Here's a minimal example that doesn't have any "shouldn't-this-be-a-tuple" aspect to it:
declare const arr: number[];
function foo(a: number, b: number) {/* ... */ }
foo(...arr); // expected 2, got 0 or more
foo(1, ...arr); // expected 2, got 1 or more
foo(1, 2, ...arr) // expected 2, got *3* or more ?!
foo(1, 2, 3, ...arr) // expected 2, got *4* or more ?!
Those are all errors; the in first two cases the compiler must error because the function may not be called with enough arguments.
In the last two cases one could argue that there should be no error because it's usually safe to call a function with more parameters than it needs (see the TypeScript FAQ for this principle being applied to assignability of function types), but the compiler produces an error here anyway.
You're not asking "why is there an error", though. You're asking "why is there that error"? After all, by rights the issue is that foo(1, 2, ...arr) is passing 2 or more arguments and the compiler is enforcing exactly 2. So why does the compiler say you're passing "3 or more" and not "2 or more"?
This seems to be an intentional decision, according to microsoft/TypeScript#20071, the pull request that introduced errors for excess spread arguments:
This PR also changes the error for excess spread arguments to be more understandable:
"Expected 3 arguments, but got least 4".
Previously the error would have been "Expected 3 arguments, but got at least 3", which is, again, technically correct, but not understandable.
I suppose that users seeing "expected 2, but got at least 2" would be confused, since "at least 2" certainly seems compatible with "2". Instead they decided to write out a message that would definitely be an error condition, if not an accurate one. That's the official answer.
You can see in these lines that, given an argument list like (a1, a2, a3, ...spreadArg) the spread argument is counted if you have enough arguments without it; otherwise it's not. So if it expects 2 arguments, then (1, ...arr) is counted as 1 argument, but (1, 2, ...arr) is counted as 3.
Frankly after researching this I'm not too happy about it, since I can imagine error messages that are correct while not being too unclear (in my opinion):
foo(...arr); // expected exactly 2 arguments, but maybe got too few or too many
foo(1, ...arr); // expected exactly 2 arguments, but maybe got too few or too many
foo(1, 2, ...arr); // expected exactly 2 arguments, but maybe got too many
foo(1, 2, 3, ...arr); // expected exactly 2 arguments, but got too many
This has been reported before, in microsoft/TypeScript#20097 (closed as "working as intended") and in microsoft/TypeScript#20372 (open as "help wanted"), so if you care a lot about it, you (or anyone reading this) could give one of those a 👍 or even submit a pull request fixing it.
Okay, hope that helps; good luck!
Playground link fwiw
