User errror or g++ disambiguation bug?

[Daniel Keller]

Hello,

If anyone can give me some insight as to why this code fails to compile, I
would be most appreciative. I have only been able to test it with gcc
3.4.4 and gcc 4.2.1, which both fail with different errors (details
below).

//////// begin test_bug.cpp

// Standard Type2Type from Alexandrescu's Modern C++ Design
template<typename T>
struct Type2Type
{
    typedef T OriginalType;
};

// Base for mix-in classes
template<typename Mixin>
struct MixinBase
{
    virtual ~MixinBase() {}
    virtual void mixinVirtual() {}

    static void mixinStatic(Type2Type<Mixin>) {}
};

struct MixinA
    : public MixinBase<MixinA>
{
};

struct MixinB
    : public MixinBase<MixinB>
{
};

// Base for mix-in classes in an object 
template <typename ObjType,typename Mixin>
struct ObjMixin
    : public Mixin
{
    void mixinVirtual()
    {
        ObjType::mixinStatic(Type2Type<Mixin>());
    }
};

// Object composed of two mix-in classes
struct TestObj
    : public ObjMixin<TestObj, MixinA>
    , public ObjMixin<TestObj, MixinB>
{
} testInstance;

//////// end test_bug.cpp

g++ 4.2.1 rejects this with the following output: ----------
% g++ test_bug.cpp
test_bug.cpp: In member function 
  'void ObjMixin<ObjType, Mixin>::mixinVirtual() 
   [with ObjType = TestObj, Mixin = MixinA]':
test_bug.cpp:44:   instantiated from here 
test_bug.cpp:35: error: reference to 'TestObj::mixinStatic' is ambiguous
test_bug.cpp:15: error: candidates are:
  static void MixinBase<Mixin>::mixinStatic(Type2Type<Mixin>)
   [with Mixin = MixinB]
test_bug.cpp:15: error:
  static void MixinBase<Mixin>::mixinStatic(Type2Type<Mixin>)
   [with Mixin = MixinA]
test_bug.cpp: In member function
  'void ObjMixin<ObjType, Mixin>::mixinVirtual() [with ObjType = TestObj,
  Mixin = MixinB]':
test_bug.cpp:44:   instantiated from here 
test_bug.cpp:35: error: reference to'TestObj::mixinStatic' is ambiguous
test_bug.cpp:15: error: candidates are: static void
  MixinBase<Mixin>::mixinStatic(Type2Type<Mixin>) 
   [with Mixin = MixinB]
test_bug.cpp:15: error:                 
  static void MixinBase<Mixin>::mixinStatic(Type2Type<Mixin>) 
   [with Mixin = MixinA]
----------

This would at least appear to be wrong, since the different values for
Mixin (MixinA and MixinB) should disambiguate the call.


g++ 3.4.4 rejects it with the following output: ----------
> g++ test_bug.cpp
test_bug.cpp: In member function 
  `void ObjMixin<ObjType,Mixin>::mixinVirtual() 
   [with ObjType = TestObj, Mixin = MixinA]':
test_bug.cpp:40:   instantiated from here
test_bug.cpp:32: error: `mixinStatic' is not a member of `TestObj'
test_bug.cpp: In member function 
  `void ObjMixin<ObjType, Mixin>::mixinVirtual()
   [with ObjType = TestObj, Mixin = MixinB]':
test_bug.cpp:40:   instantiated from here 
test_bug.cpp:32: error: `mixinStatic' is not a member of `TestObj'
----------

Again this seems wrong, removing ObjMixin<TestObj, MixinB> from the
definition of TestObj results in correct compilation under both versions,
so clearly mixinStatic is found as a member of TestObj in some cases.

Thanks in advance for any insight into what could be going on here.