));
+
+// type trait to check if erase(iterator) returns void instead of the following
+// iterator, as the object types that do not compute a successor the caller may
+// not need do
+template<typename ObjectType, typename IteratorType>
+using erase_returns_void = is_detected_exact<void, detect_erase_with_iterator, ObjectType, IteratorType>;
+
 template<typename T>
 using detect_capacity = decltype(std::declval<const T&>().capacity());
 

include/nlohmann/json.hpp

@@ -788,14 +788,14 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// (e.g., Abseil's hash maps) return void to avoid computing a successor
     /// the caller may not need. Compute it before erasing for those.
     template < typename It, detail::enable_if_t <
-                   !std::is_void<decltype(std::declval<object_t&>().erase(std::declval<It>()))>::value, int > = 0 >
+                   !detail::erase_returns_void<object_t, It>::value, int > = 0 >
     typename object_t::iterator erase_from_object(It pos)
     {
         return m_data.m_value.object->erase(pos);
     }
 
     template < typename It, detail::enable_if_t <
-                   std::is_void<decltype(std::declval<object_t&>().erase(std::declval<It>()))>::value, int > = 0 >
+                   detail::erase_returns_void<object_t, It>::value, int > = 0 >
     typename object_t::iterator erase_from_object(It pos)
     {
         auto next = std::next(pos);

single_include/nlohmann/json.hpp

@@ -4579,6 +4579,15 @@ using has_erase_with_key_type = typename std::conditional <
                                 std::true_type,
                                 std::false_type >::type;
 
+template<typename ObjectType, typename IteratorType>
+using detect_erase_with_iterator = decltype(std::declval<ObjectType&>().erase(std::declval<IteratorType>()));
+
+// type trait to check if erase(iterator) returns void instead of the following
+// iterator, as the object types that do not compute a successor the caller may
+// not need do
+template<typename ObjectType, typename IteratorType>
+using erase_returns_void = is_detected_exact<void, detect_erase_with_iterator, ObjectType, IteratorType>;
+
 template<typename T>
 using detect_capacity = decltype(std::declval<const T&>().capacity());
 
@@ -22262,14 +22271,14 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// (e.g., Abseil's hash maps) return void to avoid computing a successor
     /// the caller may not need. Compute it before erasing for those.
     template < typename It, detail::enable_if_t <
-                   !std::is_void<decltype(std::declval<object_t&>().erase(std::declval<It>()))>::value, int > = 0 >
+                   !detail::erase_returns_void<object_t, It>::value, int > = 0 >
     typename object_t::iterator erase_from_object(It pos)
     {
         return m_data.m_value.object->erase(pos);
     }
 
     template < typename It, detail::enable_if_t <
-                   std::is_void<decltype(std::declval<object_t&>().erase(std::declval<It>()))>::value, int > = 0 >
+                   detail::erase_returns_void<object_t, It>::value, int > = 0 >
     typename object_t::iterator erase_from_object(It pos)
     {
         auto next = std::next(pos);