Improve error messages when using char or char*

See #2043
Ported from 650d537b5d

CHANGELOG.md

@@ -1,6 +1,11 @@
 ArduinoJson: change log
 =======================
 
+HEAD
+----
+
+* Improve error messages when using `char` or `char*` (issue #2043)
+
 v6.21.5 (2024-01-10)
 -------
 

extras/tests/JsonVariant/converters.cpp

@@ -154,15 +154,18 @@ TEST_CASE("ConverterNeedsWriteableRef") {
 }
 
 namespace ArduinoJson {
-void convertToJson(char c, JsonVariant var) {
-  char buf[] = {c, 0};
-  var.set(buf);
-}
+template <>
+struct Converter<char> {
+  static void toJson(char c, JsonVariant var) {
+    char buf[] = {c, 0};
+    var.set(buf);
+  }
 
-void convertFromJson(JsonVariantConst src, char& dst) {
-  auto p = src.as<const char*>();
-  dst = p ? p[0] : 0;
-}
+  static char fromJson(JsonVariantConst src) {
+    auto p = src.as<const char*>();
+    return p ? p[0] : 0;
+  }
+};
 }  // namespace ArduinoJson
 
 TEST_CASE("Convert char to string") {  // issue #1922

src/ArduinoJson/Variant/ConverterImpl.hpp

@@ -20,6 +20,10 @@ 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/v6/unsupported-set/
@@ -27,6 +31,9 @@ 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/v6/non-default-constructible/
     convertFromJson(src, result);  // Error here? See https://arduinojson.org/v6/unsupported-as/
@@ -35,6 +42,9 @@ 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/v6/unsupported-is/

src/ArduinoJson/Variant/VariantRefBase.hpp

@@ -95,11 +95,9 @@ class VariantRefBase : public VariantTag {
   // Returns true if the value is of the specified type.
   // https://arduinojson.org/v6/api/jsonvariant/is/
   template <typename T>
-  FORCE_INLINE typename enable_if<!ConverterNeedsWriteableRef<T>::value &&
-                                      !is_same<T, char*>::value &&
-                                      !is_same<T, char>::value,
-                                  bool>::type
-  is() const {
+  FORCE_INLINE
+      typename enable_if<!ConverterNeedsWriteableRef<T>::value, bool>::type
+      is() const {
     return Converter<T>::checkJson(getVariantConst());
   }