GeezerVison

declare const foo: (x: string, ...args: [string] | [number, boolean]) => void;
declare const t: [string] | [number, boolean];

foo("abc", "def");  // Ok
foo("abc", 10, true);  // Ok
foo("abc", ...t);  // Ok
foo("abc", 10);  // Error, [number] not assignable to [string] | [number, boolean]
foo("abc");  // Error, [] not assignable to [string] | [number, boolean]

type Func<T extends any[]> = (x: string, ...args: T) => void;

declare const f: Func<[] | [string] | [number, ...boolean[]]>;

f("abc");
f("abc", "def");
f("abc", 10);
f("abc", 10, true);
f("abc", 10, true, false);
f();  // Error
f("abc", true);  // Error

declare let f1: (x: string, ...args: [string] | [number, boolean]) => void;
declare let f2: (x: string, y: string) => void;
declare let f3: (x: string, y: number, z: boolean) => void;
declare let f4: (...args: [string, string] | [string, number, boolean]) => void;

f2 = f1;  // Ok
f3 = f1;  // Ok
f4 = f1;  // Error, misaligned complex rest types
f1 = f2;  // Error
f1 = f3;  // Error
f1 = f4;  // Error, misaligned complex rest types