"Index signature is missing", but only when the type is inferred

[jamesknelson]

TypeScript Version: 3.0.0-dev.20180607

Search Terms:
Index signature missing inferred mapped types

Code

declare function map<FromValue>(
  from: Publisher<FromValue>,
  to: (props: FromValue) => any
);

declare function combine<CombinedValue>(children: {
  [K in keyof CombinedValue]: Publisher<CombinedValue[K]>
}): Publisher<CombinedValue>;


type Publisher<Value> = { value: Value; }
type Box<Value> = { data: Value }
type X = { a: string }
type Y = { b: number }

// ---

let a = combine({
  x: undefined as Publisher<Box<X>>,
  y: undefined as Publisher<Box<Y>>,
})
map(a, boxes => {
  // Index signature if missing in type { x: Box<X>, y: Box<Y> },
  let fail1: {
    [name: string]: Box<any>
  } = boxes

  // But not when type { x: Box<X>, y: Box<Y> } is specified manually
  // instead of inferred
  let pass1: {
    [name: string]: Box<any>
  } = boxes as {
    x: Box<X>;
    y: Box<Y>;
  }

  // And it can still be assigned to index-typed objects...
  let pass2: {
    [name: string]: any
  } = boxes

})

// Here `b` has the same type as the type inferred for `a`, but it works.
// The behavior must stem from inferred type.
let b: Publisher<{
  x: Box<X>,
  y: Box<Y>, 
}>
map(b, boxes => {
  let pass3: {
    [name: string]: Box<any>
  } = boxes
})

Expected behavior:

Should compile.

Given that boxes can be assigned to { [name: string]: any }, it feels like it should also be able to be assigned to { [name: string]: Box<any> }.

Actual behavior:

Fails with:

test.ts:24:7 - error TS2322: Type '{ x: Box<X>; y: Box<Y>; }' is not assignable to type '{ [name: string]: Box<any>; }'.
  Index signature is missing in type '{ x: Box<X>; y: Box<Y>; }'.

24   let fail1: {

Playground Link:

