Added a deprecation warning for as<char*>()

CHANGELOG.md

@@ -61,6 +61,7 @@ HEAD
 > Serial.println(doc["sensor"].as<const char*>());  // OK
 > ```
 >
+> A deprecation warning with the message "Replace `as<char*>()` with `as<const char*>()`" was added to allow a smooth transition.
 >
 > #### `DeserializationError::NotSupported` removed
 >

extras/tests/Misc/CMakeLists.txt

@@ -5,6 +5,7 @@
 add_executable(MiscTests
 	arithmeticCompare.cpp
 	conflicts.cpp
+	deprecated.cpp
 	FloatParts.cpp
 	JsonString.cpp
 	printable.cpp

extras/tests/Misc/deprecated.cpp

@@ -0,0 +1,39 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright Benoit Blanchon 2014-2021
+// MIT License
+
+#include <ArduinoJson.h>
+#include <catch.hpp>
+
+#if defined(__GNUC__)
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+#endif
+
+#ifdef _MSC_VER
+#pragma warning(disable : 4996)  // deprecation warning
+#endif
+
+TEST_CASE("Deprecated features") {
+  StaticJsonDocument<256> doc;
+  const char* s = "hello";
+  doc["s"] = s;
+  doc["a"].add(s);
+
+  SECTION("JsonVariant::as<char*>()") {
+    JsonVariant v = doc["s"];
+    REQUIRE(v.as<char*>() == s);
+  }
+
+  SECTION("JsonVariantConst::as<char*>()") {
+    JsonVariantConst v = doc["s"];
+    REQUIRE(v.as<char*>() == s);
+  }
+
+  SECTION("MemberProxy::as<char*>()") {
+    REQUIRE(doc["s"].as<char*>() == s);
+  }
+
+  SECTION("ElementProxy::as<char*>()") {
+    REQUIRE(doc["a"][0].as<char*>() == s);
+  }
+}

src/ArduinoJson/Array/ElementProxy.hpp

@@ -65,10 +65,17 @@ class ElementProxy : public VariantOperators<ElementProxy<TArray> >,
   }
 
   template <typename T>
-  FORCE_INLINE T as() const {
+  FORCE_INLINE typename enable_if<!is_same<T, char*>::value, T>::type as()
+      const {
     return getUpstreamElement().template as<T>();
   }
 
+  template <typename T>
+  FORCE_INLINE typename enable_if<is_same<T, char*>::value, const char*>::type
+  DEPRECATED("Replace as<char*>() with as<const char*>()") as() const {
+    return as<const char*>();
+  }
+
   template <typename T>
   FORCE_INLINE operator T() const {
     return getUpstreamElement();

src/ArduinoJson/Object/MemberProxy.hpp

@@ -46,20 +46,6 @@ class MemberProxy : public VariantOperators<MemberProxy<TObject, TStringRef> >,
   template <typename TValue>
   FORCE_INLINE typename enable_if<!is_array<TValue>::value, this_type &>::type
   operator=(const TValue &src) {
-    /********************************************************************
-     **                THIS IS NOT A BUG IN THE LIBRARY                **
-     **                --------------------------------                **
-     **  Get a compilation error pointing here?                        **
-     **  It doesn't mean the error *is* here.                          **
-     **  Often, it's because you try to assign the wrong value type.   **
-     **                                                                **
-     **  For example:                                                  **
-     **    char age = 42                                               **
-     **    doc["age"] = age;                                           **
-     **  Instead, use:                                                 **
-     **    int8_t age = 42;                                            **
-     **    doc["age"] = age;                                           **
-     ********************************************************************/
     getOrAddUpstreamMember().set(src);
     return *this;
   }
@@ -81,9 +67,16 @@ class MemberProxy : public VariantOperators<MemberProxy<TObject, TStringRef> >,
     return getUpstreamMember().isNull();
   }
 
-  template <typename TValue>
-  FORCE_INLINE TValue as() const {
-    return getUpstreamMember().template as<TValue>();
+  template <typename T>
+  FORCE_INLINE typename enable_if<!is_same<T, char *>::value, T>::type as()
+      const {
+    return getUpstreamMember().template as<T>();
+  }
+
+  template <typename T>
+  FORCE_INLINE typename enable_if<is_same<T, char *>::value, const char *>::type
+  DEPRECATED("Replace as<char*>() with as<const char*>()") as() const {
+    return as<const char *>();
   }
 
   template <typename T>

src/ArduinoJson/Variant/VariantRef.hpp

@@ -94,10 +94,17 @@ class VariantRef : public VariantRefBase<VariantData>,
   }
 
   template <typename T>
-  FORCE_INLINE T as() const {
+  FORCE_INLINE typename enable_if<!is_same<T, char *>::value, T>::type as()
+      const {
     return Converter<T>::fromJson(*this);
   }
 
+  template <typename T>
+  FORCE_INLINE typename enable_if<is_same<T, char *>::value, const char *>::type
+  DEPRECATED("Replace as<char*>() with as<const char*>()") as() const {
+    return as<const char *>();
+  }
+
   template <typename T>
   FORCE_INLINE bool is() const {
     return Converter<T>::checkJson(*this);
@@ -204,10 +211,17 @@ class VariantConstRef : public VariantRefBase<const VariantData>,
   }
 
   template <typename T>
-  FORCE_INLINE T as() const {
+  FORCE_INLINE typename enable_if<!is_same<T, char *>::value, T>::type as()
+      const {
     return Converter<T>::fromJson(*this);
   }
 
+  template <typename T>
+  FORCE_INLINE typename enable_if<is_same<T, char *>::value, const char *>::type
+  DEPRECATED("Replace as<char*>() with as<const char*>()") as() const {
+    return as<const char *>();
+  }
+
   template <typename T>
   FORCE_INLINE bool is() const {
     return Converter<T>::checkJson(*this);