[BUG] Constant static members not initialized inside class definition

[vladvrabie]

Describe the bug
I have a Cpp1 class with some integral const static members initialized inside class. This allows me to use them to declare other members, notable std::array. Tried to port the class to Cpp2, but the static members are initialized outside the class, so I cannot use them to declare a std::array.

To Reproduce
Steps to reproduce the behavior:

1. Sample code
   My original Cpp1 code

class Piece {};
class Board {
public:
  static const size_t kWidth{ 4 };
  static const size_t kHeight{ 4 };
  static const size_t kSize{ kWidth * kHeight };
private:  std::array<std::optional<Piece>, kSize> m_pieces;
};

Cpp2 code:

Piece: type = {}
Board: type = {
  kWidth: size_t == 4;
  kHeight: size_t == 4;
  kSize: size_t == kWidth * kHeight;

  m_pieces: std::array<std::optional<Piece>, kSize> = ();
}

main: () = {
  b: Board = ();
}

2. Command lines including which C++ compiler you are using
   For Cpp2: cppfront.exe main.cpp2 -import-std
   Visual Studio Community 2022 (64-bit) Version 17.10.5
   Microsoft (R) C/C++ Optimizing Compiler Version 19.40.33813 for x86
   Project -> Properties -> Configuration Properties -> C/C++ -> Command Line:

/JMC /experimental:module /permissive- /MP /ifcOutput "x64\Debug\" /GS /W3 /Zc:wchar_t /ZI /Gm- /Od /sdl /Fd"x64\Debug\vc143.pdb" /Zc:inline /fp:precise /D "_DEBUG" /D "_CONSOLE" /D "_UNICODE" /D "UNICODE" /errorReport:prompt /WX- /Zc:forScope /RTC1 /Gd /MDd /std:c++latest /FC /Fa"x64\Debug\" /EHsc /nologo /Fo"x64\Debug\" /Fp"x64\Debug\GameCpp2.pch" /diagnostics:column 

3. Expected result
   To compile to my original code or to compile to
   static constexpr size_t kWidth{ 4 }; // omitting the rest for brevity
4. Actual result/error
   Error: Code C2975, '_Size': invalid template argument for 'std::array', expected compile-time constant expression
   Generated Cpp1 code (normally i don't use -clean, but I'm pasting here the clean version)

#define CPP2_IMPORT_STD          Yes
#include "cpp2util.h"

class Piece;
class Board;
  
class Piece {
  public: Piece() = default;
  public: Piece(Piece const&) = delete; /* No 'that' constructor, suppress copy */
  public: auto operator=(Piece const&) -> void = delete;
};

class Board {
  public: static const size_t kWidth;
  public: static const size_t kHeight;
  public: static const size_t kSize;

  private: std::array<std::optional<Piece>,kSize> m_pieces {}; 
  public: Board() = default;
  public: Board(Board const&) = delete; /* No 'that' constructor, suppress copy */
  public: auto operator=(Board const&) -> void = delete;
};

auto main() -> int;

inline CPP2_CONSTEXPR size_t Board::kWidth{ 4 };
inline CPP2_CONSTEXPR size_t Board::kHeight{ 4 };
inline CPP2_CONSTEXPR size_t Board::kSize{ kWidth * kHeight };

auto main() -> int{
  Board b {}; 
}

Additional context
cppreference.com static members

[JohelEGP]

#706 attempts to fix this.
See also #700.