Property type guards don't narrow type of the object the property belongs to

[thorn0]

TypeScript Version: 2.9.1-insiders.20180525 (playground), 3.0.0-rc

Search Terms:

property type guards, undefined checks

Code

function myFunction(params: {
    a: Date,
    b?: Date
}) {
    if (params.b) {
        myFunctionInternal(params)
    }
}

function myFunctionInternal(params: {
    a: Date,
    b: Date
}) {}

Expected behavior:

should compile

Actual behavior:

Error: Argument of type '{ a: Date; b?: Date | undefined; }' is not assignable to parameter of type '{ a: Date; b: Date; }'.

Playground Link:

click me

[mhegazy]

Duplicate of #25631 and #18758

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.