[BUG] Unable to compile cpp2 enum

[MatthieuHernandez]

I have a compilation error with Clang 16.0.5 when I try to declare an enum in cpp2. I use cppfront compiled from main branch (ccf7011).

compiler error

clang  -IC:/Programming/Git/gopp2/../cppfront/include -O0 -g -Xclang -gcodeview -D_DEBUG -D_DLL -D_MT -Xclang --dependent-lib=msvcrtd -std=gnu++20 -MD -MT CMakeFiles/gopp2.dir/generate/main.cpp.obj -MF CMakeFiles\gopp2.dir\generate\main.cpp.obj.d -o CMakeFiles/gopp2.dir/generate/main.cpp.obj -c C:/Programming/Git/gopp2/generate/main.cpp
In file included from ../src/main.cpp2:2:
In file included from ../src/engine.h2:2:
In file included from ../src/move.h2:1:
../src/color.h2:12:18: error: constexpr constructor never produces a constant expression [-Winvalid-constexpr]
constexpr Color::Color()
                 ^
../src/color.h2:13:51: note: read of non-constexpr variable 'Black' is not allowed in a constant expression
                                        : _value{ Black._value }{}
                                                  ^
../src/color.h2:7:36: note: declared here
inline CPP2_CONSTEXPR Color Color::Black{ 0 };
                                   ^
1 error generated.

color.h2

Color: @enum<u8> type = {
    Black := 0;
    White := 1;
}

color.h

#ifndef COLOR_H_CPP2
#define COLOR_H_CPP2

#define CPP2_IMPORT_STD          Yes

//=== Cpp2 type declarations ====================================================


#include "cpp2util.h"

#line 1 "../src/color.h2"
class Color;
#line 2 "../src/color.h2"
    

//=== Cpp2 type definitions and function declarations ===========================

#line 1 "../src/color.h2"
class Color {
private: cpp2::u8 _value; private: constexpr Color(cpp2::impl::in<cpp2::i64> _val);

private: constexpr auto operator=(cpp2::impl::in<cpp2::i64> _val) -> Color& ;
public: static const Color Black;
public: static const Color White;
public: [[nodiscard]] constexpr auto get_raw_value() const& -> cpp2::u8;
public: constexpr explicit Color();
public: constexpr Color(Color const& that);
public: constexpr auto operator=(Color const& that) -> Color& ;
public: constexpr Color(Color&& that) noexcept;
public: constexpr auto operator=(Color&& that) noexcept -> Color& ;
public: [[nodiscard]] auto operator<=>(Color const& that) const& -> std::strong_ordering = default;
public: [[nodiscard]] auto to_string_impl(cpp2::impl::in<std::string_view> prefix) const& -> std::string;
public: [[nodiscard]] auto to_string() const& -> std::string;
public: [[nodiscard]] auto to_code() const& -> std::string;
public: [[nodiscard]] static auto from_string(cpp2::impl::in<std::string_view> s) -> Color;
public: [[nodiscard]] static auto from_code(cpp2::impl::in<std::string_view> s) -> Color;

#line 4 "../src/color.h2"
};


//=== Cpp2 function definitions =================================================

#line 1 "../src/color.h2"

#line 1 "../src/color.h2"

constexpr Color::Color(cpp2::impl::in<cpp2::i64> _val)
                                                          : _value{ cpp2::unchecked_narrow<cpp2::u8>(_val) } {  }
constexpr auto Color::operator=(cpp2::impl::in<cpp2::i64> _val) -> Color&  { 
                                                          _value = cpp2::unchecked_narrow<cpp2::u8>(_val);
                                                          return *this; }
inline CPP2_CONSTEXPR Color Color::Black{ 0 };

inline CPP2_CONSTEXPR Color Color::White{ 1 };

