When an interface is inherited from multiple sources this will result in undefined functions.

This occurs when a class inherits an interface from another class and from another class and from the interface itself.  See the source for the when this occurs.

This will also only occur when the inheritance is separated.  If all definitions reside in the same file then the undefined reference goes away.  

This looks like it may be thunk related.

$ gdc A.d B.d
/tmp/ccg0h4le.o:(.data+0x180): undefined reference to `_DT24_D1A3C_A1aMFZb'
collect2: error: ld returned 1 exit status

$ nm -A *.o | grep D1A3C_A1aMFZb
A.o:0000000000000000 T _D1A3C_A1aMFZb
A.o:000000000000006c T _DT16_D1A3C_A1aMFZb
B.o:                 U _D1A3C_A1aMFZb
B.o:                 U _DT24_D1A3C_A1aMFZb

$ gdc main.d
$ # Success

$ nm -A main.o | grep A1aMFZb
main.o:0000000000000000 T _D4main3C_A1aMFZb
main.o:0000000000000072 T _DT16_D4main3C_A1aMFZb
main.o:000000000000006c T _DT24_D4main3C_A1aMFZb


=== B.d === 
module B;

import A;

interface I_B : I_A
{
    void b();
}

abstract class C_B : C_A, I_B
{
        abstract void b();
}

// required to get it to try linking.
void main(){}

=== A.d ===

module A;

interface I_A
{
        bool a();
}


class C_A : I_A
{
        bool a()
        { return false; }
}


=== main.d ===
interface I_A
{
        bool a();
}


class C_A : I_A
{
        bool a()
        { return false; }
}

interface I_B : I_A
{
    void b();
}

abstract class C_B : C_A, I_B
{
        abstract void b();
}

void main(){}