Cannot iterate over union of types (used to be possible in Ts-2.3)

[apr-xaml]

TypeScript Version: 2.5.2

Code

type TypeDescription<T> = {[prop in keyof T]: undefined };

function getAllProperties<T>(typeDesc: (T|TypeDescription<T>)): ReadonlyArray<keyof T>
{
    const props: Array<keyof T> = [];

    for (var iPropName in typeDesc)
    {
        props.push(iPropName);
    }

    return props;
}

Expected behavior:
Should compile file (used to be in Ts-2.3)

Actual behavior:
Compile errors with "The right-hand side of 'for ... in' ...".

[mhegazy]

T extends object or T extends {} should avoid the error for now.

[gcnew]

Please, don't fix the new correct behaviour. It's been a bug in the compiler that has allowed this unsafe iteration.