https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65299
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
EDG rejects it for the same reason as GCC:
"c.cc", line 20: error: more than one operator "++" matches these operands:
"C::operator++" (ambiguous by inheritance)
operand types are: C ++
c++;
^
"c.cc", line 21: error: more than one operator "++" matches these operands:
"C::operator++" (ambiguous by inheritance)
operand types are: ++ C
++c;
^
