Hi Ram,
I’m afraid I don’t have a good answer regarding the vanilla version of Unreal. I would imagine that all of EA’s project do use the engine sources but I understand this can be problematic to make sure that all users of the plugin will mod the engine sources. I had a discussion with the owner of UBT yesterday and he is fine with adding an option that turn support for char8_t off through the TargetRules in a future release of the engine.
Based on my local experiment, the compiler is really picky regarding the 2 arguments and their placement on the command line. It does work fine when I hack VCToolChain.cs. Using the Additional Arguments fails as I’m assuming that the compiler is ignoring the 2nd instance of /std:c++20. The future TargetRules option will fix this.
Martin