Fix detection of char types

extras/tests/Misc/StringAdapters.cpp

@@ -86,6 +86,8 @@ TEST_CASE("custom_string") {
   CHECK(s.size() == 5);
 }
 
+struct EmptyStruct {};
+
 TEST_CASE("IsString<T>") {
   CHECK(IsString<std::string>::value == true);
   CHECK(IsString<std::basic_string<wchar_t> >::value == false);
@@ -95,6 +97,7 @@ TEST_CASE("IsString<T>") {
   CHECK(IsString<const char[8]>::value == true);
   CHECK(IsString< ::String>::value == true);
   CHECK(IsString< ::StringSumHelper>::value == true);
+  CHECK(IsString<const EmptyStruct*>::value == false);
 }
 
 TEST_CASE("stringCompare") {

src/ArduinoJson/Serialization/serialize.hpp

@@ -40,12 +40,8 @@ serialize(JsonVariantConst source, void* buffer, size_t bufferSize) {
 }
 
 template <template <typename> class TSerializer, typename TChar, size_t N>
-#if defined _MSC_VER && _MSC_VER < 1900
-typename enable_if<sizeof(remove_reference<TChar>::type) == 1, size_t>::type
-#else
-typename enable_if<sizeof(TChar) == 1, size_t>::type
-#endif
-serialize(JsonVariantConst source, TChar (&buffer)[N]) {
+typename enable_if<IsChar<TChar>::value, size_t>::type serialize(
+    JsonVariantConst source, TChar (&buffer)[N]) {
   return serialize<TSerializer>(source, buffer, N);
 }
 

src/ArduinoJson/Strings/Adapters/RamString.hpp

@@ -13,6 +13,10 @@
 
 namespace ARDUINOJSON_NAMESPACE {
 
+template <typename T>
+struct IsChar
+    : integral_constant<bool, is_integral<T>::value && sizeof(T) == 1> {};
+
 class ZeroTerminatedRamString {
  public:
   static const size_t typeSortKey = 3;
@@ -58,7 +62,7 @@ class ZeroTerminatedRamString {
 };
 
 template <typename TChar>
-struct StringAdapter<TChar*, typename enable_if<sizeof(TChar) == 1>::type> {
+struct StringAdapter<TChar*, typename enable_if<IsChar<TChar>::value>::type> {
   typedef ZeroTerminatedRamString AdaptedString;
 
   static AdaptedString adapt(const TChar* p) {
@@ -67,7 +71,7 @@ struct StringAdapter<TChar*, typename enable_if<sizeof(TChar) == 1>::type> {
 };
 
 template <typename TChar, size_t N>
-struct StringAdapter<TChar[N], typename enable_if<sizeof(TChar) == 1>::type> {
+struct StringAdapter<TChar[N], typename enable_if<IsChar<TChar>::value>::type> {
   typedef ZeroTerminatedRamString AdaptedString;
 
   static AdaptedString adapt(const TChar* p) {
@@ -128,7 +132,7 @@ class SizedRamString {
 
 template <typename TChar>
 struct SizedStringAdapter<TChar*,
-                          typename enable_if<sizeof(TChar) == 1>::type> {
+                          typename enable_if<IsChar<TChar>::value>::type> {
   typedef SizedRamString AdaptedString;
 
   static AdaptedString adapt(const TChar* p, size_t n) {