But the question is if that's actually decisive in the slow compilation problem. The solution in an eager language for evaluating too much stuff is basically "m

But the question is if that's actually decisive in the slow compilation problem. The solution in an eager language for evaluating too much stuff is basically "more if statements". Same thing in C++ metaprogramming, use more "if constexpr". If that's all it took to fix C++ compile times, it would have been done a decade ago. The actual problem is all the stuff that you do actually use that has to get repeatedly inlined and optimized away for zero cost abstraction to work.