When I write template heavy code I always miss having a REPL that deals with compile time interpretation of the template programming subsystem.

Yes one can pore over the compiler error messages to figure out what went wrong, why did it instantiate that template instead of this one, why did this overload fail, why did it not pick up this template specialization..... This is hard to do, and although error messages in g++ has improved enormously (and that I find its template related error messages more informative than Clang ones) it handsomely crosses my threshold of pain.

It would be so much better if the compiler dropped you in a REPL environment where you could explore the instantiation and overloading aspects interactively, fix the problems and then let the compiler continue from there.

I think some of this exists in bits and pieces in several personal projects, but I really wish this is available in the standard compilers.

You should check out metashell then: https://github.com/sabel83/metashell

Yes! I have been meaning to try it out, thanks for reminding.

...and then I discovered Linq https://github.com/pfultz2/Linq from your comment history. Expect movement in your karma from old posts :)