Improve error messages when using char or char*

See #2043

CHANGELOG.md

@@ -1,6 +1,11 @@
 ArduinoJson: change log
 =======================
 
+HEAD
+----
+
+* Improve error messages when using `char` or `char*` (issue #2043)
+
 v7.0.2 (2024-01-19)
 ------
 

src/ArduinoJson/Variant/ConverterImpl.hpp

@@ -21,6 +21,10 @@ ARDUINOJSON_BEGIN_PUBLIC_NAMESPACE
 
 template <typename T, typename Enable>
 struct Converter {
+  static_assert(!detail::is_same<T, char>::value,
+                "type 'char' is not supported, use 'signed char', 'unsigned "
+                "char' or another integer type instead");
+
   static void toJson(const T& src, JsonVariant dst) {
     // clang-format off
     convertToJson(src, dst); // Error here? See https://arduinojson.org/v7/unsupported-set/
@@ -28,6 +32,9 @@ struct Converter {
   }
 
   static T fromJson(JsonVariantConst src) {
+    static_assert(!detail::is_same<T, char*>::value,
+                  "type 'char*' is not supported, use 'const char*' instead");
+
     // clang-format off
     T result; // Error here? See https://arduinojson.org/v7/non-default-constructible/
     convertFromJson(src, result);  // Error here? See https://arduinojson.org/v7/unsupported-as/
@@ -36,6 +43,9 @@ struct Converter {
   }
 
   static bool checkJson(JsonVariantConst src) {
+    static_assert(!detail::is_same<T, char*>::value,
+                  "type 'char*' is not supported, use 'const char*' instead");
+
     T dummy = T();
     // clang-format off
     return canConvertFromJson(src, dummy);  // Error here? See https://arduinojson.org/v7/unsupported-is/

src/ArduinoJson/Variant/VariantRefBase.hpp

@@ -91,10 +91,8 @@ 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 &&
+  FORCE_INLINE
-                                      !is_same<T, char*>::value &&
+      typename enable_if<!ConverterNeedsWriteableRef<T>::value, bool>::type
-                                      !is_same<T, char>::value,
-                                  bool>::type
       is() const {
     return Converter<T>::checkJson(getVariantConst());
   }