Closed
Description
TypeScript Version: nightly (2.0.0-dev.201xxxxx)
Code
type ToString = {
toString(): string;
}
type BoxedValue = { kind: 'int', num: number }
| { kind: 'string', str: string }
type IntersectionFail = BoxedValue & ToString
type IntersectionInline = { kind: 'int', num: number } & ToString
| { kind: 'string', str: string } & ToString
function getValueAsString(value: IntersectionFail): string {
if (value.kind === 'int') {
// Property 'num' does not exist on type '({ kind: "int"; num: number; } | { kind: "string"; str: string; }) & { toString(): string; }'.
return '' + value.num;
}
// Property 'str' does not exist on type '({ kind: "int"; num: number; } | { kind: "string"; str: string; }) & { toString(): string; }'.
return value.str;
}If a discriminated union is enhanced via intersection, type narrowing based on the discrimination field starts to fail. This behaviour is not exhibited if a new discriminated union is created with the intersection being inlined into each of the respective options.
The function getValueAsString(value: IntersectionFail): string fails to compile, while getValueAsString(value: IntersectionInline): string compiles just fine. Unfortunately, it is IntersectionFail that is the result of natural program evolution.