The following code compiles:

import std.bigint;
import std.stdio;

public static void main() {
	writeln("Hello, world!");
	BigInt big = BigInt(0);
}

But this code causes the compiler to enter an endless loop:

import std.bigint;
import std.stdio;

public BigInt big = BigInt(0);

public static void main() {
	writeln("Hello, world!");
}

This seems to be generally true for any initialization of a BigInt variable outside a function.

Using dmd -v, with source code in bigtest.d, it hangs at "semantic2 bigtest".