From ac1d29fac0faa546c6a6464b586a62ebb935147d Mon Sep 17 00:00:00 2001
From: Benoit Blanchon <git@benoitblanchon.fr>
Date: Thu, 31 Mar 2022 11:23:24 +0200
Subject: [PATCH] Add meta function `ConverterNeedsWriteableRef<T>`

---
 extras/tests/JsonVariant/converters.cpp   | 12 ++++++++++++
 src/ArduinoJson/Variant/ConverterImpl.hpp | 14 ++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/extras/tests/JsonVariant/converters.cpp b/extras/tests/JsonVariant/converters.cpp
index 3c4ad6d3..785289e1 100644
--- a/extras/tests/JsonVariant/converters.cpp
+++ b/extras/tests/JsonVariant/converters.cpp
@@ -140,3 +140,15 @@ TEST_CASE("Custom converter with specialization") {
     REQUIRE(doc["value"]["imag"] == 3);
   }
 }
+
+TEST_CASE("ConverterNeedsWriteableRef") {
+  using namespace ARDUINOJSON_NAMESPACE;
+  CHECK(ConverterNeedsWriteableRef<int>::value == false);
+  CHECK(ConverterNeedsWriteableRef<float>::value == false);
+  CHECK(ConverterNeedsWriteableRef<JsonVariant>::value == true);
+  CHECK(ConverterNeedsWriteableRef<JsonVariantConst>::value == false);
+  CHECK(ConverterNeedsWriteableRef<JsonObject>::value == true);
+  CHECK(ConverterNeedsWriteableRef<JsonObjectConst>::value == false);
+  CHECK(ConverterNeedsWriteableRef<JsonArray>::value == true);
+  CHECK(ConverterNeedsWriteableRef<JsonArrayConst>::value == false);
+}
diff --git a/src/ArduinoJson/Variant/ConverterImpl.hpp b/src/ArduinoJson/Variant/ConverterImpl.hpp
index baf1187a..e0222592 100644
--- a/src/ArduinoJson/Variant/ConverterImpl.hpp
+++ b/src/ArduinoJson/Variant/ConverterImpl.hpp
@@ -303,4 +303,18 @@ inline bool canConvertFromJson(VariantConstRef src, const std::string_view&) {
 
 #endif
 
+template <typename T>
+struct ConverterNeedsWriteableRef {
+ protected:  // <- to avoid GCC's "all member functions in class are private"
+  typedef char Yes[1];
+  typedef char No[2];
+
+  static Yes& probe(T (*f)(VariantRef));
+  static No& probe(T (*f)(VariantConstRef));
+
+ public:
+  static const bool value =
+      sizeof(probe(Converter<T>::fromJson)) == sizeof(Yes);
+};
+
 }  // namespace ARDUINOJSON_NAMESPACE