Overload signature that uses generics breaks as of 2.7.1

[bjnsn]

TypeScript Version: 2.7.1, 2.7.2, 2.8.0-dev.20180220; issue not present in 2.6.2

Search Terms: overload, signature, generic, tuple

Code:

function foo<T1, T2>(values: [T1, T2]): [T1, T2];
function foo<T1>(values: [T1]): [T1];
function foo(values) {
	return values;
}

function fooWrapper<T1, T2>(values: [T1, T2]): [T1, T2];
/** [ts] Overload signature is not compatible with function implementation. **/
function fooWrapper<T1>(values: [T1]): [T1];
function fooWrapper(values) {
	return foo(values);
}

Expected behavior:
There should not be any type error; the wrapping function should behave identically to the wrapped function.

Actual behavior:
Compile time error on the overload signature of the wrapping function.

Playground Link: http://www.typescriptlang.org/play/#src=function%20foo%3CT1%2C%20T2%3E(values%3A%20%5BT1%2C%20T2%5D)%3A%20%5BT1%2C%20T2%5D%3B%0D%0Afunction%20foo%3CT1%3E(values%3A%20%5BT1%5D)%3A%20%5BT1%5D%3B%0D%0Afunction%20foo(values)%20%7B%0D%0A%09return%20values%3B%0D%0A%7D%0D%0A%0D%0Afunction%20fooWrapper%3CT1%2C%20T2%3E(values%3A%20%5BT1%2C%20T2%5D)%3A%20%5BT1%2C%20T2%5D%3B%0D%0A%2F**%20%5Bts%5D%20Overload%20signature%20is%20not%20compatible%20with%20function%20implementation.%20**%2F%0D%0Afunction%20fooWrapper%3CT1%3E(values%3A%20%5BT1%5D)%3A%20%5BT1%5D%3B%0D%0Afunction%20fooWrapper(values)%20%7B%0D%0A%09return%20foo(values)%3B%0D%0A%7D

Related Issues: None known

Notes: Seems potentially related to tuples fixed length change in 2.7

[DanielRosenwasser]

Seems potentially related to tuples fixed length change in 2.7

I suspect you're right about that. You can get around this by explicitly saying that fooWrapper returns an any[].

[bjnsn]

@DanielRosenwasser - yes:

function fooWrapper(values) {
	return foo(values) as any[];
}
const result = fooWrapper([1,2]); // [number, number]

or even

function fooWrapper(values) {
	return foo(values) as any;
}
const result = fooWrapper([1,2]); // [number, number]

solves the issue while retaining typings. I added this because it broke our CI and figured it might do the same for someone else later.

[mhegazy]

ideally, you would give values a type annotation to be [any] | [any, any], and expect the type of foo(values) to be [any]|[any, any]. The issue here is that overload resolution does not distribute over union properties, so that is not possible today. this is tracked by #14107

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.