Added InvalidConversion to identify invalid conversions (closes #1585)

CHANGELOG.md

@@ -8,6 +8,7 @@ HEAD
 * Fixed error `[Pe070]: incomplete type is not allowed` on IAR (issue #1560)
 * Fixed `serializeJson(doc, String)` when allocation fails (issue #1572)
 * Fixed clang-tidy warnings (issue #1574, PR #1577 by @armandas)
+* Added fake class `InvalidConversion<T1,T2>` to easily identify invalid conversions (issue #1585)
 
 v6.18.0 (2021-05-05)
 -------

src/ArduinoJson/Array/ArrayRef.hpp

@@ -193,6 +193,8 @@ struct Converter<ArrayRef> {
     return ArrayRef(pool, data != 0 ? data->asArray() : 0);
   }
 
+  static InvalidConversion<VariantConstRef, ArrayRef> fromJson(VariantConstRef);
+
   static bool checkJson(VariantConstRef) {
     return false;
   }

src/ArduinoJson/Object/ObjectRef.hpp

@@ -265,6 +265,9 @@ struct Converter<ObjectRef> {
     return ObjectRef(pool, data != 0 ? data->asObject() : 0);
   }
 
+  static InvalidConversion<VariantConstRef, ObjectRef> fromJson(
+      VariantConstRef);
+
   static bool checkJson(VariantConstRef) {
     return false;
   }

src/ArduinoJson/Variant/Converter.hpp

@@ -9,4 +9,9 @@ namespace ARDUINOJSON_NAMESPACE {
 template <typename T, typename Enable = void>
 struct Converter;
 
+// clang-format off
+template <typename T1, typename T2>
+class InvalidConversion;  // Error here? See https://arduinojson.org/v6/invalid-conversion/
+// clang-format on
+
 }  // namespace ARDUINOJSON_NAMESPACE

src/ArduinoJson/Variant/VariantRef.hpp

@@ -342,13 +342,19 @@ struct Converter<VariantRef> {
   static bool toJson(VariantRef src, VariantRef dst) {
     return variantCopyFrom(getData(dst), getData(src), getPool(dst));
   }
+
   static VariantRef fromJson(VariantRef src) {
     return src;
   }
+
+  static InvalidConversion<VariantConstRef, VariantRef> fromJson(
+      VariantConstRef);
+
   static bool checkJson(VariantRef src) {
     VariantData *data = getData(src);
     return !!data;
   }
+
   static bool checkJson(VariantConstRef) {
     return false;
   }