This looks amazing, but the fact that it is implemented in an entirely new compiler (atop LLVM, sure) is a major turn-off. Would be more useful if it were implemented as a meta compiler instead.
There is basically no way to modify clang or GCC to support executing arbitrary C++ code at compile time. Something like that would be a fork that wouldn’t get accepted upstream. According to the author the compiler front end is ~100kLoc which is an order of magnitude less than what clang is.