Intersection of multiple union types does not narrow properties

[Haubach]

TypeScript Version: 3.4.0-dev.20190307

Search Terms:
union type, intersection types,

Code

type a = {typeFoo: "a"};
type b = {typeFoo: "b"};

type c = {typeBar: "c"};
type d = {typeBar: "d", value: number};


type foo = a | b; 
type bar = c | d;

//throws build error as value does not exist on 'a & c'
const x: a & bar = {
    typeFoo:"a",
    typeBar: "c",
    value:100
}

//throws build error as value does not exist on 'b & c'
const y: b & bar = {
    typeFoo:"b",
    typeBar: "c",
    value:100
}

//does not throw a build error, but should
const z: foo & bar = {
    typeFoo:"a",
    typeBar: "c",
    value:100
}

Expected behavior:
Object literal 'z' may only specify known properties, and 'value' does not exist in type '(a & c) | (b & c)'

Actual behavior:
Typescript compiles const z without catching the error

Playground Link: https://www.typescriptlang.org/play/#src=type%20a%20%3D%20%7BtypeFoo%3A%20%22a%22%7D%3B%0D%0Atype%20b%20%3D%20%7BtypeFoo%3A%20%22b%22%7D%3B%0D%0A%0D%0Atype%20c%20%3D%20%7BtypeBar%3A%20%22c%22%7D%3B%0D%0Atype%20d%20%3D%20%7BtypeBar%3A%20%22d%22%2C%20value%3A%20number%7D%3B%0D%0A%0D%0A%0D%0Atype%20foo%20%3D%20a%20%7C%20b%3B%20%0D%0Atype%20bar%20%3D%20c%20%7C%20d%3B%0D%0A%0D%0A%2F%2Fthrows%20build%20error%20as%20value%20does%20not%20exist%20on%20'a%20%26%20c'%0D%0Aconst%20x%3A%20a%20%26%20bar%20%3D%20%7B%0D%0A%20%20%20%20typeFoo%3A%22a%22%2C%0D%0A%20%20%20%20typeBar%3A%20%22c%22%2C%0D%0A%20%20%20%20value%3A100%0D%0A%7D%0D%0A%0D%0A%2F%2Fthrows%20build%20error%20as%20value%20does%20not%20exist%20on%20'b%20%26%20c'%0D%0Aconst%20y%3A%20b%20%26%20bar%20%3D%20%7B%0D%0A%20%20%20%20typeFoo%3A%22b%22%2C%0D%0A%20%20%20%20typeBar%3A%20%22c%22%2C%0D%0A%20%20%20%20value%3A100%0D%0A%7D%0D%0A%0D%0A%2F%2Fdoes%20not%20throw%20a%20build%20error%2C%20but%20should%0D%0Aconst%20z%3A%20foo%20%26%20bar%20%3D%20%7B%0D%0A%20%20%20%20typeFoo%3A%22a%22%2C%0D%0A%20%20%20%20typeBar%3A%20%22c%22%2C%0D%0A%20%20%20%20value%3A100%0D%0A%7D%0D%0A

Related Issues: #11989

[jack-williams]

I think this can be filed under #13813 and #20863. I believe excess property checking on a union will not narrow the union if multiple fields discriminate to different types, which is exactly the case here with typeFoo and typeBar. As a consequence, field z is not considered excess in foo & bar because it appears in at least on part of the union.

[typescript-bot]

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