Typescript: 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]

@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 @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]

@sandersn hello, maybe there is any news?

[wycats]

This is a 2.8 upgrade blocker for me @DanielRosenwasser

[wycats]

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

[koloboid]

Is there any chance to get it fixed? I've got it working in my case, but only with strictNullChecks: false. We absolutely love strictNullChecks, so waiting for the fix)

[dkamyshov]

So I have this piece of code:

type D<T> = {
  [propertyName: string]: keyof T;
}

const f = <A, B, C extends D<B>>(
  a: A, b: B, c: C,
): (
  A & {
    [K in keyof C]: B[C[K]] // this line
  }
) => void 0 as any;

const result = f({
  abc: 1,
}, {
  def: true
}, {
  ghi: 'def'
});

In TS 3.0.3 result.ghi is correctly inferred as boolean and no errors are displayed.
In TS 3.1.1 result.ghi is (again) correctly inferred as boolean, but in highlighted line it says Type 'C[K]' cannot be used to index type 'B'.

I'd greatly appreciate any help.

[moretolike]

ERROR in node_modules/@amcharts/amcharts4/.internal/charts/types/XYChart.d.ts(258,33): error TS2344: Type 'this["_xAxisRendererType"]' does not satisfy the constraint 'AxisRenderer'.
Type 'AxisRendererX' is not assignable to type 'AxisRenderer'.
Types of property 'events' are incompatible.
Type 'SpriteEventDispatcher>' is not assignable to type 'SpriteEventDispatcher Type 'AMEvent' is not assignable to type 'AMEvent node_modules/@amcharts/amcharts4/.internal/charts/types/XYChart.d.ts(264,33): error TS2344: Type 'this["_yAxisRendererType"]' does not satisfy the constraint 'AxisRenderer'.
Type 'AxisRendererY' is not assignable to type 'AxisRenderer'.
Types of property 'events' are incompatible.
Type 'SpriteEventDispatcher>' is not assignable to type 'SpriteEventDispatcher Type 'AMEvent' is not assignable to type 'AMEvent node_modules/@amcharts/amcharts4/.internal/charts/types/XYChart.d.ts(503,31): error TS2344: Type 'this["_xAxisRendererType"]' does not satisfy the constraint 'AxisRenderer'.
node_modules/@amcharts/amcharts4/.internal/charts/types/XYChart.d.ts(513,31): error TS2344: Type 'this["_yAxisRendererType"]' does not satisfy the constraint 'AxisRenderer'.
node_modules/@amcharts/amcharts4/.internal/core/utils/Object.d.ts(60,117): error TS2536: Type 'Key' cannot be used to index type 'Object'.
node_modules/@amcharts/amcharts4/.internal/core/utils/Object.d.ts(67,109): error TS2536: Type 'Key' cannot be used to index type 'Object'.
node_modules/@amcharts/amcharts4/.internal/core/utils/Object.d.ts(76,116): error TS2536: Type 'Key' cannot be used to index type 'Object'.
node_modules/@amcharts/amcharts4/.internal/core/utils/Type.d.ts(25,32): error TS2304: Cannot find name 'Extract'.

Please.... Help Me...

[fabb]

Maybe related:

function getValues<T, K extends readonly (keyof T)[]>(object: T, keys: K): { [P in keyof K]: T[K[P]] } {
    return keys.map(key => object[key]) as any
}

This shows the error: Type 'K[P]' cannot be used to index type 'T'.