MSVC warning C4512 at /W4 (operator= can't be created)

[Stefan van Kessel <van_kessel@…>]

At /W4 msvc10 warns about not being able to generate the assignment operator for 4 classes.

Example code that generates the warnings:

#include <boost/bimap.hpp>
int main(){
	boost::bimap< int, char > bm;
}

As the boost warnings guidelines​https://svn.boost.org/trac/boost/wiki/Guidelines/WarningsGuidelines state "Adding the declaration (not the definition) of the appropriate operator=() as a private member does the trick as well." (disabling the warning by pragma would have to still be active at the point of bimap instantiation and thus disable the warning in user code)

[Stefan van Kessel <van_kessel@…>]

patch declaring operator= for classes for which it can't be generated

[Stefan van Kessel <van_kessel@…>]

There are template parameters for which the compiler can generate the operator= and for which just declaring it will therefore break some of the classes. A correct fix that works in all contexts is not as easy as I thought. A traits class like is_assignable for the member and base class types would be needed to only declare the operator= if either of the aforementioned is not assignable.