Type discrimination in function calls for callable types doesn't work if discriminating property is optional

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)

Start by running the provided TypeScript 3.7 reproducer and compare the behavior with the required discriminant variant. Review related issue #7294 and its reference to #29011 for context on callable-type discrimination. Done means the optional discriminating property still infers the callback parameter as number rather than any.