Type inference doesn't work with union types of tuples

[falsandtru]

separated from #22432

TypeScript Version: 2.7.0-dev.20180307

Search Terms:

Code

type A<R> = [R] | [R, string];
declare function f<T>(f: A<T>): void;
f([undefined, ''] as [never, string]); // T: string
f([undefined, ''] as [void, string]); // T: string | void

Expected behavior:

f([undefined, ''] as [never, string]); // T: never
f([undefined, ''] as [void, string]); // T: void

Actual behavior:

f([undefined, ''] as [never, string]); // T: string
f([undefined, ''] as [void, string]); // T: string | void

Playground Link:

Related Issues:

[ghost]

Worth noting that the following compiles:

f<number>([undefined, ''] as [never, string]);

because never is assignable to number.