Type <generic> cannot be used to index type <array>

[KiaraGrouwstra]

TypeScript Version:
0 2.3.1-insiders.20170416 (Playground)

Code

type X<I extends number> = ['a'][I];

Expected behavior:
No error

Actual behavior:
Type 'I' cannot be used to index type '["a"]'.

I thought this may have worked before. Potential regression?

[WanderWang]

I have a simular issue .

type ValueType = { 0: { name: string }, 36: { id: string } };
type KeyType = 0 | 36

function getValue<T extends KeyType>(key: T): ValueType[T] {
    let data: ValueType;
    let result = data[key.toString()];
    return result;
}
let a = getValue(0).name;
let b = getValue(36).id;

Expected behavior:
No error

Actual behavior:
Type 'T' cannot be used to index type 'ValueType'.

[KiaraGrouwstra]

Temp workaround from here: throw the generic through another type that outputs a number.

[KiaraGrouwstra]

Hm, not having as much luck with that workaround anymore on nightlies, maybe related to #17455.
That now makes this error account for the majority of errors in my type lib.

Here's another bigger example showing it fails when generics are involved even if separated by operations that would work:

type NumberToString = { [i: number]: string; };
type StringToNumber = {
  // [k: string]: number;
  '0': 0;
};

type A = NumberToString[StringToNumber[string]];
// -> ok, string

type B<T extends '0'> = NumberToString[StringToNumber[T]];
// -> Type 'StringToNumber[T]' cannot be used to index type 'NumberToString'.
type C<T extends '1'> = NumberToString[StringToNumber[T]];
// ^ if StringToNumber has no index, infers any
// ^ if StringToNumber has an index, "Type 'StringToNumber[T]' cannot be used to index type 'NumberToString'."

[KiaraGrouwstra]

I think the cause here is checkIndexedAccessIndexType in the TS compiler's checker.ts, which does a TypeVariable check for the object type yet not for the index type.

[KiaraGrouwstra]

PR at #17767.