[TypeScript] 5.4.5 - if(true) {return -1} throws off control-flow analysis

[riven8192]

🔎 Search Terms

none

🕗 Version & Regression Information

All tested versions of TypeScript (tested from 3.3.3 until 5.4.5 in the playground)

⏯ Playground Link

https://www.typescriptlang.org/play?target=9&jsx=0&module=7&ts=5.4.5#code/JYOwLgpgTgZghgYwgAgLIQM4bgcxQbwFgAoZZBAe3AnAH4AuZDMKUHAbhIF8SSEAbOFjSZseAMIUAtgAcqNMMiKlkU0bggBCRuiwbOxEmQAmAVylSAngAoAlIxDmARtCVGyyAPSfkAOQB0AEL+jHBgkLJgGMhgFDEQshRQcKz8lsjGwNhO-ChqYAAWFMYAtMCy-LwqZMAw1iymELZu1R7e5HCmGJiM0FBJyAAqAMoATACsAMyjjADyTgBWEAiKWchyWMA56QDkpiDGEDCgEMY7-u4eXj5XV1TI-CfIAO7AhcighwAeszAANMgIAA3BRFUw4AofRQCCApNIZCiYS5tG63MhwcgFZYAa2QMCSjE0AF4ich9odjiBTh8QDEsQ8ni58VAUG8SABIK4ssCmKC0koARgMVx4rWR3lq9SgjWQQmQTgoFFycBAzXw1xeSWx0UpEHFN25vP5Qv1oqu+slApJRIFao1zy1OpO+rIhr5yEFwpRovZLsKKCSwBwoDg-FUEEKxTKFWRksKWX8aj0eH8lGo4GQxNJ5KOJ2MauRXIjRrpCaTYggqaokHA-k+EB+dR2JRbO1sXrIZo8yLdxq9otFfCoGCVlf4FBw1h2cCcCHoO2QAGpkFTnsgACJhCB2fx4MCDcrb2y2IA

💻 Code

interface Message {
  content?: string;
}

class MessageComponent {
  message!: Message;

  dummy(): number {
    // N.B.: attempts to temporarily disable method-impl

    if(true) {
      // causes: error TS2532: Object is possibly 'undefined'.
      //         on line with indexOf, eventhough it clearly does
      //         a check for: !== undefined in the line before it
	    return -1;
    }
    
    //if(true as boolean) { // works fine
    //  return -1;
    //}
    
    //if(1===1) { // works fine
    //  return -1;
    //}
	
    // the original method-impl
    if(this.message.content !== undefined) {
        return this.message.content.indexOf('---');
    }
    
    return -1;
  }
}

🙁 Actual behavior

It throws an compile-time error that in the following (unreachable) code:

    if(this.message.content !== undefined) {
        return this.message.content.indexOf('---');
    }

the variable this.message.content may be undefined, when attempting to call indexOf.

🙂 Expected behavior

There probably should be merely a warning of unreachable code, not a blocking compile-time error about a variable potentially being undefined, when one can clearly see it cannot be undefined (whether or not the code was reachable)

Additional information about the issue

No response

[RyanCavanaugh]

#26914