One problem with this, is that your code often ends up in a state where only step-through debugging works anymore. It might become too complex to reason about j

One problem with this, is that your code often ends up in a state where only step-through debugging works anymore. It might become too complex to reason about just looking at the code, from the types alone, or by printing data. Same problem happens with other methods too: If you develop solely with unit and integration tests, it might actually be quite difficult to get a step-through debugger set up to debug your application. I worked at a company where I was the only one who used a step-through debugger, and some uses of compile-time metaprogramming would frequently break the debugger. And if you use multiple techniques as appropriate(unit tests, printf or equivalent, step-through debugging), it's generally easy to use any of them as appropriate.