Type inferrence breaks on nested generic indexes

[asmundg]

TypeScript Version:

2.7.1
2.8.0-dev.20180208

Search Terms:

inferred generic index

Code

type Dict<T extends string> = { [key in T]: number }
type DictDict<V extends string, T extends string> = { [key in V]: Dict<T> }

function f<V extends string, T extends string>(dd: DictDict<V, T>, k1: V, k2: T): number {
 // Type 'DictDict<V, T>[V][T]' is not assignable to type 'number'.
 return dd[k1][k2]
}

// works
function f1<V extends string, T extends string>(dd: DictDict<V, T>, k1: V, k2: T): number {
 const d: Dict<T> = dd[k1]
 return d[k2]
}

Expected behavior:

The code above works on typescript 2.6.2, where dd[k1][k2] in f appears to correctly infer number as the type.

Actual behavior:

In >= 2.7.1, typescript now appears to need help figuring out the type of dd[k1].

Playground Link:

https://www.typescriptlang.org/play/#src=type%20Dict%3CT%20extends%20string%3E%20%3D%20%7B%20%5Bkey%20in%20T%5D%3A%20number%20%7D%0D%0Atype%20DictDict%3CV%20extends%20string%2C%20T%20extends%20string%3E%20%3D%20%7B%20%5Bkey%20in%20V%5D%3A%20Dict%3CT%3E%20%7D%0D%0A%0D%0Afunction%20f%3CV%20extends%20string%2C%20T%20extends%20string%3E(dd%3A%20DictDict%3CV%2C%20T%3E%2C%20k1%3A%20V%2C%20k2%3A%20T)%3A%20number%20%7B%0D%0A%20%20%20%20%2F%2F%20Type%20'DictDict%3CV%2C%20T%3E%5BV%5D%5BT%5D'%20is%20not%20assignable%20to%20type%20'number'.%0D%0A%20%20%20%20return%20dd%5Bk1%5D%5Bk2%5D%0D%0A%7D%0D%0A%0D%0A%2F%2F%20works%0D%0Afunction%20f1%3CV%20extends%20string%2C%20T%20extends%20string%3E(dd%3A%20DictDict%3CV%2C%20T%3E%2C%20k1%3A%20V%2C%20k2%3A%20T)%3A%20number%20%7B%0D%0A%20%20%20%20const%20d%3A%20Dict%3CT%3E%20%3D%20dd%5Bk1%5D%0D%0A%20%20%20%20return%20d%5Bk2%5D%0D%0A%7D

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