Conflicting declaration using constexpr and auto in C++11

问题

I'm experimenting with C++11, constexpr and auto.

I'm failing to understand why this code does not compile:

  template<class T, T t>
  struct TestEle2
    {

    };

template<class T, T t>
  struct DStruct {int a;};

  template<char t>
  struct TestEle2<char, t>
    {
    static constexpr auto f = 5;

    static constexpr auto g = &DStruct<char, t>::a;
    };

  template<char c>
  constexpr decltype(TestEle2<char, c>::f)
    TestEle2<char, c>::f ; // This compiles

  template<char c>
  constexpr decltype(TestEle2<char, c>::g)
    TestEle2<char, c>::g ; // This does not compile

Without definition I have linking problems. I know this problem has been fixed in C++17 but better understand fully C++11 now

[Edit] Error message is:

error: conflicting declaration ‘constexpr decltype (TestEle2<char, t>::g) TestEle2<char, t>::g’
         TestEle2<char, c>::g ;
                            ^
error: ‘TestEle2<char, t>::g’ has a previous declaration as ‘constexpr const auto TestEle2<char, t>::g’
         static constexpr auto g = &DStruct<char, t>::a;
                               ^
error: declaration of ‘constexpr const auto TestEle2<char, t>::g’ outside of class is not definition [-fpermissive]

[Edit 2] I'm using GCC 4.8.5

回答1:

Consider the following a long comment instead of an answer (sorry).

I don't know who is right (MSVS that accept all, g++ that accept f but refuse g or clang++ that refuse both f and g) but, if I understand correctly, this is a simplification of a more complex problem that you can't solve simply using int and int * instead of auto.

So I propose to insert a couple of types inside TestEle2

using typeF = decltype(5);
using typeG = decltype(&DStruct<char, t>::a);

and use them instead of auto and decltype() for f and g types.

The following is a full compiling (both g++ and c++; I don't know MSVS (sorry)) example

template <class T, T t>
struct TestEle2
 { };

template <class T, T t>
struct DStruct
 { int a; };

template <char t>
struct TestEle2<char, t>
 {
   using typeF = decltype(5);
   using typeG = decltype(&DStruct<char, t>::a);

   static constexpr typeF f = 5;
   static constexpr typeG g = &DStruct<char, t>::a;
 };

// This compiles
template <char c>
constexpr typename TestEle2<char, c>::typeF TestEle2<char, c>::f; 

// This also compile
template <char c>
constexpr typename TestEle2<char, c>::typeG TestEle2<char, c>::g;

int main()
 {
 }

来源：https://stackoverflow.com/questions/54345310/conflicting-declaration-using-constexpr-and-auto-in-c11