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Unexpected intersection type constraint error related to union, generic type and private field #38236
Description
TypeScript Version: 3.9.1-rc/4.0.0-dev.20200428
3.9.0-beta seems to be fine.
Search Terms: intersection,private,generic,constraint,union
Code
type Type = {foo: string} | {bar: number};
class SyncableObject {
private foo: unknown;
}
interface SyncableRef<T extends ISyncableObject> {
}
interface ISyncableObject<T = object> extends SyncableObject {
}
type __ValueDescriptorType<
T extends Type | undefined
> = T extends ISyncableObject
? SyncableRef<T>
: T;Expected behavior: No error
Actual behavior:
Type 'ISyncableObject<object> & T' does not satisfy the constraint 'ISyncableObject<object>'.
Property 'foo' is missing in type 'ISyncableObject<object> & T' but required in type 'ISyncableObject<object>'.(2344)
Playground Link: Playground
Related Issues:
Activity
DanielRosenwasser commentedon Apr 29, 2020
I'm guessing related to #37762.
[-]Unexpected intersection type constraint error related to primitive union, generic type and private field[/-][+]Unexpected intersection type constraint error related to union, generic type and private field[/+]ahejlsberg commentedon Apr 30, 2020
Actually, looks to be caused by #37537. We currently say that unions of object types never contain private or protected members. But that isn't quite right. A union of object types should contain any private or protected member that is present in every constituent and originates in exactly the same declaration. This rarely matters, but in this particular case we end up structurally relating the union
ISyncableObject & object | ISyncableObject & stringtoISyncableObject, and that fails (but shouldn't) because of the private member.