[Bug c++/68903] missing default initialization of member when combined with virtual inheritance

[spam at trzeci dot pl]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68903

--- Comment #1 from trzeci <spam at trzeci dot pl> ---
Created attachment 40290
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40290&action=edit
simpler testcase

should output: 3 3 3
usually outputs: 3 3 0

C::c remains uninitialized after construction