Try to understand compiler error message: default member initializer required before the end of its enclosing class

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.

As of 2021 it's still not fixed, but as a work-around I'm using something like this:

namespace detail {
struct DownloaderHints
{       
    int32_t numOfMaxEasyHandles = 8;
};
}

class Downloader {
public:
    using Hints = details::DownloaderHints;
    static Downloader *Create(const Hints &hints = Hints());
};