Union types are not allowed in indexers #1715

I have declared function getter(key: string|number).
Inside I try to do var x = someObject[key]
I get a TS error saying "An index expression argument must be of type 'string', 'number' or 'any'.
Obviously key satisfies this constraint as it is either a string or a number. But I guess the TS compiler does hard type equality checks in this case.