[Bug c++/66698] New: Multiple inheritance from instantiations of template class and what about access to member functions

[apyszczuk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66698

            Bug ID: 66698
           Summary: Multiple inheritance from instantiations of template
                    class and what about access to member functions
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: apyszczuk at gmail dot com
  Target Milestone: ---

Let's take a look at the code:

template <typename C>
class S {
public:
    void add (C c) { ++cnt; }
    size_t size () const { return cnt; }

private:
    size_t cnt {}; 
};

struct Foo1 {};
struct Foo2 {};
struct Foo3 {};

class Z : public S<Foo1>, public S<Foo2>, public S<Foo3> {
public:
    using S<Foo1>::add;
    using S<Foo2>::add;
    using S<Foo3>::add;

    using S<Foo1>::size;    // (1)
    using S<Foo2>::size;    // (2)
    using S<Foo3>::size;    // (3)
};

And usage looks like this:

Z z;

z.add (Foo1 {});
z.add (Foo1 {});
z.add (Foo2 {});

cout << z.size () << endl;

This code compiles fine with gcc-5.1 (c++11), but this code does not compile
under clang-3.5 (c++11 - sorry, I do not have newer version of clang).

Clang produces "error: call to member function 'size' is ambiguous" which is
basically (from my point of view) correct, but gcc compiles it and returns 2.

Ok, but here is much more fun, if I switch the order of lines marked with
comments (1) and (2), to get something like this:

using S<Foo2>::size;    // (2)
using S<Foo1>::size;    // (1)

The code still compiles on gcc and the result is: 1.

As you can imagine, if you write line (3) before these two, you will get 0.

So, from what I see, gcc gets first using declaration of S<C>::size, ignores
rest of them and uses this one.

Is it a bug?

This question (bug) can be seen at
http://stackoverflow.com/questions/31104690/multiple-inheritance-from-instantiations-of-template-class-and-what-about-access
but I didn't receive reasonable answer there.

Best,

Artur