New rule: ensure that all code paths return a value

[adidahiya]

tsc doesn't report any errors for code like this:

function foo(): string {
  if (false) {
    return "abc";
  }
}

It'd be nice to have a tslint rule that does control flow analysis to warn against that.

[aluanhaddad]

@adidahiya

This is an error since TypeScript 1.8.

See microsoft/TypeScript#4788 and http://www.typescriptlang.org/play/#src=function%20foo()%3A%20string%20%7B%0D%0A%20%20if%20(false)%20%7B%0D%0A%20%20%20%20return%20%22abc%22%3B%0D%0A%20%20%7D%0D%0A%7D

[andy-hanson]

Can this be closed then?