type dosen't infered correctly in loop when strictNullChecks turns on

[disjukr]

TypeScript Version: 3.4.0-dev.20190310

Search Terms: loop, for, while, strictNullChecks, implicitly has type 'any' because it does not have a type annotation and is referenced directly or indirectly in its own initializer.

Code

class A { constructor(a: string) { } }
let a: A | null = null;
while (true) {
    const why = a ? 'a' : 'b';
    a = new A(why);
}

Expected behavior: infer why as string

Actual behavior: why inferred as any

Playground Link: https://www.typescriptlang.org/play/#src=class%20A%20%7B%20constructor(a%3A%20string)%20%7B%20%7D%20%7D%0D%0Alet%20a%3A%20A%20%7C%20null%20%3D%20null%3B%0D%0Awhile%20(true)%20%7B%0D%0A%20%20%20%20const%20why%20%3D%20a%20%3F%20'a'%20%3A%20'b'%3B%0D%0A%20%20%20%20a%20%3D%20new%20A(why)%3B%0D%0A%7D%0D%0A

Related Issues:

[RyanCavanaugh]

The error message means what it says: why does witness itself in its own initializer (in the second iteration of the loop).

[saschanaz]

@RyanCavanaugh Can you clarify more? why should be string (or 'a' | 'b'?) in this case even if it does witness itself, because both branches give string anyway.

[saschanaz]

This is still broken only with strictNullChecks 🤷‍♀️

[RyanCavanaugh]

@saschanaz SNC matters here because otherwise a isn't a union

[saschanaz]

@RyanCavanaugh Yeah, but how does that make a potentially not a string?