Unexpected intersection inferred from indexed type

[shigma]

TypeScript Version: 3.7.2

Search Terms: type alias index signature intersection TS2345

Expected behavior: pass the type checks

Actual behavior: fail with a TS2345 error

data[key]
// expected: (value: DataTypes[K]) => void
// actual: Data[K], with value: never

Related Issues: #31445, #35613

Note: Although #35613 is marked as duplicated, I think a different approach than #31445 can be used to address this issue. Actually data[key] can be inferred as (value: DataTypes[K]) => void, which solves the problem.

Code

interface DataTypes {
  a: string
  b: number
}

type Data = {
  [K in keyof DataTypes]: (value: DataTypes[K]) => void
}

function myFunc <K extends keyof DataTypes> (data: Data, key: K, value: DataTypes[K]) {
  data[key](value)
}

Output

"use strict";
function myFunc(data, key, value) {
    data[key](value);
}

Compiler Options

{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "useDefineForClassFields": false,
    "alwaysStrict": true,
    "allowUnreachableCode": false,
    "allowUnusedLabels": false,
    "downlevelIteration": false,
    "noEmitHelpers": false,
    "noLib": false,
    "noStrictGenericChecks": false,
    "noUnusedLocals": false,
    "noUnusedParameters": false,
    "esModuleInterop": true,
    "preserveConstEnums": false,
    "removeComments": false,
    "skipLibCheck": false,
    "checkJs": false,
    "allowJs": false,
    "declaration": true,
    "experimentalDecorators": false,
    "emitDecoratorMetadata": false,
    "target": "ES2017",
    "module": "ESNext"
  }
}

Playground Link: Provided

[dragomirtitian]

Not really unexpected. Inside the function K can be any keyof DataTypes. This means that data[key] could be ((value: string) => void) | ((value: number) => void). Such a union is invokable only with an intersection of all possible parameter types (see PR). So this means the parameter would need to be string & number. Intersections of incompatible primitive types reduce to never so you end up with data[key] being effectively (value: never) => void.

To get this to work you would need some kind of correlated types such as @jcalz proposes here.

[shigma]

@dragomirtitian Thanks for your detailed explanation.

I know the inner logic now but my question is can the inferred type of data[key] be just more specific (from Data[K] to the definition of Data[K]), which may be easier to implement? I think it's different from #31445 and #30581.

[shigma]

Or, in other words, can we get actual typings for type parameters from indexed types (or inferences, however an index signature parameter type cannot be a union type)?

[dragomirtitian]

@shigma I don't think there is any way to get the implementation types to work out except for a type assertion or using a separate implementation signature:

function myFunc<K extends keyof DataTypes>(data: Data, key: K, value: DataTypes[K]): void {
  (data[key] as (value: DataTypes[K]) => void)(value);
}

Playground Link

[RyanCavanaugh]

This is a correct error; a legal call to myFunc is

declare const data: Data;
declare const ab: "a" | "b";
myFunc(data, ab, "");

which corrupts data when ab is "b"

There isn't a sound way to write the function signature, thus no way to correctly implement the body. You can use a type assertion if you pinky-swear to only call it with unit keys.

[shigma]

Thanks. I will close this issue.