Partial type fails to narrow in spite of check for undefined #29496
Description
The code below works in TS 3.0.
TypeScript Version: 3.2.1.0
Search Terms: is:issue 3.2 Partial
Code
declare function never(value: never): never;
const enum BarEnum {
bar1 = 1,
bar2 = 2,
}
type UnionOfBar = TypeBar1 | TypeBar2;
type TypeBar1 = {
type: BarEnum.bar1;
};
type TypeBar2 = {
type: BarEnum.bar2;
};
const value: Partial<UnionOfBar> = {} as any;
if (value.type !== undefined) {
switch (value.type) {
case BarEnum.bar1: break;
case BarEnum.bar2: break;
default: never(value.type); // Error: Type 'undefined' is not assignable to parameter of type 'never'
}
}Expected behavior:
No compiler error
Actual behavior:
Type 'undefined' is not assignable to parameter of type 'never'
Related Issues:
#28748