[RFC] libstdc++/31440

[Paolo Carlini <pcarlini at suse dot de>]

Hi,

we just got this PR: Maude doesn't compile anymore, as un unintended 
effect of some recent work of mine on _Rb_tree::lower_bound, 
upper_bound, etc. Shame on me, I have a patch ready and tested (the 
issue is only in mainline). However, before going ahead wanted to make 
sure we all agree we want to accept this kind of code:

#include <map>

// libstdc++/31440
struct DagNode
{ };

class MemoTable
{
public:
 void memoRewrite();

private:
 struct dagNodeLt;
 class MemoMap;

 MemoMap* memoMap;
};

struct MemoTable::dagNodeLt
{
 bool operator()(const DagNode*, const DagNode*);
};

class MemoTable::MemoMap
: public std::map<DagNode*, DagNode*, MemoTable::dagNodeLt>
{ };

void
MemoTable::memoRewrite()
{
 memoMap->find(0);
}

Indeed, the interesting observation is that at least another common 
implementation doesn't accept it: in such cases, we are always at risk 
of encouraging non-portability (assuming of course the Standard isn't 
100% clear about the case at issue). The problem pointed out is the same 
we have in mainline and the fix (constify operator()) also the same:

.../bits/stl_tree.h:938: error: passing 'const MemoTable::dagNodeLt' as 
'this' argument of 'bool MemoTable::dagNodeLt::operator()(const 
DagNode*, const DagNode*)' discards qualifiers

Opinions?

Thanks,
Paolo.