Using a char value >= 128

[Per Nordlöw via Digitalmars-d-learn]

In which circumstances can a `char` be initialized a non-7-bit 
value (>= 128)? Is it possible only in non-@safe code?

And, if so, what will be the result of casting such a value to 
`dchar`? Will that result in an exception or will it interpret 
the `char` using a 8-bit character encoding?

I'm asking because I'm pondering about how to specialize the 
non-7-bit `needle`-case of the following array-overload of 
`startsWith` when `T` is `char`:

bool startsWith(T)(scope const(T)[] haystack,
                   scope const T needle)
{
    static if (is(T : char)) { assert(needle < 128); } // TODO 
convert needle to `char[]` and call itself
    if (haystack.length >= 1)
    {
        return haystack[0] == needle;
    }
    return false;
}