https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53360
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
If we add this to expression it compiles OK:
expression& operator=(const expression&) { return *this; }
So the invalid instantiation of column::operator=<column>(const column&) comes
from the compiler-generated copy assignment operator. That presumably gets
defined during overload resolution.
I don't know what the difference is between C++98 and C++11 that causes that
(or if it's reasonable to expect a c++11-compat warning).
