http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51633
--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> 2012-01-11 13:52:11 UTC --- The testcase is valid because A's implicitly-declared default constructor is constexpr. We just need to fix build_constexpr_member_initializers to deal with the trees that we get for B().
