Typescript union type with singleton types does not compile

[bomgar]

Search Terms:
I did ask here https://stackoverflow.com/questions/54593103/typescript-union-type-with-singleton-types-does-not-compile

Code

type X = {
  id: "primary"
  inverted?: boolean
} | {
  id: "secondary"
  inverted?: boolean
} | {
  id: "tertiary"
  inverted?: false
}


const a = true
const x: X = {
  id: a ? "primary" : "secondary"
}

Expected behavior:
Compiles

Actual behavior:

» node_modules/typescript/bin/tsc test.ts
test.ts:14:7 - error TS2322: Type '{ id: "primary" | "secondary"; }' is not assignable to type 'X'.
  Type '{ id: "primary" | "secondary"; }' is not assignable to type '{ id: "tertiary"; inverted?: false; }'.
    Types of property 'id' are incompatible.
      Type '"primary" | "secondary"' is not assignable to type '"tertiary"'.
        Type '"primary"' is not assignable to type '"tertiary"'.

14 const x: X = {
         ~

Playground Link:
https://www.typescriptlang.org/play/#src=type%20X%20%3D%20%7B%0D%0A%20%20id%3A%20%22primary%22%0D%0A%20%20inverted%3F%3A%20boolean%0D%0A%7D%20%7C%20%7B%0D%0A%20%20id%3A%20%22secondary%22%0D%0A%20%20inverted%3F%3A%20boolean%0D%0A%7D%20%7C%20%7B%0D%0A%20%20id%3A%20%22tertiary%22%0D%0A%20%20inverted%3F%3A%20false%0D%0A%7D%0D%0A%0D%0A%0D%0Aconst%20a%20%3D%20true%0D%0Aconst%20x%3A%20X%20%3D%20%7B%0D%0A%20%20id%3A%20a%20%3F%20%22primary%22%20%3A%20%22secondary%22%0D%0A%7D

[ahejlsberg]

This is working as intended. In general we don't consider a type { x: A | B } assignable to { x: A } | { x: B }, even though in the rare case where all other properties have identical types we potentially could.

[bomgar]

If this is working as intended (I don't think it should be) the error message could be improved. I read Type '"primary"' is not assignable to type '"tertiary" and thought to myself: I doesn't have to be assignable to "tertiary". I took me a while to figure out the error is not caused by something I did but something I assumed typescript would do. The object matches the type definition (in my head) and it still doesn't fit. So a better error message could help me and others to understand this without wasting time.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.