Looks like a bug, since the following quotes prove that they are indeed compatible, and I couldn't find anything to disallow it.
12.3.2 Conversion functions [class.conv.fct]
2) A conversion function may be
explicit
[...]
[...]
5) Conversion functions can bevirtual
.
and
7.1.2 Function specifiers [dcl.fct.spec]
5) The
virtual
specifier shall be used only in the initial declaration of a non-static class member function; see 10.3.
6) Theexplicit
specifier shall be used only in the declaration of a constructor or conversion function within its class definition; see 12.3.1 and 12.3.2.