MSetIterator traits mismatch operator*, breaks libc++

[Daniel Colascione]

Xapian::MSetIterator has typedefs that mark it STL-compatible, but they're incorrect.

Current master xapian-core/include/xapian/mset.h:

Xapian::docid operator*() const;
...
typedef std::string value_type;
typedef std::string& reference;

So operator* returns a numeric docid, but traits claim string/reference-to-string.

On libc++ this breaks things like std::insert into a std::vector<Xapian::docid>: libc++ validates against iterator traits and rejects the overload in confusion when it sees reference = std::string&.

#include <xapian.h>
#include <vector>

void f(const Xapian::MSet& mset) {
    std::vector<Xapian::docid> ids;
    ids.insert(ids.end(), mset.begin(), mset.end()); // fails on libc++
}

error: no matching member function for call to 'insert'
... candidate template ignored: requirement
'is_constructible<unsigned int, std::string &>::value' was not satisfied

MSetIterator traits should match dereference type (docid) so standard STL iterator operations work consistently.

[Olly Betts]

Thanks for spotting and reporting. Looks like this was probably copied from ESetIterator and has been wrong since it was added in 9d334768b8f94b6fd8d9c3445be1f5c051cbebac.

[Olly Betts]

Fixed in git master by c1ff519bda1dba5635b95ea78e4e0c4085ebdbc1; backported for 1.4.31 in 55c54246cc889a92b67357df77d9e881ff9a5d21.

As part of that I've added a testcase which checks value_type, reference and pointer are consistent with dereferencing for all our iterator types (this was the only incorrect set).

[Olly Betts]

Your code also fails to compile with libstdc++ if compiling with -D_GLIBCXX_DEBUG (as one of our CI jobs does), and similar testcases for our other iterators also break in this configuration - I've pushed (and backported) a fix for all of these too.