TypeScript Version: 3.1.0-dev.20180821

Search Terms: function argument infer overload return type rest tuple

Code

function foo (a: string): string;
function foo (a: number, b: string): boolean;
function foo (a: boolean, ...b: object[]): object;
function foo (...args: any[]): any {
}

type A = ReturnType<typeof foo>;

Expected behavior:

Type A is a union of the return types of every overload, or an error along the lines of "cannot infer things about an overloaded function" is generated.

Actual behavior:

Type A is object.

This also happens when inferring function params:

type Parameters<T extends (...args: any[]) => any> = T extends (...args: infer U) => any ? U : never;

type B = Parameters<typeof foo>; // [boolean, ...object[]]

Playground Link:

http://www.typescriptlang.org/play/#src=function%20foo%20(a%3A%20string)%3A%20string%3B%0D%0Afunction%20foo%20(a%3A%20number%2C%20b%3A%20string)%3A%20boolean%3B%0D%0Afunction%20foo%20(a%3A%20boolean%2C%20...args%3A%20object%5B%5D)%3A%20object%3B%0D%0Afunction%20foo%20(...args%3A%20any%5B%5D)%3A%20any%20%7B%0D%0A%7D%0D%0A%0D%0Atype%20A%20%3D%20ReturnType%3Ctypeof%20foo%3E%3B%0D%0A%0D%0Atype%20Parameters%3CT%20extends%20(...args%3A%20any%5B%5D)%20%3D%3E%20any%3E%20%3D%20T%20extends%20(...args%3A%20infer%20U)%20%3D%3E%20any%20%3F%20U%20%3A%20never%3B%0D%0A%0D%0Atype%20B%20%3D%20Parameters%3Ctypeof%20foo%3E%3B%20%2F%2F%20%5Bboolean%2C%20...object%5B%5D%5D