编译器报告“已删除”运算符 = ，但它在那里

问：

我遇到了一个令人讨厌的问题，编译器声称删除了一个，但它就在那里。经过几个小时的尝试，我产生了一个重现该问题的最小解决方案。我正在使用 MSVC 社区版 2017 15.7.5（截至今天的最新版本，2018-07-20），并将其设置为“C++17”operator=

代码不是微不足道的;这个概念是模板类 TT 用于强制在一组类中强制存在静态成员函数。这与工厂模式非常相似，只是此解决方案不创建类实例，而是报告有关类的静态详细信息。fooFn

在最后一行的赋值中报告了错误，内容如下（底部有完整的错误列表）：

“错误 C2280：'C &C：：operator =（const C &）'：尝试引用 删除功能”

但是第 5 行定义了这个运算符，对那些装饰器吗？

赋值失败时会尝试将返回的变量赋值给类成员变量。
我以为正在产生问题，但是删除每个函数中的所有功能没有任何区别;同一行中的相同错误。const std::vector<C>&constconst

问：为什么编译器会报告此问题，可能的修复方法是什么？
我想这一定是我想念的傻东西，但我找不到它。

#include <vector>

class C
{
public:
  C(int ii) : i(ii) {}
  C& operator=(const C&) = default;    /// HERE is the assignment operator
  const int i;
};
typedef std::vector<C> CVec;   // shorthand for a vector of C's

template <class T>   // this template forces classes F1, F2, ... to have a static member function 'foo'
class TT {
public:
  static const CVec& foo(void) { return T::foo(); }
};

class F1  // one of many Fn classes
{
public:
  static const CVec& foo(void) { static CVec cv{ C{ 1 }, C{ 2 } }; return cv; }    // static member as forced by template
  //...
};
class F2  // another one of many Fn classes
{
public:
  static const CVec& foo(void) { static CVec cv{ C{ 3 } }; return cv; }     // static member as forced by template
  //...
};

class D    // controller class
{
public:
  CVec cv;
  const CVec& bar(int z)   // function to select one of the subclasses
  {
    switch (z)
    {
      case 1: return TT<F1>::foo();
      case 2: return TT<F2>::foo();
        //...
    }
  }

  void foobar(void)  //selector (from user input)
  {
    int z = 2; // user input
    cv = bar(z);   // THIS assignment produces the error
  }
};

错误全文：

------ Build started: Project: BG, Configuration: Debug Win32 ------
bgcore.cpp
c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.14.26428\include\xutility(2443): error C2280: 'C &C::operator =(const C &)': attempting to reference a deleted function
d:\projects\bg\core\bgcore.h(8): note: see declaration of 'C::operator ='
c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.14.26428\include\xutility(2462): note: see reference to function template instantiation '_OutIt std::_Copy_unchecked1<_InIt,_OutIt>(_InIt,_InIt,_OutIt,std::_General_ptr_iterator_tag)' being compiled
        with
        [
            _OutIt=C *,
            _InIt=C *
        ]
c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.14.26428\include\vector(1430): note: see reference to function template instantiation '_OutIt *std::_Copy_unchecked<_Iter,C*>(_InIt,_InIt,_OutIt)' being compiled
        with
        [
            _OutIt=C *,
            _Iter=C *,
            _InIt=C *
        ]
c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.14.26428\include\vector(1448): note: see reference to function template instantiation 'void std::vector<C,std::allocator<_Ty>>::_Assign_range<_Iter>(_Iter,_Iter,std::forward_iterator_tag)' being compiled
        with
        [
            _Ty=C,
            _Iter=C *
        ]
c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.14.26428\include\vector(1448): note: see reference to function template instantiation 'void std::vector<C,std::allocator<_Ty>>::_Assign_range<_Iter>(_Iter,_Iter,std::forward_iterator_tag)' being compiled
        with
        [
            _Ty=C,
            _Iter=C *
        ]
c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.14.26428\include\vector(1471): note: see reference to function template instantiation 'void std::vector<C,std::allocator<_Ty>>::assign<C*,void>(_Iter,_Iter)' being compiled
        with
        [
            _Ty=C,
            _Iter=C *
        ]
c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.14.26428\include\vector(1471): note: see reference to function template instantiation 'void std::vector<C,std::allocator<_Ty>>::assign<C*,void>(_Iter,_Iter)' being compiled
        with
        [
            _Ty=C,
            _Iter=C *
        ]
c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.14.26428\include\vector(1457): note: while compiling class template member function 'std::vector<C,std::allocator<_Ty>> &std::vector<_Ty,std::allocator<_Ty>>::operator =(const std::vector<_Ty,std::allocator<_Ty>> &)'
        with
        [
            _Ty=C
        ]
d:\projects\bg\core\bgcore.h(49): note: see reference to function template instantiation 'std::vector<C,std::allocator<_Ty>> &std::vector<_Ty,std::allocator<_Ty>>::operator =(const std::vector<_Ty,std::allocator<_Ty>> &)' being compiled
        with
        [
            _Ty=C
        ]
d:\projects\bg\core\bgcore.h(22): note: see reference to class template instantiation 'std::vector<C,std::allocator<_Ty>>' being compiled
        with
        [
            _Ty=C
        ]
Done building project "BG.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========