This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 112e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
According to 11.9.3 [class.base.init] paragraph 7,
A mem-initializer where the mem-initializer-id denotes a virtual base class is ignored during execution of a constructor of any class that is not the most derived class.
Presumably “ignored” here means that there will be no runtime effect but that semantic restrictions such as access checking and the ODR must still be applied, but this is not completely clear.
Rationale (October, 2015):
The fact that “ignored” applies only to runtime effects is indicated by the phrase “during execution” in the existing wording. This seems clear enough.