Type string does not narrow when checked for equality with a string literal.

[aaronlahey]

TypeScript Version: 2.1.0-dev.20160918

Code

export function returnAnyString(): string {
  return "";
}

export function acceptsOnlyFoo(value: "foo"): void {
  // ...
}

const value = returnAnyString();

if(value === "foo")
  acceptsOnlyFoo(value); // error TS2345: Argument of type 'string' is not assignable to parameter of type '"foo"'.

Expected behavior:

The === to narrow type string to a string literal type when checking for equality with a string literal. I would expect the above example to be the same as the following example which does work...

export function returnAnyString(): string {
  return "";
}

export function acceptsOnlyFoo(value: "foo"): void {
  // ...
}

function isFoo(s: string): s is "foo" {
  return s === "foo";
}

const value = returnAnyString();

if(isFoo(value))
  acceptsOnlyFoo(value);

Actual behavior:

The type is still string.

[kitsonk]

Essential a dupe of #7447 which was supposed to be implemented in #9407 but doesn't appear to be working.