[[nodiscard]] constexpr auto Color::get_raw_value() const& -> cpp2::u8 { return _value; }
constexpr Color::Color()
                                        : _value{ Black._value }{}
constexpr Color::Color(Color const& that)
                                              : _value{ that._value }{}
constexpr auto Color::operator=(Color const& that) -> Color& {
                                              _value = that._value;
                                              return *this;}
constexpr Color::Color(Color&& that) noexcept
                                              : _value{ std::move(that)._value }{}
constexpr auto Color::operator=(Color&& that) noexcept -> Color& {
                                              _value = std::move(that)._value;
                                              return *this;}
[[nodiscard]] auto Color::to_string_impl(cpp2::impl::in<std::string_view> prefix) const& -> std::string{

auto pref {cpp2::to_string(prefix)}; 
if ((*this) == Black) {return pref + "Black"; }
if ((*this) == White) {return cpp2::move(pref) + "White"; }
return "invalid Color value"; 
}

[[nodiscard]] auto Color::to_string() const& -> std::string { return to_string_impl(""); }
[[nodiscard]] auto Color::to_code() const& -> std::string { return to_string_impl("Color::"); }
[[nodiscard]] auto Color::from_string(cpp2::impl::in<std::string_view> s) -> Color{

auto x {s}; 
if ("Black" == x) {return Black; }
else {if ("White" == cpp2::move(x)) {return White; }
#line 1 "../src/color.h2"
}
CPP2_UFCS(report_violation)(cpp2::type_safety, CPP2_UFCS(c_str)(("can't convert string '" + cpp2::to_string(s) + "' to enum of type Color")));
return Black; 
}

[[nodiscard]] auto Color::from_code(cpp2::impl::in<std::string_view> s) -> Color{
std::string str {s}; return from_string(cpp2::string_util::replace_all(cpp2::move(str), "Color::", "")); }
#endif

[gregmarr]

Is it only when you create the separate header? There are no issues compiling that here: https://cpp2.godbolt.org/z/81jTWGM63

[JohelEGP]

It might be this:

cppfront/include/cpp2util.h

Lines 371 to 376 in ccf7011

	#if defined(_MSC_VER)
	// MSVC can't handle 'inline constexpr' variables yet in all cases
	#define CPP2_CONSTEXPR const
	#else
	#define CPP2_CONSTEXPR constexpr
	#endif

[gregmarr]

@JohelEGP Does clang ever set _MSC_VER?

[gregmarr]

I compared color.h above to the output of my cpp2.godbolt.org link, and found that the only differences are the include guard, the #define CPP2_IMPORT_STD Yes and the #line directives in color.h, and the main function in the godbolt output, so no clues there unless it's the CPP2_IMPORT_STD but I don't see how that would be.

[MatthieuHernandez]

@JohelEGP and @gregmarr thank you for helping me.
I compile with Clang but the STD comes from MSVC.
The CPP2_CONSTEXPR instruction is correctly replaced, however it's the const that doesn't work and the constexpr that does contrary to what the commentary says.

This doesn't work:

inline CPP2_CONSTEXPR Color Color::Black{ 0 }; // error: constexpr constructor never produces a constant expression

Because it become this:

inline const Color Color::Black{0}; // same error

But this work perfectly:

inline const Color Color::Black{0}; // same error

[gregmarr]

I took the command line shown in the original post, removed the clang -IC:/Programming/Git/gopp2/../cppfront/include and -MT CMakeFiles/gopp2.dir/generate/main.cpp.obj -MF CMakeFiles\gopp2.dir\generate\main.cpp.obj.d -o CMakeFiles/gopp2.dir/generate/main.cpp.obj -c C:/Programming/Git/gopp2/generate/main.cpp and put it in godbolt.org using x86-64 clang 16.0.0 with the color.h provided and replaced the include of cpp2util.h with its current contents, and it still builds fine.

[gregmarr]

@MatthieuHernandez I assume your last code block should have constexpr instead of const. Yes, just const isn't working, but the question is how are you getting that const? You should only get that if _MSC_VER is set.

[MatthieuHernandez]

I'm on Windows and my standard library comes from MSVC even though I compile with Clang, I think that's where it comes from..

[gregmarr]

_MSC_VER is supposed to come from the compiler, not the library. Ah, clang does set those. https://clang.llvm.org/docs/UsersManual.html#microsoft-extensions

That workaround for MSVC will need to be updated to not do it for clang. There is an example just above those lines of having the clang defines used first.

[MatthieuHernandez]

Perhaps a Clang under Windows CI pipeline should be added? I saw that there wasn't any.