成员和成员函数的模糊部分模板特化

我不明白为什么这个代码在使用 &T::b 和 &T::c 时由于"ambiguous class template instantiation"而无法在 main() 中编译 . 这是g 4.6.1的错误吗？

#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：

这是完整的错误消息：

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.real COLLECT_LTO_WRAPPER = / usr / lib / gcc / x86_64-linux-gnu / 4.6.1 / lto-wrapper目标：x86_64-linux-gnu配置为：../ src / configuration -v --with-pkgversion ='Ubuntu / Linaro 4.6.1-9ubuntu3'--with-bugurl = file：///usr/share/doc/gcc-4.6/README.Bugs --enable-languages = c， c，fortran，objc，obj-c，go --prefix = / usr --program-suffix = -4.6 --enable-shared --enable-linker-build-id --with-system-zlib --libexecdir = / usr / lib --without-included-gettext --enable-threads = posix --with-gxx-include-dir = / usr / include / c /4.6 --libdir = / usr / lib --enable-nls - -with-sysroot = / --enable-clocale = gnu --enable-libstdcxx-debug --enable-libstdcxx-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-gnu线程模型：posix gcc版本4.6.1（Ubuntu / Linaro 4.6.1-9ubuntu3）