This narrowing typeguard effect bleeds into subsequent statments on a type with bivariant type-parameter

TypeScript Version: 3.4.0-dev.201xxxxx (although 3.3 is also impacted)
Search Terms: this type guards

If we create a type-guard that narrows this on a type that has a type-parameter that is present only in a bivariant position, the effect of the type guard persists outside of the guarded block.

Code

type GetKnownKeys<G> = G extends GuardedMap<infer KnownKeys>  ? KnownKeys: never;
interface GuardedMap<KnownKeys extends string> {
    get(k: KnownKeys): number;
    has<S extends string>(k: S): this is GuardedMap<S | GetKnownKeys<this>>;
}

declare let map: GuardedMap<never>;

map.get('bar') // err, as expected
if (map.has('foo')) {
    map.get('foo').toExponential(); // ok as expected
    if(map.has('bar')) 
    {
        map.get('foo').toExponential(); // ok as expected
        map.get('bar').toExponential(); // ok as expected
    }
        
    map.get('bar').toExponential(); /// OK!?!?! WHY ?!
    
}
map.get('bar') // OK ?!

Expected behavior:
Type guard only impacts the guarded block.
Actual behavior:
The effect of the type guard bleads into all subsequent statements. (marked with OK!?!?! and OK?!)

Note: With strictFunctionTypes on, declaring get as get: (k: KnownKeys) => number; makes the code work as expected.

Playground Link: link

Related Issues: Similar to #14817

Found this while playing with a solution for #9619

Tái hiện lỗi rò rỉ narrowing trong TypeScript Playground được liên kết bằng ví dụ GuardedMap được cung cấp. Theo dõi cách trình biên dịch xử lý control flow cho các type guard của this và các tham số kiểu bivariant; hoàn thành khi narrowing vẫn bị giới hạn trong block được bảo vệ và map.get('bar') sau block bị từ chối, kèm theo coverage hồi quy.