You want: (Achievable with virtual inheritance)
A
/ \
B C
\ /
D
And not: (What happens without virtual inheritance)
A A
| |
B C
\ /
D
Virtual inheritance means that there will be only 1 instance of the base A class not 2.
Your type D would have 2 vtable pointers (you can see them in the first diagram), one for B and one for C who virtually inherit A. D‘s object size is increased because it stores 2 pointers now; however there is only one A now.
So B::A and C::A are the same and so there can be no ambiguous calls from D. If you don’t use virtual inheritance you have the second diagram above. And any call to a member of A then becomes ambiguous and you need to specify which path you want to take.
Wikipedia has another good rundown and example here