They were possible and widely used, but a lot less ergonomic. You "just" had to copy-paste some number of instantiatons for the number of arguments between 0 and, say, 50.

Constraints and concepts don't increase the scope or feature set of templates, which was GP's broader point. That is, all the additions are around things that are a smaller, but easier to use, subset of the broader feature. `if constexpr` for example just being a way more approachable std::enable_if, not an expansion of the feature set. And concepts then are about clarifying the contract of the template, they don't actually add new capabilities.

It'd be I think reflection, however that ends up looking, that's the next actual feature set expansion of templates.