Searching through closed Core issues
I found that DMC compiles:

------------a.cpp----------------
namespace NS {
       struct A { friend struct B; };
    }
    struct NS::B { };


int main()
{
}
--------------------------------------

which should fail according to resolved issue #165
(their explanation: Entities whose names are "invisibly injected" into
a namespace as a result of friend declarations are not "declared"
in that namespace until an explicit declaration of the entity
appears at namespace scope.)

Comeau fails to compile it.

/Pavel