C++20 isn't a standard yet. You can chose to enable the latest supported features from the working draft in Visual Studio, however this is obviously neither complete nor guaranteed to be in any way stable:
Right click your project -> Properties -> General -> C++ Language Standard: Select "Preview - Features from the Latest C++ Working Draft (std:c++latest)"
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
