Bug or Feature: unsigned integer overflow

[Tobias Pankrath]

void main()
{
     auto x = 9223372036854775808; // long.max + 1
}

> onlineapp.d(3): Error: signed integer overflow

According to spec x should be of type ulong and this should 
compile? It indeed compiles if I add the uL postfix.

Is this a bug or indented behaviour?