Union of string literals is incorrectly widened in error message when compared to never

[Aprillion]

TypeScript Version: 4.1.2, 4.0.5 (worked better in 3.9.7)

Search Terms: "string is not assignable to type never" (all 15 issues I found look slightly different I think)

Code

function assertUnreachable(_: never): void {}

type ActionTypes = 'a' | 'b'

function reducer(state: {}, action: {type: ActionTypes}) {
  switch(action.type) {
    case 'a':
      return state
    default: {
      assertUnreachable(action.type) // <-- incorrect widening on this line
      return state
    }
  }
}

Expected behavior: like in 3.9.7, the error message should be Argument of type '"b"' is not assignable to parameter of type 'never'

Actual behavior: Argument of type 'string' is not assignable to parameter of type 'never' - which is too wide to be actually helpful :(

Playground Link: https://www.typescriptlang.org/play?ts=4.1.2#code/GYVwdgxgLglg9mABAQwM6oKYCcoFUxYbIQAWyARgDYYAUA+gFyJgYBu2AlE63DACaIA3gF8AUKID0ExCFTIA5hlFQAngAcMiAILR4YACrqMqRAF5EAcmQXEAH0vkL40JFgJEhPiAjYaqKMhQGEwiADQougghqhpMOm4GRqjCHEKiiIioAO4wUKQ0xAkAdDEYqYLpGYgQaJpWFgyVVR4YUCBYSP6BSs18GMDIIJRQIU1VaJg4+ITEZFS0hXolRhxjGYRtHZkBQWNiGWJiohAIqHDURZRw8jSe3r5hQqVMFo4pHEA (change the version number to see the previous error)

Related Issues: ?

[hyzyla]

I've found a quick workaround for this problem for anyone who has the same issue:

Typescript Playground

// Use this literal instead of "never" to ensure that a switch statement is exhaustive.
// See issue: https://github.com/microsoft/TypeScript/issues/41707
type NeverStringLiteral = "__THIS_SHOULD_BE_UNREACHABLE__";

export function assertUnreachable(value: NeverStringLiteral): never {
  throw new Error(`Unreachable: ${value}`);
}

// usage
type ActionTypes = 'a' | 'b' | 'c'

function reducer(state: {}, action: {type: ActionTypes}) {
  switch(action.type) {
    case 'a':
      return state
    default: {
      // 🚫 Argument of type '"b" | "c"' is not assignable to parameter of type '"__THIS_SHOULD_BE_UNREACHABLE__"'
      assertUnreachable(action.type);
    }
  }
}

console.log(reducer({}, {type: 'b'}))