The following code does not compile

    char[] unsignedToTempString(uint radix = 10)(ulong value, return scope char[] buf) @safe
        if (radix >= 2 && radix <= 16)
    {
        size_t i = buf.length;
        do
        {
            uint x = void;
            if (value < radix)
            {
                x = cast(uint)value;
                value = 0;
            }
            else
            {
                x = cast(uint)(value % radix);
                value /= radix;
            }
            buf[--i] = cast(char)((radix <= 10 || x < 10) ? x + '0' : x - 10 + 'a');
        } while (value);
        return buf[i .. $];
    }

    char[] myToString(ulong n)
    {
        char[20] buf;
        auto s = unsignedToTempString(n, buf);
        return s ~ (n > uint.max ? "UL" : "U");
    }

    enum a = myToString(5);

Fails with 

Error: array concatenation of expression `cast(const(char)[])s ~ (n > 4294967295LU ? "UL" : "U")` requires the GC which is not available with -betterC