Test case:
a.d:
----
import b;
class Sub : Super
{
        int fail()
        {
                return Super.fail();
        }

}
----
b.d:
----
class Super
{
        int fail()
        out (result) {}
        body
        {
                assert(0);
        }
}
----
Compile with dmd -c a.d. If you compile with dmd a.d b.d the ICE does not occur. DMD outputs:
b.d(5): Error: function __ensure forward declaration
linkage = 0
dmd: tocsym.c:407: virtual Symbol* FuncDeclaration::toSymbol(): Assertion `0' failed.