PR : microsoft/TypeScript#8652
Issue : microsoft/TypeScript#3076
Concept : https://en.wikipedia.org/wiki/Bottom_type

Add bottom.md in tips.

Use case

// Without a bottom type we would error : 
// - Not all code paths return a value
// - Unreachable code detected
function foo(x: string | number) { 
  if (typeof x === "string") { 
    return true; 
  } else if (typeof x === "number") { 
    return false; 
  }
  fail("Unexhaustive!"); 
} 

function fail(message: string) { throw message; }

Confusion with void

void is a Unit. never is a falsum.

A function that returns nothing returns a Unit void. However a function that never returns (or always throws) returns never. void is something that can be assigned (wihout strictNullChecking) but never can never be assigned to anything other than never.