In templates, where and why do I have to put `typename` and `template` on dependent names?   
What exactly are dependent names anyway?  

I have the following code:

    template &lt;typename T, typename Tail&gt; // Tail will be a UnionNode too.
    struct UnionNode : public Tail {
        // ...
        template&lt;typename U&gt; struct inUnion {
            // Q: where to add typename/template here?
            typedef Tail::inUnion&lt;U&gt; dummy; 
        };
        template&lt; &gt; struct inUnion&lt;T&gt; {
        };
    };
    template &lt;typename T&gt; // For the last node Tn.
    struct UnionNode&lt;T, void&gt; {
        // ...
        template&lt;typename U&gt; struct inUnion {
            char fail[ -2 + (sizeof(U)%2) ]; // Cannot be instantiated for any U
        };
        template&lt; &gt; struct inUnion&lt;T&gt; {
        };
    };

The problem I have is in the `typedef Tail::inUnion&lt;U&gt; dummy` line. I&#39;m fairly certain that `inUnion` is a dependent name, and VC++ is quite right in choking on it.   
I also know that I should be able to add `template` somewhere to tell the compiler that inUnion is a template-id. But where exactly? And should it then assume that inUnion is a class template, i.e. `inUnion&lt;U&gt;` names a type and not a function?

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Where and why do I have to put the "template" and "typename" keywords?