cached typeof does not narrow down types of a union

[AndyOGo]

TypeScript Version: v4.0.5

Search Terms: typeof, cached typeof

Code

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.
interface TestHash {
    [index: string]: boolean;
}
type TestType = boolean | TestHash;

function testTypeOf(value: TestType) {
    const valueType = typeof value;

    if (valueType === "boolean") {
        return value;
    }

    if (valueType === "object") {
        return Object.keys(value).every((key) => value[key]);
    }

    return !!value
}

Expected behavior:
The cached result of typeof should also be used to narrow down the union type.

Actual behavior:

It errors with:

Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'TestType'. No index signature with a parameter of type 'string' was found on type 'TestType'.

Playground Link:

Broken: https://www.typescriptlang.org/play?#code/JYOwLgpgTgZghgYwgAgCoQM5gBJwwC2QG8AoZc5AbVABMIAPALmSylAHMBdZgIwHs+AGwhwQAbhIBfEmACeABxTosqBSgC8yfkJEhkAHzSYcefBJIwAriARhgfPZBVqA8jAAUANziDLEZspgqooAlMRkFAgOWMjevhDBGshyinwwsT5+5hTIwOlemQlqyOqlyABE2sKi5WGkOTlQEGCWUHpxWRHk0l25+R1FiiVl5Xw8AFYQtrXhDRRNLW3ILhNTYAB0ANYQshgF8SHrEJ7Qsu7u27Jh6gB8GfGUl5whEjk9jc2tegCE3wNSJEB5CAA

Working: https://www.typescriptlang.org/play?#code/JYOwLgpgTgZghgYwgAgCoQM5gBJwwC2QG8AoZc5AbVABMIAPALmSylAHMBdZgIwHs+AGwhwQAbhIBfEmACeABxTosqBSgC8yfkJEhkAHzSYcefBJIwAriARhgfPZBVqA8jAAUANziDLEZspgqooAlMRkFMAwyO5yinzR3r4a6poARNrComlhpBT5yFAQYJZQekl+EvnSEeRRMXEQCcgVKel8PABWELY54QUURSVlyC5dPWAAdADWELIYXj5+IZMQntCy7u6zsmHqAHwtSxCUO5whVRQ1+UOlegCE961SJK-kQA

Related Issues:

[MartinJohns]

Duplicate of #12184.