Incorrect types within false condition

[cns-solutions-admin]

Bug Report

Typescript does not evaluate all conditions to determine actual types, if the condition is always false because of a literal "false".
Adding a "false &&" to a condition is often done during debugging.

🔎 Search Terms

false condition, type

🕗 Version & Regression Information

4.7.3, 4.7.4

⏯ Playground Link

none

💻 Code

function test(param?: string) {
  if (param) {
    return param.length;
  }
  return 0;
}

function brokenTest(param?: string) {
  if (false && param) {
    return param.length; // <= TS2532: Object is possibly 'undefined'.
  }
  return 0;
}

function workaroundTest(param?: string) {
  if (!true && param) {
    return param.length; // compiles
  }
  return 0;
}

🙁 Actual behavior

When adding a "false &&" to a condition of an if, the original condition is ignored for the type checks in the if block.

🙂 Expected behavior

It still compiles.

[MartinJohns]

Duplicate of #26914.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.