“no viable conversion” with lemon for clang but valid for g++

Question

Currently, I try to incorporate lemon library in our project. Most developers are on Windows, they compile with MSVC, but I am in charge (for this part) to compile with gcc and clang.

I came across an error with clang that gcc does not reproduce and I managed to reduce the code:

#include <lemon/dfs.h>

int main() {
    lemon::ListDigraph g{};
    lemon::ListDigraph::Node node = g.nodeFromId(42);
    lemon::Dfs<lemon::ListDigraph> dfs{g};
    lemon::SimplePath<lemon::ListDigraph> path = dfs.path(node);
    return 0;
}  

With gcc, no errors.

/usr/bin/g++-5 -std=c++11 -Wall -O3  -I${SRC_ROOT}/external/lemon/latest -I${BIN_ROOT}/lemon -o ${TMP_ROOT}/core/src/core.cpp.o -c ${SRC_ROOT}/core/src/core.cpp

But with clang:

/usr/bin/clang++-3.7 -std=c++11 -Wall -stdlib=libc++ -O3 -I${SRC_ROOT}/external/lemon/latest -I${BIN_ROOT}/lemon -o ${TMP_ROOT}/core/src/core.cpp.o -c ${SRC_ROOT}/core/src/core.cpp

In file included from ${SRC_ROOT}/core/src/core.cpp:1:
In file included from ${SRC_ROOT}/external/lemon/latest/lemon/dfs.h:31:
${SRC_ROOT}/external/lemon/latest/lemon/path.h:408:23: error: no viable
      conversion from 'typename PredMapPath<ListDigraph, NodeMap<Arc> >::RevArcIt' to
      'lemon::ListDigraphBase::Arc'
        data[index] = it;;
                      ^~

Notes:

• SRC_ROOT, BIN_ROOT, TMP_ROOT are replaced for readability
• the snippet source code won't work, but should compile (I will correct the real big code)
• I really need the c++11 features for the real source code.
• gcc 5
• clang 3.7
• lemon 1.3.1

Questions :

• Did I forget to clang a flag?
• Does lemon is fully compatible with clang?
• How to solve this error?

Answer 1

It's a double dispatch related problem

The code in http://lemon.cs.elte.hu/hg/lemon/file/9fd86ec2cb81/lemon/path.h#l443

template <typename CPath>
void buildRev(const CPath& path) {
    int len = path.length();
    data.resize(len);
    int index = len;
    for (typename CPath::RevArcIt it(path); it != INVALID; ++it) {
        --index;
        data[index] = it;; // sic!
    }
}

relies on this user-defined cast-operator of the iterator on the right hand side

http://lemon.cs.elte.hu/hg/lemon/file/9fd86ec2cb81/lemon/bits/path_dump.h#l139

operator const typename Digraph::Arc() const {
    return path->predMatrixMap(path->source, current);
}

but the left hand type of the assigment expression

http://lemon.cs.elte.hu/hg/lemon/file/9fd86ec2cb81/lemon/list_graph.h#l89

class Arc {
    friend class ListDigraphBase;
    friend class ListDigraph;
protected:
    int id;
    explicit Arc(int pid) { id = pid;}
public:
    Arc() {}
    Arc (Invalid) { id = -1; }
    bool operator==(const Arc& arc) const {return id == arc.id;}
    bool operator!=(const Arc& arc) const {return id != arc.id;}
    bool operator<(const Arc& arc) const {return id < arc.id;}
};

doesn't have a custom assignment operator, but a single-argument custom ctor that clang tries to match against conversions of the right side. And fails.

Patching the right side of above shown lemon/path.h, line #443 with an simple explicit cast operator invocation

   data[index] =  it.operator const typename Digraph::Arc();; 

makes the code at least compile with clang (3.5).

The lemon-developers must decide whether this is desired behavior; a bug report should be filed for this.