Error: Type cannot be used to index type after two indexes

[tenorok]

TypeScript Version: 2.7.1, 2.8.0-dev.20180208

Search Terms:
Type cannot be used to index type

Code

interface IExample {
 foo: {
 bar: {
 baz: number;
 }
 }
}

type F = <
 name extends keyof IExample,
 val extends keyof IExample[name]
>() => IExample[name][val]['baz']; // ← Type '"baz"' cannot be used to index type 'IExample[name][val]'.

Expected behavior:
In version 2.7.0-dev.20171115 this code was checking without errors.

Actual behavior:
Now in throws error: Type '"baz"' cannot be used to index type 'IExample[name][val]'.

Playground Link:
https://www.typescriptlang.org/play/index.html#src=interface%20IExample%20%7B%0D%0A%20%20%20%20foo%3A%20%7B%0D%0A%20%20%20%20%20%20%20%20bar%3A%20%7B%0D%0A%20%20%20%20%20%20%20%20%20%20%20%20baz%3A%20number%3B%0D%0A%20%20%20%20%20%20%20%20%7D%0D%0A%20%20%20%20%7D%0D%0A%7D%0D%0A%0D%0Atype%20F%20%3D%20%3C%0D%0A%20%20%20%20name%20extends%20keyof%20IExample%2C%0D%0A%20%20%20%20val%20extends%20keyof%20IExample%5Bname%5D%0D%0A%3E()%20%3D%3E%20IExample%5Bname%5D%5Bval%5D%5B'baz'%5D%3B

[ajafff]

Likely related to #21368

[mhegazy]

Nathan Shively-Sanders (@sandersn) can you take a look.

[sandersn]

The nut of the problem is that the base constraint of U extends keyof IExample[T] is string, even though we would like it be "bar". This means that IExample[T][U] doesn't have a constraint, and therefore can't be indexed by anything except keyof IExample[T][U], which "baz" is not.

I think it would be possible to fix this case by changing the rule for keyof types in computeBaseConstraint to return getIndexType(getBaseConstraint(t.type)) instead of just return stringType. I'll try it when I have time to see if it works.

[sandersn]

Here's the change I made. It re-broke #15371, so I'm not sure it's the right change. I'll discuss with Anders Hejlsberg (@ahejlsberg) tomrrow.

 if (t.flags & TypeFlags.Index) {
- return stringType;
+ const baseType = getBaseConstraint((t as IndexType).type);
+ const baseIndex = baseType ? getIndexType(baseType) : undefined;
+ return baseIndex && baseIndex !== unknownType ? getBaseConstraint(baseIndex) : undefined;
 }

[tenorok]

Nathan Shively-Sanders (@sandersn) hello, maybe there is any news?

[wycats]

This is a 2.8 upgrade blocker for me Daniel Rosenwasser (@DanielRosenwasser)

[wycats]

Actually, it's really weird! I'm getting type errors, but TS is getting the derived type right anyway!