Mapped tuples types iterates over all properties

TypeScript Version: 3.2.0-dev.20181019

Search Terms: mapped tuples reify

Code

type Foo = ['a', 'b'];
interface Bar
{
	a: string;
	b: number;
}

type Baz = { [K in keyof Foo]: Bar[Foo[K]]; }; // Expected Baz to be [string, number]

Expected behavior: Baz should be [string, number]

Actual behavior: Type '["a", "b"][K]' cannot be used to index type 'Bar'.

Playground Link: https://www.typescriptlang.org/play/index.html#src=type%20Foo%20%3D%20%5B'a'%2C%20'b'%5D%3B%0D%0Ainterface%20Bar%0D%0A%7B%0D%0A%09a%3A%20string%3B%0D%0A%09b%3A%20number%3B%0D%0A%7D%0D%0A%0D%0Atype%20Baz%20%3D%20%7B%20%5BK%20in%20keyof%20Foo%5D%3A%20Bar%5BFoo%5BK%5D%5D%3B%20%7D%3B%20%2F%2F%20Expected%20Baz%20to%20be%20%5Bstring%2C%20number%5D%0D%0A%0D%0Atype%20WorkingBaz%20%3D%20%7B%20%5BK%20in%20Exclude%3Ckeyof%20Foo%2C%20keyof%20any%5B%5D%3E%5D%3A%20Foo%5BK%5D%20extends%20keyof%20Bar%20%3F%20Bar%5BFoo%5BK%5D%5D%20%3A%20never%3B%20%7D%20%26%20%7B%20length%3A%20Foo%5B'length'%5D%3B%20%7D%20%26%20any%5B%5D%3B

Related Issues: https://github.com/Microsoft/TypeScript/issues/25947

Given the Mapped tuple types feature (#25947). I'd expect the code above to work cleanly.

However, I still need to do:

type WorkingBaz = { [K in Exclude<keyof Foo, keyof any[]>]: Foo[K] extends keyof Bar ? Bar[Foo[K]] : never; } & { length: Foo['length']; } & any[];

To have an equivalent type. As far as I understand, the "K" in a mapped type on a tuple should iterate only on numeric keys of this tuple. Therefore, Foo[K] should always be a valid key for Bar...

Start with the minimal TypeScript repro in the issue and compare it with Related Issue #25947. Trace mapped tuple type checking in the compiler; done means Baz is accepted as [string, number] without the Exclude/intersection workaround.