Checking that a template parameter is an enum

[Nordlöw via Digitalmars-d-learn]

How do I check that a template parameter is a CT-value or an enum 
symbol?

I want this to restrict the following template:
/** Returns: true iff all values $(D V) are the same. */
template allSame(V...)          // TODO restrict to values only
{
    static if (V.length <= 1)
        enum bool allSame = true;
    else
        enum bool allSame = V[0] == V[1] && allSame!(V[1..$]);
}

unittest
{
    static assert(!allSame!(41, 42));
    static assert(allSame!(42, 42, 42));
}