Reflect.has fails to act as type guard (should act same as "in" operator)

TypeScript Version: ^3.4.0-dev.20190330

Search Terms:
Reflect.has
in operator

Code

const test1 = (a: { field: number } | {}) => (("field" in a) ? a.field : 0);

const test2 = (a: { field: number } | {}) => Reflect.has(a, "field") ? a.field : 0;

Expected behavior:
Both compile successfully. "in" operator acts as a type guard per https://github.com/Microsoft/TypeScript/issues/10485. Reflect.has should act the same as "in" operator here.

We'd like to use Reflect.has a lot more and this case holds us back. Thank you!

Actual behavior:
"in" operator line compiles; Reflect.has line does not compile. Error:

allPasos.ts:445:31 - error TS2339: Property 'field' does not exist on type '{} | { field: number; }'.
Property 'field' does not exist on type '{}'.

445 Reflect.has(a, "field") ? a.field : 0;

Playground Link:

https://goo.gl/2R4dkS

Related Issues:

https://github.com/Microsoft/TypeScript/issues/10485

リンク先の Playground の再現から始め、既存の in 演算子の narrowing の挙動を Reflect.has と比較してください。背景については関連する issue #10485 を読んでください。示されている Reflect.has 式がコンパイルされ、issue が期待する通りに動作し、回帰に対するカバレッジがあることが完了条件です。