The declaration of a special member can require overload resolution to be

performed, to determine whether that special member is deleted or constexpr.
That overload resolution process can in turn trigger the instantiation of a
template, which can do anything, including triggering the declaration of that
very same special member function. When this happens, do not try to recursively
declare the special member -- that's impossible. Instead, only try to realise
the truth. There is no special member.

llvm-svn: 168847