成员和成员函数的模糊部分模板专门化

我不明白为什么在使用&T::b和&T::c时，由于"类模板实例化不明确"，此代码无法在main()中编译。是g++4.6.1的bug吗？

#include <iostream>
#include <string>
using namespace std;
struct T{
    int a;
    void b(){}
    int c()
    { 
        return 1; 
    }
};
template<typename CT, CT> struct member_helper;
template<typename FT, FT(T::*mem)> 
struct member_helper<FT(T::*), mem> {
    static string worker()
    { 
        return "for members"; 
    }
};
template<typename Return, typename... Args, Return(T::*fun)(Args...)> 
struct member_helper<Return(T::*)(Args...), fun> {
    static string worker()
    { 
        return "for member functions returning non void"; 
    }
};
template<typename... Args, void(T::*fun)(Args...)> 
struct member_helper<void(T::*)(Args...), fun> {
    static string worker()
    { 
        return "for member functions returning void"; 
    }
};
int main() {
    cout << member_helper<decltype(&T::a), &T::a>::worker();    //prints for members, ok
    cout << member_helper<decltype(&T::b), &T::b>::worker();    //cannot distinguish between all of the three
    cout << member_helper<decltype(&T::c), &T::c>::worker();    //cannot distinguish between member function returning non void and member
}

编辑：

这是完整的错误信息：

g++ -O0 -g3 -Wall -c -fmessage-length=0 -std=c++0x -MMD -MP -MF"main.d" -MT"main.d" -o "main.o" "../main.cpp" 
../main.cpp: In function ‘int main()’: 
../main.cpp:27:45: error: ambiguous class template instantiation for ‘struct member_helper’ 
../main.cpp:13:43: error: candidates are: struct member_helper 
../main.cpp:17:78: error: struct member_helper 
../main.cpp:21:59: error: struct member_helper 
../main.cpp:27:8: error: incomplete type ‘member_helper’ used in nested name specifier
../main.cpp:28:45: error: ambiguous class template instantiation for ‘struct member_helper’ 
../main.cpp:13:43: error: candidates are: struct member_helper 
../main.cpp:17:78: error: struct member_helper 
../main.cpp:28:8: error: incomplete type ‘member_helper’ used in nested name specifier make: * [main.o] Errore 1

这是copmiler版本：

使用内置规格。COLLECT_GCC=/usr/bin/g++-4.6.realCOLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.6.1/LTO-WRAPPER目标：x86_64-linux-gnu配置为：/src/configure-v--with pkgversion="Ubuntu/Linaro 4.6.1-9ubuntu3"--带有bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs--启用语言=c，c++，fortran，objc，obj-c++，go--prefix=/usr/--program suffix=-4.6--enable shared--enable linker build id--带系统zlib--libexecdir=/usr/lib--不带gettext--enable threads=posix--带gxx include dir=/usr/include/c++/4.6--libdir=/usr/lib--enable nls--带sysroot=/--enable clocale=gnu--enable libstdcxx debug--enable librstdcxx time=yes--enable plugin--enable objc gc--disable werror--with-arch-32=i686--with-tune=generic--enable-checking=release--build=x86_64-linux-gnu--host=x86_64-linux-gnu--target=x86~64-linux-g努线程模型：posix gcc 4.6.1版（Ubuntu/Linaro 4.6.1-9ubuntu3）