Invalid operands to binary expression when performing lstFolders.sort()

The last kopano-core snapshot (1281) does not build with clang on my OpenBSD/amd64 system. The snapshot before did. Here is the error. I am investigating this, but in the meantime I thought someone more knowledgeable from the inside might fix it faster.

In file included from provider/libserver/ECConvenientDepthObjectTable.cpp:18:
In file included from ./common/include/kopano/platform.h:30:
In file included from ./common/include/kopano/platform.linux.h:26:
In file included from /usr/include/c++/v1/vector:266:
In file included from /usr/include/c++/v1/__bit_reference:15:
/usr/include/c++/v1/algorithm:708:71: error: invalid operands to binary expressi
on ('const KC::FOLDERINFO' and 'const KC::FOLDERINFO')
    bool operator()(const _T1& __x, const _T1& __y) const {return __x < __y;}
                                                                  ~~~ ^ ~~~
/usr/include/c++/v1/list:2238:13: note: in instantiation of member function 'std
::__1::__less<KC::FOLDERINFO, KC::FOLDERINFO>::operator()' requested here
        if (__comp(*--__e2, *__f1))
            ^
/usr/include/c++/v1/list:2224:5: note: in instantiation of function template spe
cialization 'std::__1::list<KC::FOLDERINFO, std::__1::allocator<KC::FOLDERINFO>
>::__sort<std::__1::__less<KC::FOLDERINFO, KC::FOLDERINFO> >' requested here
    __sort(begin(), end(), base::__sz(), __comp);
    ^
/usr/include/c++/v1/list:2215:5: note: in instantiation of function template spe
cialization 'std::__1::list<KC::FOLDERINFO, std::__1::allocator<KC::FOLDERINFO>
>::sort<std::__1::__less<KC::FOLDERINFO, KC::FOLDERINFO> >' requested here
    sort(__less<value_type>());
    ^
provider/libserver/ECConvenientDepthObjectTable.cpp:179:13: note: in instantiati
on of member function 'std::__1::list<KC::FOLDERINFO, std::__1::allocator<KC::FO
LDERINFO> >::sort' requested here
        lstFolders.sort();
                   ^

Making the compare operator const fixed the error for me (which makes sense since we are calling the function from a const object).

bool operator <(const FOLDERINFO &a) const {

Diff here.