Clingis a C++ interpreter built on top of Clang/LLVM technology stack by Cern (the Swiss Physics lab). There is a long video of a talk about it given at Google.
Would it be possible to embed something like this in the Unreal Editor, combined with a simple text editor and project manager? Clang already has code completion available through it’s API so it should be possible to integrate basic C++ development functionality into the Unreal Editor.