functions returning never are not treated the same as raw throw calls in control-flow analysis

[rkirov]

TypeScript Version: 2.0.0-beta

Code

function fail(): never {
  throw 'error';
}

enum E {
  A,
  B
}
function f(t: E) {
  let s: string;
  switch (t) {
    case E.A:
      s = 'a';
      break;
    case E.B:
      s = 'b';
      break;
    default:
      fail();  // but replacing with "throw 'error"' removes the TS error.
  }
  s += '';
}

Expected behavior:
s += '' is allowed because all terminating (non-throwing) branches assign to s.

In general, I am working under the assumption that calling functions that returning never are equivalent to raw throw calls during control-flow analysis.

Actual behavior:
ERROR(21,3): : Variable 's' is used before being assigned.

[RyanCavanaugh]

The limitation here has to do with the way the compiler is designed -- control flow analysis happens before typechecking, but we would need type information (as well as identifier resolution) to determine that the fail() call points to a function that is : never, so the s += ''; node is marked as reachable via a path that doesn't assign to s.

The intended fix is to write return fail(); instead of fail();.