error: 'String' does not name a type. (base64.h)

[FlorSanders]

Basic Infos

• This issue complies with the issue POLICY doc.
• I have read the documentation at readthedocs and the issue is not addressed there.
• I have tested that the issue is present in current master branch (aka latest git).
• I have searched the issue tracker for a similar issue.
• If there is a stack dump, I have decoded it.
• I have filled out all fields below.

Platform

• Hardware: ESP-12
• Core Version: 2.7.1
• Development Env: Arduino IDE
• Operating System: Windows

Settings in IDE

• Module: NodeMCU 1.0
• Flash Mode: ?
• Flash Size: 4MB
• lwip Variant: v2 Lower Memory
• Reset Method: nodemcu
• Flash Frequency: 40MHz
• CPU Frequency: 80MHz
• Upload Using: SERIAL
• Upload Speed: 115200

Problem Description

Compiler error stemming from the \cores\esp8266\base64.h.
Can be fixed by adding #include <WString.h> in base64.h though I suppose that's not the prefered fix.
Didn't find any other mention of this issue at first sight, so I thought I'd mention it.

The issue occurs when working with the KPN_SenML library, which has this Base64 library as a dependency, and occurs after applying the first step of the workaround mentioned by SimonKlausLudwig.

MCVE Sketch

#include <kpn_senml.h>

void setup(){
}

void loop(){
}

Debug Messages

In file included from C:\Users\flors\Documents\Arduino\libraries\arduino-base64-master\MyBase64.cpp:1:0:

C:\Users\flors\Documents\ArduinoData\packages\esp8266\hardware\esp8266\2.7.1\cores\esp8266/Base64.h:36:12: error: 'String' does not name a type

static String encode(const uint8_t * data, size_t length, bool doNewLines = true);

C:\Users\flors\Documents\ArduinoData\packages\esp8266\hardware\esp8266\2.7.1\cores\esp8266/Base64.h:37:12: error: 'String' does not name a type

static String inline encode(const String& text, bool doNewLines = true)

exit status 1

[d-a-v]

Can be fixed by adding #include <WString.h> in base64.h though I suppose that's not the prefered fix.

Because String is used in this file, I think it is perfectly legal and harmless to include its declaration.
the #ifndef/#define/#endif triplets in every .h files are there to allow it.