Strange narrowing behavior for types assigned to in while loop

[midiusRed]

TypeScript Version: 3.2.0+

Code

class AInfo {
	a_count:number = 1;
}
class BInfo {
	b_count:number = 1;
}

class Base {
	next:Base;
	id:number = 0;
}
class A extends Base {
	constructor(readonly info:AInfo) {
		super();
	}
}
class B extends Base {
	constructor(readonly info:BInfo) {
		super();
	}
}

let target:Base = ...;
while (target) {
	if (target instanceof A) {
		target.info.a_count = 3;
	} else if (target instanceof B) {
		target.info.b_count = 3;
	}
	target = target.next;
}

Expected behavior:
TS2551: Property 'b_count' does not exist on type 'AInfo'. Did you mean 'a_count'?
When code place in WHILE

Actual behavior:
no error

[RyanCavanaugh]

Slightly smaller repro:

class AInfo {
	a_count:number = 1;
}
class BInfo {
	b_count:number = 1;
}

class Base {
	id:number = 0;
}
class A extends Base {
 info: AInfo;
}
class B extends Base {
 info: BInfo;
}

let target: Base = null as any;

while (target) {
	if (target instanceof A) {
		target.info.a_count = 3;
 } else if (target instanceof B) {
 // Error, but shouldn't be
 const j: BInfo = target.info;
	}
}

Note that this depends on the code being in the while loop

[DanielRosenwasser]

This whole thing is so strange - the IDE experience is totally inconsistent with the errors being reported.

class Base {
 id: number = 0;
}
class A extends Base {
 info: { aProp: number };
}
class B extends Base {
 info: { bProp: number };
}

let target: Base = null as any;

while (target) {
 if (target instanceof A) {
 target.info.aProp = target.info.aProp;
 }
 else if (target instanceof B) {
 let x: string = target.info.aProp;
 }
}

Here the language service says that the second use of target.info.aProp has type any (sort of correct, but should provide an error), but gives an error on x that number is not assignable to string (weird that it accessed that).

In this example, there's an error assigning to bProp (correct) but there's no problem assigning to aProp (incorrect):

class Base {
 id: number = 0;
}
class A extends Base {
 info: { aProp: number };
}
class B extends Base {
 info: { bProp: number };
}

let target: Base = null as any;

while (target) {
 if (target instanceof A) {
 target.info.bProp = 100;
 }
 if (target instanceof B) {
 target.info.aProp = 100;
 }
}

[ahejlsberg]

This looks to be an effect of #28112. The change in that PR isn't quite right, but having thought about it some more I think I have a better fix.