Replace ConverterNeedsWriteableRef with function_traits

extras/tests/JsonVariant/converters.cpp

@@ -140,15 +140,3 @@ TEST_CASE("Custom converter with specialization") {
     REQUIRE(doc["value"]["imag"] == 3);
   }
 }
-
-TEST_CASE("ConverterNeedsWriteableRef") {
-  using namespace ArduinoJson::detail;
-  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);
-}

extras/tests/JsonVariantConst/as.cpp

@@ -17,3 +17,24 @@ TEST_CASE("JsonVariantConst::as<T>()") {
   REQUIRE(var.as<const char*>() == std::string("hello"));
   REQUIRE(var.as<std::string>() == std::string("hello"));
 }
+
+TEST_CASE("Invalid conversions") {
+  using namespace ArduinoJson::detail;
+
+  JsonVariantConst variant;
+
+  CHECK(is_same<decltype(variant.as<int>()), int>::value);
+  CHECK(is_same<decltype(variant.as<float>()), float>::value);
+  CHECK(is_same<decltype(variant.as<JsonVariantConst>()),
+                JsonVariantConst>::value);
+  CHECK(
+      is_same<decltype(variant.as<JsonObjectConst>()), JsonObjectConst>::value);
+  CHECK(is_same<decltype(variant.as<JsonArrayConst>()), JsonArrayConst>::value);
+
+  CHECK(is_same<decltype(variant.as<JsonVariant>()),
+                InvalidConversion<JsonVariantConst, JsonVariant>>::value);
+  CHECK(is_same<decltype(variant.as<JsonObject>()),
+                InvalidConversion<JsonVariantConst, JsonObject>>::value);
+  CHECK(is_same<decltype(variant.as<JsonArray>()),
+                InvalidConversion<JsonVariantConst, JsonArray>>::value);
+}

extras/tests/Misc/conflicts.cpp

@@ -58,5 +58,10 @@
 // issue #1914
 #define V7 7
 
+// STM32, Mbed, Particle
+#define A0 16
+#define A1 17
+#define A2 18
+
 // catch.hpp mutes several warnings, this file also allows to detect them
 #include "ArduinoJson.h"

src/ArduinoJson/Polyfills/type_traits.hpp

@@ -6,6 +6,7 @@
 
 #include "type_traits/conditional.hpp"
 #include "type_traits/enable_if.hpp"
+#include "type_traits/function_traits.hpp"
 #include "type_traits/integral_constant.hpp"
 #include "type_traits/is_array.hpp"
 #include "type_traits/is_base_of.hpp"

src/ArduinoJson/Polyfills/type_traits/function_traits.hpp

@@ -0,0 +1,27 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright © 2014-2024, Benoit BLANCHON
+// MIT License
+
+#pragma once
+
+#include <ArduinoJson/Namespace.hpp>
+
+ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
+
+template <typename Sig>
+struct function_traits;
+
+template <typename ReturnType, typename Arg1>
+struct function_traits<ReturnType (*)(Arg1)> {
+  using return_type = ReturnType;
+  using arg1_type = Arg1;
+};
+
+template <typename ReturnType, typename Arg1, typename Arg2>
+struct function_traits<ReturnType (*)(Arg1, Arg2)> {
+  using return_type = ReturnType;
+  using arg1_type = Arg1;
+  using arg2_type = Arg2;
+};
+
+ARDUINOJSON_END_PRIVATE_NAMESPACE

src/ArduinoJson/Variant/Converter.hpp

@@ -20,7 +20,4 @@ template <typename T1, typename T2>
 class InvalidConversion;  // Error here? See https://arduinojson.org/v7/invalid-conversion/
 // clang-format on
 
-template <typename T>
-struct ConverterNeedsWriteableRef;
-
 ARDUINOJSON_END_PRIVATE_NAMESPACE

src/ArduinoJson/Variant/ConverterImpl.hpp

@@ -305,19 +305,6 @@ inline bool canConvertFromJson(JsonVariantConst src, const std::string_view&) {
 
