Remove misconception about what never is assignable to.

[dwjohnston]

And because never is only assignable to another never

This isn't true!

never is assignable to a number for example, see for yourself.

function myFunction(a: number) {

}


function returnsNever() : never {
    throw new Error(); 
}


const a= returnsNever(); 
myFunction(a); // No type error here!

https://www.typescriptlang.org/play?ssl=15&ssc=1&pln=1&pc=1#code/FDBmFcDsGMBcEsD2kAEBbAngMSnJkAKAQwC4VJw0AjAUwCcBKFAbxAF8QxcFkU6bY4OpADOAORoA3egSZlIU+i2ApVKWAAs6iAO7kaegKJ1tdWQG4UwDiGjIRsFEQC8fAUNETpZhpeCYcGB5CIl8UAHpwlDFEdQwABxoUelMUDXoaAEJgIA

See documentation here:
https://www.typescriptlang.org/docs/handbook/basic-types.html#never

The never type is a subtype of, and assignable to, every type;

[dwjohnston]

I think you want to update your fail function to what I've got here for a proper exhaustiveness check:

https://stackoverflow.com/a/67684250/1068446