unreachable code detected not work for if clause.

[ShuiRuTian]

TypeScript Version: 3.9.2

Search Terms: unreachable

Code

const hel2 = (x: number) => {
  switch (typeof x) {
    case 'number': return 0
  }
  x    // Unreachable code detected
}

const hel12 = (x: number) => {
  if(typeof x === 'number'){
    return 0;
  }
  x    // expect same error here!
}

Expected behavior:
in commnet

Actual behavior:
in commnet

Playground Link:
https://www.staging-typescript.org/play?#code/MYewdgzgLgBAFgUwDYCYYF4YAoAeAuGMAVwFsAjBAJwEoMA+GAbwCgYYIB3ASymDmygBPAA4IQAMxg5aLNm2ABDCAhgByYuSqqClBFCKUwMAAysYAXzM5ml5qEixESAIxpMuAhoo16TM13EsIVEJKQx0THVSb1VqWTldfUMTAG4zSzZrSyA

Related Issues:

[tom-sherman]

This is very confusing, the type of x is narrowed to never which will raise a compiler error only if you return it. Side effects for example will not be marked as unreachable for example, as they should.

[safareli]

Unreachable code detection works great with switch/case

declare const x: 1 | 0

function f0() {
    switch(x){
        case 1: return null
        case 0: return null
    }
    return null
//  ^^^^^^^^^^^ Unreachable code detected.(7027)
}

But it doesn't work well with if/else + return or assertion:

function f1() {
    if (x === 1) { return null }
    if (x === 0) { return null }
    // const x: never
    x;
    return null
}

function f2() {
    if (x === 1) { throw "" }
    if (x === 0) { throw "" }
    // const x: never
    x;
    return null
}

Even tho in both cases type of x is properly inferred to be never, we do not get the Unreachable code detected.(7027) compilation error.

I think it shouldn't be super hard to fix, probably there should be added a check that will raise error in cases some variable is deduced to never and there is still some code left, so that both f1 and f2 result in compiler error.