 #endif
 
-namespace detail {
-template <typename T>
-struct ConverterNeedsWriteableRef {
- protected:  // <- to avoid GCC's "all member functions in class are private"
-  static int probe(T (*f)(ArduinoJson::JsonVariant));
-  static char probe(T (*f)(ArduinoJson::JsonVariantConst));
-
- public:
-  static const bool value =
-      sizeof(probe(Converter<T>::fromJson)) == sizeof(int);
-};
-}  // namespace detail
-
 template <>
 struct Converter<JsonArrayConst> : private detail::VariantAttorney {
   static void toJson(JsonArrayConst src, JsonVariant dst) {
@@ -354,13 +341,6 @@ struct Converter<JsonArray> : private detail::VariantAttorney {
     return JsonArray(data != 0 ? data->asArray() : 0, resources);
   }
 
-  static detail::InvalidConversion<JsonVariantConst, JsonArray> fromJson(
-      JsonVariantConst);
-
-  static bool checkJson(JsonVariantConst) {
-    return false;
-  }
-
   static bool checkJson(JsonVariant src) {
     auto data = getData(src);
     return data && data->isArray();
@@ -403,13 +383,6 @@ struct Converter<JsonObject> : private detail::VariantAttorney {
     return JsonObject(data != 0 ? data->asObject() : 0, resources);
   }
 
-  static detail::InvalidConversion<JsonVariantConst, JsonObject> fromJson(
-      JsonVariantConst);
-
-  static bool checkJson(JsonVariantConst) {
-    return false;
-  }
-
   static bool checkJson(JsonVariant src) {
     auto data = getData(src);
     return data && data->isObject();

src/ArduinoJson/Variant/JsonVariant.hpp

@@ -53,17 +53,10 @@ struct Converter<JsonVariant> : private detail::VariantAttorney {
     return src;
   }
 
-  static detail::InvalidConversion<JsonVariantConst, JsonVariant> fromJson(
-      JsonVariantConst);
-
   static bool checkJson(JsonVariant src) {
     auto data = getData(src);
     return !!data;
   }
-
-  static bool checkJson(JsonVariantConst) {
-    return false;
-  }
 };
 
 template <>

src/ArduinoJson/Variant/JsonVariantConst.hpp

@@ -27,6 +27,12 @@ class JsonVariantConst : public detail::VariantTag,
                          public detail::VariantOperators<JsonVariantConst> {
   friend class detail::VariantAttorney;
 
+  template <typename T>
+  using ConversionSupported =
+      detail::is_same<typename detail::function_traits<
+                          decltype(&Converter<T>::fromJson)>::arg1_type,
+                      JsonVariantConst>;
+
  public:
   // Creates an unbound reference.
   JsonVariantConst() : data_(nullptr), resources_(nullptr) {}
@@ -62,23 +68,35 @@ class JsonVariantConst : public detail::VariantTag,
   // Casts the value to the specified type.
   // https://arduinojson.org/v7/api/jsonvariantconst/as/
   template <typename T>
-  typename detail::enable_if<!detail::is_same<T, char*>::value &&
-                                 !detail::is_same<T, char>::value,
-                             T>::type
-  as() const {
+  typename detail::enable_if<ConversionSupported<T>::value, T>::type as()
+      const {
     return Converter<T>::fromJson(*this);
   }
 
+  // Casts the value to the specified type.
+  // https://arduinojson.org/v7/api/jsonvariantconst/as/
+  template <typename T>
+  typename detail::enable_if<
+      !ConversionSupported<T>::value,
+      detail::InvalidConversion<JsonVariantConst, T>>::type
+  as() const;
+
   // Returns true if the value is of the specified type.
   // https://arduinojson.org/v7/api/jsonvariantconst/is/
   template <typename T>
-  typename detail::enable_if<!detail::is_same<T, char*>::value &&
-                                 !detail::is_same<T, char>::value,
-                             bool>::type
-  is() const {
+  typename detail::enable_if<ConversionSupported<T>::value, bool>::type is()
+      const {
     return Converter<T>::checkJson(*this);
   }
 
