Type guard fails to narrow type with generic parameters

[hesselink]

The following code type checks:

class C
{
    foo: number;
}
function foo (x : C | Array<C>) : C
{
    if (x instanceof C)
      return x;
    return x[0];
}

However, if we add a type parameter to C, the following fails:

class C<A>
{
    foo: A;
}
function foo<A> (x : C<A> | Array<C<A>>) : C<A>
{
    if (x instanceof C)
      return x;
    return x[0];
}

There is an error on line 8 (return x;):

Type 'C<A> | C<A>[]' is not assignable to type 'C<A>'. Type 'C<A>[]' is not assignable to type 'C<A>'. Property 'foo' is missing in type 'C<A>[]'.

I've tried a couple of other things, but can't get this to type check. Is this a bug, or am I missing an obvious solution?

[mhegazy]

looks like the same issue in #2388