'always false' field now narrowed in 'if' true-branch

[SQReder]

TypeScript Version: 3.9.2, Nightly

Search Terms:
true|false boolean controlled flow
boolean type narrow

Code

function absurd(_: never) { throw new Error("Must not be called"); }

interface Foo {
    bar: false;
}

function fooled(foo: Foo) {
    if (foo.bar) {
        // foo expected to be narrowed to 'never' type
        absurd(foo);
    } else {
        const bar: false = foo.bar; 
    }
}

Expected behavior:
foo type in the 'true' branch narrowed to never

Actual behavior:
foo type is Foo, which is nonsense

Playground Link: https://www.typescriptlang.org/play/?ts=4.0.0-dev.20200615#code/GYVwdgxgLglg9mABAQwEYGcQCcAmAKAfQC5EwBTANzKwEpEBvRKACyzgHdSzOBRLNrHgBEAWRDoopOJNRlEEZABtFZHEJoBuRAF8AULphgo1YMghyAYnDgNdie4lTIsJU4vRkNuvbtCRYCIjA1ir4wXAkVnB09HYOMMCIeOEAdE60tg5ZiAD0OUHWiGQAHgAOZNCqTDaypM5s7FVQNgDk5FRYLUwAnuVx2SgY2GHWmv06Re5ysQP2EAgSjs6uSh6IALwFcGnOWuN6ekA

Related Issues:
No

[ShuiRuTian]

Not pretty sure, but I think this is similar to #39110 and #39114

[fatcerberus]

Foo is not a union, it's just a single object type. Therefore you don't get discriminated union behavior, and TS won't narrow arbitrary objects based on their properties. It works the way you expect if you use a proper discriminated union:

function absurd(_: never) { throw new Error("Must not be called"); }

interface Foo {
    bar: false;
}

interface Bar {
    bar: true;
}

function fooled(foo: Foo | Bar) {
    if (foo.bar) {
    } else if (!foo.bar) {
    } else {
        absurd(foo);
    }
}

[fatcerberus]

See also #29827

[SQReder]

The problem described in detail in #38963. I wanted to avoid complete union discriminator, however, there are not possible for now.
Thanks, guys 😉