Compiler fails to infer that a Date variable is not null even though it's obvious #41266

TypeScript Version: 4.03

Search Terms: Date, truthy, inference

Code

const date = getDate();

function getDate() {
    return Date.now() > 1000 ? new Date() : null;
}

const y = !date || date.valueOf() > 1000;

if (y) {
    console.log("y is truthy");
} else {
    console.log(getMonth(date));
}

function getMonth(value: Date) {
    return value.getMonth();
}

Expected behavior:

No error trying to pass date to getMonth, since it has been proved to be a non-null date due to variable y being false.

Actual behavior:

The following error is displayed for date:

Argument of type 'Date | null' is not assignable to parameter of type 'Date'. Type 'null' is not assignable to type 'Date'.

Note that if you do if (!date || date.valueOf() > 1000 instead of if (y), the error disappears, so that one small bit of indirection seems to throw off the compiler.

Playground Link: https://www.typescriptlang.org/play?#code/MYewdgzgLgBAJgQygUxgXhgc2VAIk5ACgEoBuAKHIDMBXMYKAS3Cx3xRJgG9yY+YATjhoCwMdsgB0YEAHdOAPhgBGAAzqYAfhhhks8QU4AuHTQA2ZigF9KoSLACe6GAEJEKGAB9P8ApIBuCGY0yADyVIoq6qoU5IxUMIQOxNy8-HYQIGZSZiCYhABETowQMFACNFAAFg4FZORWMMhmEKg8-DAZWTl5hNhQALLg1YTuyMT1NtR0DMxi-UNgI4HByCYSKe38QlAiYishkgvDVSTWQA

Related Issues: (not able to find any -- not sure what other key words to search for)