Fair enough, but that addition wasn't to solve the problem of templates being too complex, but to support usages that were previously impossible.

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.

Wasn't it also at the cost of compilation memory? I still remember the time when it was easy to crash compilers by just giving them hard enough templates.

I mean, that's just the tip of the iceberg. Templates have evolved a great deal over time, and continue to do so (C++20 added Concepts).

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.

They added a whole new keyword just to support it. Just because C++ templates are Turing complete doesn't mean new features don't count as new features.