From 1d24caf066035d52eb67dfc6be4440db31213251 Mon Sep 17 00:00:00 2001
From: Benoit Blanchon <git@benoitblanchon.fr>
Date: Fri, 11 Jun 2021 11:46:20 +0200
Subject: [PATCH] Added InvalidConversion to identify invalid conversions
 (closes #1585)

---
 CHANGELOG.md                           | 1 +
 src/ArduinoJson/Array/ArrayRef.hpp     | 2 ++
 src/ArduinoJson/Object/ObjectRef.hpp   | 3 +++
 src/ArduinoJson/Variant/Converter.hpp  | 5 +++++
 src/ArduinoJson/Variant/VariantRef.hpp | 6 ++++++
 5 files changed, 17 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 206cbd50..4a3bc949 100644
--- a/CHANGELOG.md
+++ b/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)
 -------
diff --git a/src/ArduinoJson/Array/ArrayRef.hpp b/src/ArduinoJson/Array/ArrayRef.hpp
index a991db0d..9a6f82d8 100644
--- a/src/ArduinoJson/Array/ArrayRef.hpp
+++ b/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;
   }
diff --git a/src/ArduinoJson/Object/ObjectRef.hpp b/src/ArduinoJson/Object/ObjectRef.hpp
index c945fb6c..cb541bdb 100644
--- a/src/ArduinoJson/Object/ObjectRef.hpp
+++ b/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;
   }
diff --git a/src/ArduinoJson/Variant/Converter.hpp b/src/ArduinoJson/Variant/Converter.hpp
index 476ca8fd..88afa526 100644
--- a/src/ArduinoJson/Variant/Converter.hpp
+++ b/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
diff --git a/src/ArduinoJson/Variant/VariantRef.hpp b/src/ArduinoJson/Variant/VariantRef.hpp
index 10c3fdb7..06615920 100644
--- a/src/ArduinoJson/Variant/VariantRef.hpp
+++ b/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;
   }