tsc misunderstands possible value of variable (TS2367)

[smallkirby]

Bug Report

In the below code, tsc emmits the error toward flag = flag === 'B' ? 'B' : 'A';.

type TestType = 'A' | 'B' | 'C';

const f = (strs: string) => {
  let flag: TestType = 'A';
  let other_flag = false;

  for (const line of strs.split('\n')) {
    if (line.length !== 0 && flag === 'B') {
      flag = 'A';
    }

    if (line.startsWith('#')) {
      flag = 'C';
      other_flag = false;
    } else if (line.length !== 0 && other_flag) {
      flag = 'B';
      other_flag = false;
    } else if (line.length !== 0) {
      flag = 'A';
      other_flag = false;
    } else if (line.length === 0) {
      console.log(`flag: ${flag}`);
      flag = flag === 'B' ? 'B' : 'A';
      other_flag = true;
    }

    if (flag === 'C') {
      // DO WORK
    }
  }
};

f(`
# Hello

hello

hello
`);

error message:

index.ts:23:14 - error TS2367: This condition will always return 'false' since the types '"A"' and '"B"' have no overlap.

23       flag = flag === 'B' ? 'B' : 'A';

However, if I set ts-ignore at the line, compile it, and then run it, the condition can actually be true.

$ node ./index.js
flag: A
flag: C
flag: B
flag: B

🔎 Search Terms

This condition will always return

🕗 Version & Regression Information

I tested it by tsc v4.5.5 on local, and tsc nightly on Playground.

⏯ Playground Link

Playground link with relevant code

[smallkirby]

Oh, it might be duplicate of #46475 , I guess? I'm sorry for my poor search, if it's just same.

[MartinJohns]

It does sound like a duplicate of #46475 to me too.

[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.