> Simple example: a function has a parameter whose type is "variable-length tuple of int". You can pass any tuple in that is known to have 0..n elements, all of type int.

And n is fixed at the calling site, right? I wonder if something like "TypeVar, but for a list of type arguments" could solve your problem.

What's funny is that this is already kind of implemented in `typing.Concatenate`, but only for function parameters [1], not for type hint parameters.

Anyway, I would have written "a well-typed function that concatenates two arbitrary tuples whose size is statically known at the call site". Can't really remove "at the call site" or "statically known" without being ambiguous.

Edit: just found out about `TypeVarTuple`. So really we're only missing `concatenate`.

[1] https://docs.python.org/3/library/typing.html#typing.Concate...