unreachable code detected not work for if clause.

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:

从 TypeScript 代码示例及其 Playground 链接开始，比较 switch 和 if 语句之后的不可达代码诊断。跟踪编译器针对这两种控制流形式的不可达代码分析，并验证等效情况下在更改前后都会产生预期的诊断。