std::string no longer available for 2.x (regression from 1.x)

[easytarget]

In the IDE 1.0.6 the following code will compile and run properly

#include "Arduino.h"

String someText = "This is my example";

void setup() {
    Serial.begin(115200);
    Serial.println("Started");
}

void loop() {
    String match = "NO";
    // put your main code here, to run repeatedly:
    if (someText.indexOf(match) == std::string::npos) {
        Serial.println("No match");
    } else { 
        Serial.println("Match");
    }
    delay(1000);
}

However, with 2.0.0-alpha1 compilation fails with:

/home/owen/Arduino/std.string.npos.reproduction/std.string.npos.reproduction.ino: In function 'void loop()':
std.string.npos.reproduction:14:41: error: 'std::string' has not been declared
     if (someText.indexOf(match) == std::string::npos) {
                                         ^~~~~~
exit status 1
'std::string' has not been declared

So; starting from 2.0.0 the esp32 toolchain no longer supports the std::string class. This is actually in line with other boards; for instance the esp8266 and offiicial Arduino boards fail to compile this code giving the same error.

However, while this brings the ESP32 code into line with other Arduino toolchains, it will break some legacy projects that have taken advantage of std::string being present.

I'm submitting this issue not because I think this needs 'fixing' (I'll modify my code) but because I forsee quite a few older esp32 projects that will be broken by this, and having a placeholder for the issues will be useful. I could not see any existing discussion of this here or elsewhere.

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