Fixed value returned by serializeXxx() when writing to a String

CHANGELOG.md

@@ -14,6 +14,7 @@ HEAD
 * Added `measureJson`, `measureJsonPretty`, and `measureMsgPack` to `keywords.txt`
   (This file is used for syntax highlighting in the Arduino IDE) 
 * Fixed `variant.is<nullptr_t>()`
+* Fixed value returned by `serializeJson()`, `serializeJsonPretty()`, and `serializeMsgPack()` when writing to a `String`
 
 > ### BREAKING CHANGES
 > 

extras/tests/Helpers/WString.h

@@ -0,0 +1,39 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2020
+// MIT License
+
+#pragma once
+
+#include <string>
+
+// Reproduces Arduino's String class
+class String {
+ public:
+  String& operator+=(char c) {
+    _str += c;
+    return *this;
+  }
+  String& operator+=(int);  // no used, just to add ambiguity
+
+  unsigned char reserve(size_t capacity) {
+    _str.reserve(capacity);
+    return 1;
+  }
+
+  size_t length() const {
+    return _str.size();
+  }
+
+  const char* c_str() const {
+    return _str.c_str();
+  }
+
+ private:
+  std::string _str;
+};
+
+class StringSumHelper;
+
+bool operator==(const std::string& lhs, const ::String& rhs) {
+  return lhs == rhs.c_str();
+}

extras/tests/Misc/StringWriter.cpp

@@ -2,6 +2,7 @@
 // Copyright Benoit Blanchon 2014-2020
 // MIT License
 
+#define ARDUINOJSON_ENABLE_ARDUINO_STRING 1
 #include <ArduinoJson.h>
 #include <catch.hpp>
 #include "custom_string.hpp"
@@ -55,6 +56,12 @@ TEST_CASE("Writer<std::string>") {
   common_tests(sb, output);
 }
 
+TEST_CASE("Writer<String>") {
+  ::String output;
+  Writer< ::String> sb(output);
+  common_tests(sb, output);
+}
+
 TEST_CASE("Writer<custom_string>") {
   custom_string output;
   Writer<custom_string> sb(output);

src/ArduinoJson/Serialization/Writers/ArduinoStringWriter.hpp

@@ -22,9 +22,8 @@ class Writer< ::String, void> {
     // CAUTION: Arduino String doesn't have append()
     // and old version doesn't have size() either
     _str->reserve(_str->length() + n);
-    while (n > 0) {
+    for (size_t i = 0; i < n; i++) {
       _str->operator+=(static_cast<char>(*s++));
-      n--;
     }
     return n;
   }