Disable String reader when Stream is enabled and String derives from Stream

[d-a-v]

This is related to PR esp8266/Arduino#6979 (specifically this).
It is opened for discussion.

This would solve this error:

sketch/StringExample.ino.cpp:
In file included from ArduinoJson/src/ArduinoJson/Json/JsonDeserializer.hpp:7,
                 from ArduinoJson/src/ArduinoJson.hpp:34,
                 from ArduinoJson/src/ArduinoJson.h:9,
                 from ArduinoJson/examples/StringExample/StringExample.ino:13:
ArduinoJson/src/ArduinoJson/Deserialization/deserialize.hpp: In instantiation of 'ArduinoJson6161_11::DeserializationError ArduinoJson6161_11::deserialize(ArduinoJson6161_11::JsonDocument&, TStream&, ArduinoJson6161_11::NestingLimit, TFilter) [with TDeserializer = ArduinoJson6161_11::JsonDeserializer; TStream = String; TFilter = ArduinoJson6161_11::AllowAllFilter]':
ArduinoJson/src/ArduinoJson/Json/JsonDeserializer.hpp:676:39:   required from 'ArduinoJson6161_11::DeserializationError ArduinoJson6161_11::deserializeJson(ArduinoJson6161_11::JsonDocument&, TInput&, ArduinoJson6161_11::NestingLimit) [with TInput = String]'
ArduinoJson/examples/StringExample/StringExample.ino:22:29:   required from here
ArduinoJson/src/ArduinoJson/Deserialization/deserialize.hpp:61:19: error: ambiguous template instantiation for 'struct ArduinoJson6161_11::Reader<String, void>'
   61 |   Reader<TStream> reader(input);
      |                   ^~~~~~
In file included from ArduinoJson/src/ArduinoJson/Deserialization/Reader.hpp:43,
                 from ArduinoJson/src/ArduinoJson/Deserialization/deserialize.hpp:10,
                 from ArduinoJson/src/ArduinoJson/Json/JsonDeserializer.hpp:7,
                 from ArduinoJson/src/ArduinoJson.hpp:34,
                 from ArduinoJson/src/ArduinoJson.h:9,
                 from ArduinoJson/examples/StringExample/StringExample.ino:13:
ArduinoJson/src/ArduinoJson/Deserialization/Readers/ArduinoStreamReader.hpp:12:8: note: candidates are: 'template<class TSource> struct ArduinoJson6161_11::Reader<TSource, typename ArduinoJson6161_11::enable_if<ArduinoJson6161_11::is_base_of<Stream, TSource>::value>::type> [with TSource = String]'
   12 | struct Reader<TSource,
      |        ^~~~~~~~~~~~~~~
   13 |               typename enable_if<is_base_of<Stream, TSource>::value>::type> {
      |               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from ArduinoJson/src/ArduinoJson/Deserialization/Reader.hpp:47,
                 from ArduinoJson/src/ArduinoJson/Deserialization/deserialize.hpp:10,
                 from ArduinoJson/src/ArduinoJson/Json/JsonDeserializer.hpp:7,
                 from ArduinoJson/src/ArduinoJson.hpp:34,
                 from ArduinoJson/src/ArduinoJson.h:9,
                 from ArduinoJson/examples/StringExample/StringExample.ino:13:
ArduinoJson/src/ArduinoJson/Deserialization/Readers/ArduinoStringReader.hpp:10:8: note:                 'template<class TSource> struct ArduinoJson6161_11::Reader<TSource, typename ArduinoJson6161_11::enable_if<ArduinoJson6161_11::is_base_of<String, TSource>::value>::type> [with TSource = String]'
   10 | struct Reader<TSource,
      |        ^~~~~~~~~~~~~~~
   11 |               typename enable_if<is_base_of< ::String, TSource>::value>::type>
      |               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from ArduinoJson/src/ArduinoJson/Json/JsonDeserializer.hpp:7,
                 from ArduinoJson/src/ArduinoJson.hpp:34,
                 from ArduinoJson/src/ArduinoJson.h:9,
                 from ArduinoJson/examples/StringExample/StringExample.ino:13:
ArduinoJson/src/ArduinoJson/Deserialization/deserialize.hpp:61:19: error: 'ArduinoJson6161_11::Reader<String, void> reader' has incomplete type
   61 |   Reader<TStream> reader(input);
      |                   ^~~~~~
Using library ArduinoJson at version 6.16.1 in folder: ArduinoJson 

[bblanchon]

You mean String derives from Stream 😮
What the hell is this?!

[coveralls]

Coverage remained the same at 99.289% when pulling a70411f on d-a-v:streamTo6 into 61a5273 on bblanchon:6.x.

[d-a-v]

What the hell is this?!

Only a proposal on esp8266 Arduino for now.

Is that really such a hell ?
Wouldn't a String be a Stream from which we could read, or to which we could write ?
That'd make Strings usable on any virtual (the arduino way) function designed for Streams, without needing to overload, or copy to intermediate buffer.

[bblanchon]

You don't seem to realize that the ESP8266 Core was copied from the AVR core, and there are dozens of other duplicates out there.
If you change the interface like this, you're going to break a lot of code.

If you want to use a String as a Stream, simply use an adapter like StringStream.