+  // Always returns false for the unsupported types.
+  // https://arduinojson.org/v7/api/jsonvariantconst/is/
+  template <typename T>
+  typename detail::enable_if<!ConversionSupported<T>::value, bool>::type is()
+      const {
+    return false;
+  }
+
   template <typename T>
   operator T() const {
     return as<T>();

src/ArduinoJson/Variant/VariantRefBase.hpp

@@ -46,16 +46,7 @@ class VariantRefBase : public VariantTag {
   // Casts the value to the specified type.
   // https://arduinojson.org/v7/api/jsonvariant/as/
   template <typename T>
-
-  typename enable_if<!ConverterNeedsWriteableRef<T>::value, T>::type as()
-      const {
-    return Converter<T>::fromJson(getVariantConst());
-  }
-
-  // Casts the value to the specified type.
-  // https://arduinojson.org/v7/api/jsonvariant/as/
-  template <typename T>
-  typename enable_if<ConverterNeedsWriteableRef<T>::value, T>::type as() const;
+  T as() const;
 
   template <typename T,
             typename = typename enable_if<!is_same<T, TDerived>::value>::type>
@@ -83,18 +74,7 @@ class VariantRefBase : public VariantTag {
   // Returns true if the value is of the specified type.
   // https://arduinojson.org/v7/api/jsonvariant/is/
   template <typename T>
-  FORCE_INLINE
-      typename enable_if<ConverterNeedsWriteableRef<T>::value, bool>::type
-      is() const;
-
-  // Returns true if the value is of the specified type.
-  // https://arduinojson.org/v7/api/jsonvariant/is/
-  template <typename T>
-  FORCE_INLINE
-      typename enable_if<!ConverterNeedsWriteableRef<T>::value, bool>::type
-      is() const {
-    return Converter<T>::checkJson(getVariantConst());
-  }
+  FORCE_INLINE bool is() const;
 
   // Copies the specified value.
   // https://arduinojson.org/v7/api/jsonvariant/set/
@@ -298,6 +278,18 @@ class VariantRefBase : public VariantTag {
     return ArduinoJson::JsonVariantConst(getData(), getResourceManager());
   }
 
+  template <typename T>
+  FORCE_INLINE typename enable_if<is_same<T, JsonVariantConst>::value, T>::type
+  getVariant() const {
+    return getVariantConst();
+  }
+
+  template <typename T>
+  FORCE_INLINE typename enable_if<is_same<T, JsonVariant>::value, T>::type
+  getVariant() const {
+    return getVariant();
+  }
+
   ArduinoJson::JsonVariant getOrCreateVariant() const;
 };
 

src/ArduinoJson/Variant/VariantRefBaseImpl.hpp

@@ -17,9 +17,10 @@ inline JsonVariant VariantRefBase<TDerived>::add() const {
 
 template <typename TDerived>
 template <typename T>
-inline typename enable_if<ConverterNeedsWriteableRef<T>::value, T>::type
-VariantRefBase<TDerived>::as() const {
-  return Converter<T>::fromJson(getVariant());
+inline T VariantRefBase<TDerived>::as() const {
+  using variant_type =  // JsonVariantConst or JsonVariant?
+      typename function_traits<decltype(&Converter<T>::fromJson)>::arg1_type;
+  return Converter<T>::fromJson(getVariant<variant_type>());
 }
 
 template <typename TDerived>
@@ -109,9 +110,10 @@ inline JsonVariant VariantRefBase<TDerived>::getOrCreateVariant() const {
 
 template <typename TDerived>
 template <typename T>
-inline typename enable_if<ConverterNeedsWriteableRef<T>::value, bool>::type
-VariantRefBase<TDerived>::is() const {
-  return Converter<T>::checkJson(getVariant());
+inline bool VariantRefBase<TDerived>::is() const {
+  using variant_type =  // JsonVariantConst or JsonVariant?
+      typename function_traits<decltype(&Converter<T>::checkJson)>::arg1_type;
+  return Converter<T>::checkJson(getVariant<variant_type>());
 }
 
 template <typename TDerived>