The language used to implement the compiler and runtime system for a new language has no impact on what is possible in the new language. If the implementation language does not natively support something you want in the new language, you can just implement that yourself, and it is then available to the new language.

For example, C does not natively support garbage collection, but Python certainly does. The most popular Python implementation is implemented in C.

Thanks for pointing that out. I'm not a language or compiler engineer so my sight wasn't big enough. I tend to think of a new language as a gigantic library sitting on top of another language, and just assumed that if the base language can't handle something like a persistent data structure, then the new language would struggle with that as well.

Only a very small handful of languages are actually implemented like this.

Its worth also pointing out that some languages that compile to C, still support features that C does not (for example, Chicken Scheme compiles to C and supports both garbage collection and continuations, neither of which C supports natively).

> for example, Chicken Scheme compiles to C and supports both garbage collection and continuations, neither of which C supports natively

And GHC still allows compiling Haskell (lazy, functional, garbage-collected) via C (eager, procedural, manually managed).

That is almost never an accurate way to think about it.

> I tend to think of a new language as a gigantic library sitting on top of another language

That's a bad default assumption

> it is about what is possible in the language based on how it is implemented (which is C++)

What is possible to express in a language is determined by its specification, not its implementation. Of course, it's up to the implementor to correctly adhere to the spec, but this is also independent of language choice. It's certainly possible, and has been done, to implement a language with GC, TCO, lazy evaluation, purity, etc etc, in a language like C++ which has none of those things. For example, the nix language, which has all of those things except TCO (which it certainly could have in theory) is implemented in C++.