Control flow analysis should "work" in unreachable code

TypeScript Version: master

Search Terms: unreachable throw return narrowing

Code

function fn() {
    // TODO: Finish writing this function!
    return;

    const h = Math.random() > 0.5 ? 'h' : undefined;

    if (h) {
        // Error, wat
        const a: string = h;
    }
}

Expected behavior: No error

Actual behavior: Error - h is possibly undefined

We get "bug" reports like this every once in a while, and it's hard to understand why the narrowing just stops working because the code isn't reachable. This also makes it difficult to insert debugging throws/returns earlier in the function.

e.g. https://twitter.com/aendrew/status/1037301561062514688

Playground Link: http://www.typescriptlang.org/play/#src=function%20fn()%20%7B%0A%20%20%20%20%2F%2F%20Not%20done%20yet%0A%20%20%20%20return%3B%0A%0A%20%20%20%20const%20h%20%3D%20Math.random()%20%3E%200.5%20%3F%20'h'%20%3A%20undefined%3B%0A%20%20%20%20const%20d%20%3D%20(arg%3A%20string)%20%3D%3E%20arg%3B%0A%0A%20%20%20%20if%20(h)%20%7B%0A%20%20%20%20%20%20%20%20const%20a%20%3D%20d(h)%3B%0A%20%20%20%20%7D%0A%7D%0A

Related Issues: Believe there's one out there somewhere

Start with the supplied TypeScript Playground reproduction and confirm that the narrowing error occurs after an unreachable return. Trace the compiler's control-flow analysis for unreachable statements, then add or update a regression test so the narrowed value is accepted and the existing reachable-code behavior remains unchanged.