noUnusedLocals throws errors for "never" type.

[technoligest]

Search Terms:
nounusedlocals never
nounusedlocals
never
never variable is not used error

Code

//NOTE: nounusedlocals is set to true in tsconfig.
enum Color {
  A,
  B,
}
function f(color: Color) {
  switch(color) {
    case Color.A:
      console.log("White");
      break;
    case Color.B:
      console.log("Black");
      break;
    default:
      const _:never = color;
  }
}

Expected behavior:
Compiles fine, prints nothing

Actual behavior:
Given variable _ is unused. _ is of type never which means it is expected to be never used. It is being declared as a compile time safety to ensure that we have cases over all the Color enum values.

[acutmore]

It is being declared as a compile time safety to ensure that we have cases over all the Color enum values.

An alternative way to do this which will not cause a nounusedlocals error is to use a helper function.

function isNever(_: never): never {
   throw new Error(`Unreachable code was run`);
}

enum Color {
  A,
  B,
}
function f(color: Color) {
  switch(color) {
    case Color.A:
      console.log("White");
      break;
    case Color.B:
      console.log("Black");
      break;
    default:
      isNever(color);
  }
}

[technoligest]

Thanks @acutmore This is actually the workaround I am using right now, but I still think it is a bug on Typescript side as a never variable should not be expected to be used anyways.

[fatcerberus]

Even assuming this didn't trigger noUnusedLocals, shouldn't this trigger an unreachable code warning?

[RyanCavanaugh]

It literally is an unused local, though 😕. I think it'd be best if flags did what they're called, but we can hear more feedback on this