Unexpected exhaustive check result for switch statement

[lriuui0x0]

TypeScript Version: [email protected]

Search Terms: switch

Code

const assertUnreachable = (x: never): void => {}

const f = (x: 'a' | 'b') => {
  switch (x) {
    case 'a':
      {
      }
      break;

    case 'a':
      {
      }
      break;
  }
};

const g = (x: { t1: 'a'; t2: 'a1' | 'a2' } | { t1: 'b'; t2: 'b1' | 'b2' }) => {
  switch (x.t1) {
    case 'a':
      {
        switch (x.t2) {
          case 'a1':
            {
            }
            break;

          case 'a2':
            {
            }
            break;

          default: {
            assertUnreachable(x.t2);
          }
        }
      }
      break;
  }
};

Expected behavior:

Function f should not pass the typecheck at the second case block.
Function g should pass the typecheck at assertUnreachable.

Actual behavior:

Function f passes the typecheck at the second case block.
Function g doesn't pass the typecheck at assertUnreachable.

Note that if we put assertUnreachable in function f, it will work. So this seems to be only happening for the nested switch.

[ahejlsberg]

This has been fixed by #29563.