http://www.typescriptlang.org/play/#src=declare%20function%20map%3CFromValue%3E(%0D%0A%20%20from%3A%20Publisher%3CFromValue%3E%2C%0D%0A%20%20to%3A%20(props%3A%20FromValue)%20%3D%3E%20any%0D%0A)%3B%0D%0A%0D%0Adeclare%20function%20combine%3CCombinedValue%3E(children%3A%20%7B%0D%0A%20%20%5BK%20in%20keyof%20CombinedValue%5D%3A%20Publisher%3CCombinedValue%5BK%5D%3E%0D%0A%7D)%3A%20Publisher%3CCombinedValue%3E%3B%0D%0A%0D%0A%0D%0Atype%20Publisher%3CValue%3E%20%3D%20%7B%20value%3A%20Value%3B%20%7D%0D%0Atype%20Box%3CValue%3E%20%3D%20%7B%20data%3A%20Value%20%7D%0D%0Atype%20X%20%3D%20%7B%20a%3A%20string%20%7D%0D%0Atype%20Y%20%3D%20%7B%20b%3A%20number%20%7D%0D%0A%0D%0A%2F%2F%20---%0D%0A%0D%0Alet%20a%20%3D%20combine(%7B%0D%0A%20%20x%3A%20undefined%20as%20Publisher%3CBox%3CX%3E%3E%2C%0D%0A%20%20y%3A%20undefined%20as%20Publisher%3CBox%3CY%3E%3E%2C%0D%0A%7D)%0D%0Amap(a%2C%20boxes%20%3D%3E%20%7B%0D%0A%20%20%2F%2F%20Index%20signature%20if%20missing%20in%20type%20%7B%20x%3A%20Box%3CX%3E%2C%20y%3A%20Box%3CY%3E%20%7D%2C%0D%0A%20%20let%20fail1%3A%20%7B%0D%0A%20%20%20%20%5Bname%3A%20string%5D%3A%20Box%3Cany%3E%0D%0A%20%20%7D%20%3D%20boxes%0D%0A%0D%0A%20%20%2F%2F%20But%20not%20when%20type%20%7B%20x%3A%20Box%3CX%3E%2C%20y%3A%20Box%3CY%3E%20%7D%20is%20specified%20manually%0D%0A%20%20%2F%2F%20instead%20of%20inferred%0D%0A%20%20let%20pass1%3A%20%7B%0D%0A%20%20%20%20%5Bname%3A%20string%5D%3A%20Box%3Cany%3E%0D%0A%20%20%7D%20%3D%20boxes%20as%20%7B%0D%0A%20%20%20%20x%3A%20Box%3CX%3E%3B%0D%0A%20%20%20%20y%3A%20Box%3CY%3E%3B%0D%0A%20%20%7D%0D%0A%0D%0A%20%20%2F%2F%20And%20it%20can%20still%20be%20assigned%20to%20index-typed%20objects...%0D%0A%20%20let%20pass2%3A%20%7B%0D%0A%20%20%20%20%5Bname%3A%20string%5D%3A%20any%0D%0A%20%20%7D%20%3D%20boxes%0D%0A%0D%0A%7D)%0D%0A%0D%0A%2F%2F%20Here%20%60b%60%20has%20the%20same%20type%20as%20the%20type%20inferred%20for%20%60a%60%2C%20but%20it%20works.%0D%0A%2F%2F%20The%20behavior%20must%20stem%20from%20inferred%20type.%0D%0Alet%20b%3A%20Publisher%3C%7B%0D%0A%20%20x%3A%20Box%3CX%3E%2C%0D%0A%20%20y%3A%20Box%3CY%3E%2C%20%0D%0A%7D%3E%0D%0Amap(b%2C%20boxes%20%3D%3E%20%7B%0D%0A%20%20let%20pass3%3A%20%7B%0D%0A%20%20%20%20%5Bname%3A%20string%5D%3A%20Box%3Cany%3E%0D%0A%20%20%7D%20%3D%20boxes%0D%0A%7D)%0D%0A

Related Issues:

• Type params constrained to Record<string, ...> require type args to have index signatures #15161
• Index signature is missing in type (only on interfaces, not on type alias) #15300

[mhegazy]

This is partially the same underlying issue as #15300.

[jamesknelson]

The thing that confused me is that the error message says Index signature if missing in type { x: Box<X>, y: Box<Y> }, however { x: Box<X>, y: Box<Y> } can still be assigned - just not when inferred.

If this is the intended behavior, perhaps the error message could indicate that the inferred { x: Box<X>, y: Box<Y> } somehow differs from the manually specified { x: Box<X>, y: Box<Y> }?

[jamesknelson]

Found a temporary solution that allows inferred types to be treated as having an index signature:

type Publisher<Value> = { value: Value; }
type Box<Value> = { data: Value }
type X = { a: string }
type Y = { b: number }

declare function map<FromValue>(
  from: Publisher<FromValue>,
  to: (props: FromValue) => any
);

declare function combine<CombinedValue>(children: {
  [K in keyof CombinedValue]: Publisher<CombinedValue[K]>
}): Publisher<CombinedValue>;

declare function processBoxes<T extends { [name: string]: Box<any> }>(boxes: T): T;

// ---

let combinedPublisher = combine({
  x: undefined as Publisher<Box<{ a: string }>>,
  y: undefined as Publisher<Box<{ b: number }>>,
})
map(combinedPublisher, boxes => {
  // Error: Index signature if missing in type { x: Box<X>, y: Box<Y> },
  let a = processBoxes(boxes)

  // Compiles
  let b = processBoxes(boxes as {
    [K in keyof typeof boxes]: typeof boxes[K]
  })

  // "typeof c" is string
  let c = b.x.data.a
})

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.