Well, not just some library. (The 'standard library' should be seen as part of the language.)

True, but there is a huge difference between a standard library feature and a keyword.

Not really... Not in the case of the standard library anyway.

A keyword is a more core part of the language than the standard library. I could re-implement my own vector. I cannot add my own keyword.

No, you cannot/wouldn't reimplement std::vector (not in the sense you are implying).

Compiler writers often extend the language with new keywords.

Can you elaborate on the last point? Do you mean things like macros they use, e.g., concepts lite, or __builtin* functions?

I still feel like a standard container is less fundamental than keywords, but I value your perspective.

Yes, built-ins, intrinsics, and extensions.

My original point was simply that std::vector, say, is just as good a "keyword" as any - perhaps, not because you cannot redefine it but because you wouldn't, and if you didn't like what it does, you'd use/implement something else.