Rest tuple parameter with intersection/union places error on wrong argument

[jcalz]

TypeScript Version: 3.2.0-dev.20181113

Search Terms:
rest tuple, intersection, union, error, argument, misplaced

Code

declare function boop<T extends any[]>(...args: T & [number, string]): void;
boop(123, 456); // error
//   ~~~ <-- error on first argument
// Argument of type '[123, 456]' is not assignable to parameter 
//   of type '[number, number] & [number, string]'.
// Type '[123, 456]' is not assignable to type '[number, string]'.
// Type '456' is not assignable to type 'string'.

Expected behavior:
I expect an error on the second argument (456), since that's the argument that causes the failure, as made evident by the error message. (Or possibly I expect an error on both arguments, if it's interpreted as a failure to match the rest parameter.)

Actual behavior:
The actual error is on the first argument (123). This is a minor problem, and it only seems to show up in the intersection-with-type-parameter situation above (concrete types like ...args: [number, string] & [number, string] work fine), but I figured I'd report it in case it has an easy fix.

(Note to self or interested non-selves: ran into this in a Stack Overflow answer)

Playground Link:
🔗

Related Issues:
Haven't found anything. Anyone find anything?

[jack-williams]

Think the same happens with unions too:

declare function boop<T extends [number, string]>(...args: T | [number, boolean]): void;
boop(123, 456); // error
//   ~~~ <-- error on first argument
// Argument of type '[123, 456]' is not assignable to parameter 
//   of type '[number, string] & [number, boolean]'.
// Type '[123, 456]' is not assignable to type '[number, string]'.
// Type '456' is not assignable to type 'string'.

[jcalz]

I see generics aren't even needed to reproduce this:

declare function f(...args: [x: string, y: number] | []): void;
f("a", "b"); // error
//~~~ <--- error here due to a problem with the *second* argument 😕

Playground link