Switch over discriminated union inside while loop is broken

[WiseBird]

TypeScript Version: 3.2.0-dev.20181002

Search Terms: tagged/discriminated unions unreachable/conditions

Code

enum Types { Str = 1, Num = 2 }
type Instance = StrType | NumType;
interface StrType {
    type: Types.Str;
    value: string;
    length: number;
}
interface NumType {
    type: Types.Num;
    value: number;
}

function func(inst: Instance) {
    if (false) {
        switch (inst.type) {
            case Types.Str: {
                console.log(inst.value.length);
                break;
            }
            case Types.Num: {
                console.log(inst.value.toExponential);
                break;
            }
        }
    }
}

Expected behavior:
No errors

Actual behavior:

Error:(17, 40) TS2339: Property 'length' does not exist on type 'string | number'.
  Property 'length' does not exist on type 'number'.
Error:(21, 40) TS2339: Property 'toExponential' does not exist on type 'string | number'.
  Property 'toExponential' does not exist on type 'string'.

Code

function func2(inst: Instance) {
    while (true) {
        switch (inst.type) {
            case Types.Str: {
                console.log(inst.value.length);
                break;
            }
            case Types.Num: {
                console.log(inst.value.toExponential);
                break;
            }
        }
    }
}

Expected behavior:
No errors

Actual behavior:

Error:(21, 40) TS2339: Property 'toExponential' does not exist on type 'string'.

Playground Link:
Playground

Related Issues:
Not found

[RyanCavanaugh]

Duplicate #26914 - search for "unreachable" 🙃

[WiseBird]

@RyanCavanaugh, what about second case?