TypeScript Version: 3.7.0-dev.20191015

Search Terms: type discrimination, callable, signatures, implicit any

Code

function test( a: | { type?: 1; property: (a: number) => void } | { type: 2; property: (a: string) => void }, ) { return a.property } test({ property: (x) => { // complaining that x implicitly has type any console.log('hi') }, })

Expected behavior:

x should be inferred to be number.

Actual behavior:

x is any.

If you change type?: 1 to type: 1, it works as expected.

Playground Link: link

Related Issues: #7294 (closed with #29011 - see second half of the original post, I believe it describes why this issue occurs)