Type guards don't work for interfaces with only optional properties

[domoritz]

For some reason type guards to not work if an interface has only optional properties.

interface A {
  foo : number;
}

function isA(object: any): object is A {
  return true;
}

let a: A | boolean;

if (isA(a)) {
  a.foo;   // compiles
}



interface B {
  foo? : number;
}

function isB(object: any): object is B {
  return true;
}

let b: B | boolean;

if (isB(b)) {
  b.foo;  // does not compile because type inference says its `B | boolean`
}

[mhegazy]

For a type guard, it will look through all the union type constituents (B and boolean in the example), and will filter out types that are not assignable to the target type. since B has only optional properties, booelan is assignable to, and it is not filtered out from the union. looks like a good candidate for #3842.

a work around is to tag type B, see #4895