flag to check for type-unreachable code

feature request

the allowUnreachableCode flag (when disabled) only checks for code that's syntactically unreachable. however, the check doesn't work at all when looking at types.

i propose an additional flag (as @RyanCavanaugh suggested keeping these two concerns separate, see https://github.com/microsoft/TypeScript/issues/45267#issuecomment-901297945) that checks for code that the type checker believes is unreachable. for example:

declare function foo(): never

const a = foo()
console.log(1) //no error

foo()

console.log(1) //error

assertNever

in order to allow the assertNever pattern, the flag could still allow unreachable code where an argument of type never is passed, but not code after never is returned. this way, the following code would still be valid:

declare function assertNever(value: never): never
declare const foo: 1|2|3
switch(foo) {
    case 1:
        break
    case 2:
        break
    case 3:
        break
    default:
        assertNever(foo)
}

🔎 Search Terms

unreachable code

⏯ Playground Link

Playground link with relevant code

💻 Code

if (false) console.log("asdf") //error
if (false as false) console.log("asdf") //no error

Start with the linked TypeScript Playground and reproduce both examples using the existing allowUnreachableCode behavior. Read the type-checking and unreachable-code diagnostics around these cases, then define the flag's behavior for never returns and assertNever calls. Done means the proposed type-based unreachable checks and exceptions are covered by agreed compiler tests.