Type inference of the type union and intersection

[AntoineEsteve]

TypeScript Version: 2.7.2

Search Terms: Type union, type intersection, type inference

Code

interface A {
    a: boolean
}

interface A1 extends A {
    a1: boolean
}

interface A2 extends A {
    a2: boolean
}

type A1andA2 = A1 & A2

type A1orA2 = A1 | A2

type A1orA2orA1andA2 = A1orA2 | A1andA2

// Expected
const test1: A1orA2 = {
    a: true,
    a1: true,
}

console.log(test1) // Type inference: A1

// Expected
const test2: A1orA2 = {
    a: true,
    a2: true,
}

console.log(test2) // Type inference: A2

// Expected
const test3: A1orA2orA1andA2 = {
    a: true,
    a1: true,
    a2: true,
}

console.log(test3) // Type inference: A1andA2 <<< good

// Expected => union accept the intersections
const test4: A1orA2 = {
    a: true,
    a1: true,
    a2: true,
}

console.log(test4) // Type inference: A1 | A2 <<< Should be A1 & A2, not good
console.log(test4.a1) // Error 
console.log(test4.a2) // Error

It is not very clear in the documentation if the union contains the intersection, but both assumptions seems to have issues

Expected behavior:

• Assumption: union contains the intersection
  A1 | A2 equals A1 or A2 or A1 & A2: the type intefence of { a: true, a1: true, a2: true } should be A1 & A2
• Assumption: union excludes the intersection
  A1 | A2 equals A1 or A2 excluding A1 & A2: { a: true, a1: true, a2: true } should not be assignable to A1 | A2

Actual behavior:

• Assumption: union contains the intersection
  The type inference of { a: true, a1: true, a2: true } is A1 | A2 and we can not use the properties a1 and a2 without type guards
• Assumption: union excludes the intersection
  { a: true, a1: true, a2: true } is still assignable to A1 | A2

Playground Link:
https://www.typescriptlang.org/play/index.html#src=interface%20A%20%7B%0D%0A%20%20%20%20a%3A%20boolean%0D%0A%7D%0D%0A%0D%0Ainterface%20A1%20extends%20A%20%7B%0D%0A%20%20%20%20a1%3A%20boolean%0D%0A%7D%0D%0A%0D%0Ainterface%20A2%20extends%20A%20%7B%0D%0A%20%20%20%20a2%3A%20boolean%0D%0A%7D%0D%0A%0D%0Atype%20A1andA2%20%3D%20A1%20%26%20A2%0D%0A%0D%0Atype%20A1orA2%20%3D%20A1%20%7C%20A2%0D%0A%0D%0Atype%20A1orA2orA1andA2%20%3D%20A1orA2%20%7C%20A1andA2%0D%0A%0D%0A%2F%2F%20Expected%0D%0Aconst%20test1%3A%20A1orA2%20%3D%20%7B%0D%0A%20%20%20%20a%3A%20true%2C%0D%0A%20%20%20%20a1%3A%20true%2C%0D%0A%7D%0D%0A%0D%0Aconsole.log(test1)%20%2F%2F%20Type%20inference%3A%20A1%0D%0A%0D%0A%2F%2F%20Expected%0D%0Aconst%20test2%3A%20A1orA2%20%3D%20%7B%0D%0A%20%20%20%20a%3A%20true%2C%0D%0A%20%20%20%20a2%3A%20true%2C%0D%0A%7D%0D%0A%0D%0Aconsole.log(test2)%20%2F%2F%20Type%20inference%3A%20A2%0D%0A%0D%0A%2F%2F%20Expected%0D%0Aconst%20test3%3A%20A1orA2orA1andA2%20%3D%20%7B%0D%0A%20%20%20%20a%3A%20true%2C%0D%0A%20%20%20%20a1%3A%20true%2C%0D%0A%20%20%20%20a2%3A%20true%2C%0D%0A%7D%0D%0A%0D%0Aconsole.log(test3)%20%2F%2F%20Type%20inference%3A%20A1andA2%20%3C%3C%3C%20good%0D%0A%0D%0A%2F%2F%20Expected%20%3D%3E%20union%20accept%20the%20intersections%0D%0Aconst%20test4%3A%20A1orA2%20%3D%20%7B%0D%0A%20%20%20%20a%3A%20true%2C%0D%0A%20%20%20%20a1%3A%20true%2C%0D%0A%20%20%20%20a2%3A%20true%2C%0D%0A%7D%0D%0A%0D%0Aconsole.log(test4)%20%2F%2F%20Type%20inference%3A%20A1%20%7C%20A2%20%3C%3C%3C%20Should%20be%20A1%20%26%20A2%2C%20not%20good%0D%0Aconsole.log(test4.a1)%20%2F%2F%20Error%20%0D%0Aconsole.log(test4.a2)%20%2F%2F%20Error

[jack-williams]

Union includes the intersection, otherwise you would get weird behaviour such as 0 not being assignable to 0 | number. The operator behaves much the same as the set operation.

This issue seems to be similar to #20863.

I wouldn't expect the inferred type of test4 to be A1 & A2 because of the structural nature of TypeScript. Partitioning the inferred type into A1 and A2 is arbitrary and it would be equally valid to infer {a: boolean; a1: boolean; a2; boolean}. You get this behaviour for test3 because you include it explicitly in the union type and it uses discrimination to remove A1orA2.

[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.