Bond generated headers don't work for clang in c++20

[tiagomacarios]

Bond generates code as follows:
https://godbolt.org/z/a4qnbcjeh

#include <vector>

struct forward_decl;

struct S{
    std::vector <forward_decl> m;

    S(){};
};

struct forward_decl{};

But this does not work in C++20 (in clang) because the constructor will be instantiated with the forward declaration and compilation will fail.

A possible fix is to move the constructor body until after all entities have bee fully defined:
https://godbolt.org/z/6YjarYx3c

#include <vector>

struct forward_decl;

struct S {
    std::vector<forward_decl> m;

    S();
};

struct forward_decl {};

inline S::S() {}

Office is currently hitting this when trying to update to C++20.

[chwarr]

This change makes sense and should be backward compatible to C++11 as well. Feel free to submit a PR with the changes to code generation.

Does S::S() {} need to be marked inline as it won't be inside a class definition?

[tiagomacarios]

yes it will need inline. I am not familiar with Haskell, nor do I have an environment set. I presume this is a simple change, could you author it? This is currently blocking us. If you prefer I can create an internal bug on you team - just provide me the correct area path.