Btukfyl

I try next code with three compilers (msvc2017, gcc8.2, clang7.0) and msvc2017 works all the way, but gcc and clang not. I want to understand what is wrong with my code, and why compiler can't compile it.

#include <cassert>

#include <iostream>

#include <cstdlib>



class Downloader

{

public:



    struct Hints

    {       

        int32_t numOfMaxEasyHandles = 8;

        //Hints(){}          // <= if I uncomment this all works gcc+clang+msvc

        //Hints() = default; // <= if I uncomment this neither clang no gcc works (msvc - works)

    };



    static Downloader *Create(const Hints &hints = Hints());

};



Downloader* Downloader::Create(const Hints &hints)

{

    std::cout << hints.numOfMaxEasyHandles << std::endl;

    return nullptr;

}



int main()

{

    return 0;

}

You can play with this code yourself on https://wandbox.org/ and see error:

prog.cc:16:58: error: default member initializer for 'Downloader::Hints::numOfMaxEasyHandles' required before the end of its enclosing class

     static Downloader *Create(const Hints &hints = Hints());

                                                          ^

prog.cc:11:37: note: defined here

         int32_t numOfMaxEasyHandles = 8;

                                     ^~~~

Why gcc and clang not compile this code even with uncomment Hints() = default?
My compile commands:

1. $ g++ prog.cc -std=gnu++2a


2. $ clang++ prog.cc -std=gnu++2a

But if I uncomment Hints(){} all three compilers works. Maybe it is compiler bug? Thanks in advance.

This is a clang and gcc bug, we have a clang bug report for this: default member initializer for 'm' needed within definition of enclosing class for default argument of function which has the following example:

#include <limits>

class A

{

   public:

      class B

      {

         public:

            explicit B() = default;

            ~B() = default;



         private:

            double m = std::numeric_limits<double>::max();

      };



   void f(double d, const B &b = B{}) {}

};



int main()

{

   A a{};

   a.f(0.);

}

which produces the following similar diagnostic:

t.cpp(15,34):  error: default member initializer for 'm' needed within definition of enclosing class 'A' outside of member functions

   void f(double d, const B &b = B{}) {}

                                 ^

t.cpp(12,20):  note: default member initializer declared here

            double m = std::numeric_limits<double>::max();

                   ^

Richard Smith indicates this is a bug:

Regarding comment#0: if we want to fix this once-and-for-all, we should use the same technique we use for delayed template parsing: teach Sema to call back into the parser to parse the delayed regions on-demand. Then we would only reject the cases where there's an actual dependency cycle.

Although does not explain why in details.