> What is this mythical subset of C++

Ladybird inherits its C++ from SerenityOS. Ladybird has an almost completely homegrown standard library including their own pointer classes and a couple of different string classes that do some interesting things with memory. But perhaps the most novel stuff are things like TRY and MUST: https://github.com/SerenityOS/serenity/blob/master/Documenta...

You see this reflected all the way back to the main function. Here is the main entry function for the entire browser:

ErrorOr<int> ladybird_main(Main::Arguments arguments).

https://github.com/LadybirdBrowser/ladybird/blob/master/UI/Q...

If Ladybird is successful, I would not be surprised to see its standard library take off with other projects. Again, it is really the SerenityOS standard library but the SerenityOS founder left the project to focus on Ladybird. So, that is where this stuff evolves now.