OMG THANK YOU!
I’m also running the engine built from source and I’m on 4.21.2, which by all accounts should be free of this infamous 4.20 bug. I could not figure out why I was still getting intellisense errors on my includes for “generated.h” files as well as certain Unreal Macros and Super calls on Pawn classes. I had tried EVERYTHING.
Setting BuildEnvironment = TargetBuildEnvironment.Shared in the MyGame*.Target.cs files SOLVED IT!!!
As pointed out by FeiSuZZ further down this page, it’s a bug with the “source version”. By now the code in UEBuildTarget.cs that determines the default BuildEnvironment (e.g. what will be set if you DON’T make the above fix) has actually been moved to RulesAssembly.cs. But as it is the result is the 4.20 include/generation bug persists even in 4.21.x.