Error in detecting type if there is a OR logic in if

Bug Report

🔎 Search Terms

• detect type
• OR logic
• ||

🕗 Version & Regression Information

This appear in all recent TS. (demo is with 4.4.4)

⏯ Playground Link

Playground link with demo of bug

💻 Code

All code with comment is already in playground link watch error on line 19 of index.ts But here:

import { TypeOfMain, getMainObj } from "./external";

let mainObject: false | TypeOfMain = false;

mainObject = getMainObj();

const randomAlternative = Math.random();

// with OR in IF error on line 20

if (
  randomAlternative > 0.5 ||
  (mainObject && Number(mainObject.value) <= 250)
) {
  if (randomAlternative > 0.5) {
    console.log(`Error in random ${randomAlternative}`);
  } else {
    // error mainObject must be TypeOfMain not false
    console.log(`Error in main ${mainObject.value}`); // <--- Error
  }
}

// with double IF no error on line 31

if (randomAlternative > 0.5) {
  if (mainObject && Number(mainObject.value) <= 250) {
    if (randomAlternative > 0.5) {
      console.log(`Error in random ${randomAlternative}`);
    } else {
      // no error
      console.log(`Error in main ${mainObject.value}`);
    }
  }
}

🙁 Actual behavior

Error on line 19 mainObject must be TypeOfMain not false

🙂 Expected behavior

Like line 31

[RyanCavanaugh]

If we could do that we would just do the right thing instead. Detecting that condition is the problem in the first case.

[r1si]

@RyanCavanaugh Could you consider including a specific warning in all errors related to the data type within an if (scope) where its condition is written with an OR (and thus not determinable by design pattern)? Reasoning about this, I understand the limitation or at least the difficulty on the implementation of the check; but if not written it could mislead the user, who would expect (as is the case of other situations) a precise warning, especially because a TS error of this type compromises the compilation of the code.