Jinja2Cpp: ParseError and assignment/swap -- Note: nonstd::expected uses swap() in operator=( expected const & other ).

Jinja2Cpp-ParseError.cpp

//
// If you write
//    ParseError(ParseError&& other) noexcept(true) = default;
// the type won't be treated as nothrow move-constructable.
// But if you write this ctor manually - everything is ok.

#include <initializer_list>
#include <utility>
#include <vector>

struct Token{};
struct ErrorCode{};
;

struct ParseError
{
    ParseError() = default;

    ParseError( ErrorCode code, Token tok )
    : errorCode( code )
    , errorToken( tok )
    {}

    ParseError( ErrorCode code, Token tok, std::initializer_list<Token> toks )
    : errorCode( code )
    , errorToken( tok )
    , relatedTokens( toks )
    {}

    ParseError( const ParseError& ) = default;

#if 1
    ParseError( ParseError&& other ) noexcept(true) = default;
#else
    ParseError( ParseError&& other ) noexcept
    : errorCode( std::move( other.errorCode ) )
    , errorToken( std::move( other.errorToken ) )
    , relatedTokens( std::move( other.relatedTokens ) )
    {}
#endif

    ErrorCode errorCode;
    Token errorToken;
    std::vector<Token> relatedTokens;
};

#include <type_traits>

// C++ language version detection (C++20 is speculative):
// Note: VC14.0/1900 (VS2015) lacks too much from C++14.

#ifndef   nsel_CPLUSPLUS
# if defined(_MSVC_LANG ) && !defined(__clang__)
#  define nsel_CPLUSPLUS  (_MSC_VER == 1900 ? 201103L : _MSVC_LANG )
# else
#  define nsel_CPLUSPLUS  __cplusplus
# endif
#endif

#define nsel_CPP98_OR_GREATER  ( nsel_CPLUSPLUS >= 199711L )
#define nsel_CPP11_OR_GREATER  ( nsel_CPLUSPLUS >= 201103L )
#define nsel_CPP14_OR_GREATER  ( nsel_CPLUSPLUS >= 201402L )
#define nsel_CPP17_OR_GREATER  ( nsel_CPLUSPLUS >= 201703L )
#define nsel_CPP20_OR_GREATER  ( nsel_CPLUSPLUS >= 202000L )

// type traits C++17:

namespace std17 {

#if nsel_CPP17_OR_GREATER

using std::conjunction;
using std::is_swappable;
using std::is_nothrow_swappable;

#else // nsel_CPP17_OR_GREATER

namespace detail {

using std::swap;

struct is_swappable
{
    template< typename T, typename = decltype( swap( std::declval<T&>(), std::declval<T&>() ) ) >
    static std::true_type test( int );

    template< typename >
    static std::false_type test(...);
};

struct is_nothrow_swappable
{
    // wrap noexcept(epr) in separate function as work-around for VC140 (VS2015):

    template< typename T >
    static constexpr bool test()
    {
        return noexcept( swap( std::declval<T&>(), std::declval<T&>() ) );
    }

    template< typename T >
    static auto test( int ) -> std::integral_constant<bool, test<T>()>{}

    template< typename >
    static std::false_type test(...);
};

} // namespace detail

// is [nothow] swappable:

template< typename T >
struct is_swappable : decltype( detail::is_swappable::test<T>(0) ){};

template< typename T >
struct is_nothrow_swappable : decltype( detail::is_nothrow_swappable::test<T>(0) ){};

// conjunction:

template< typename... > struct conjunction : std::true_type{};
template< typename B1 > struct conjunction<B1> : B1{};

template< typename B1, typename... Bn >
struct conjunction<B1, Bn...> : std::conditional<bool(B1::value), conjunction<Bn...>, B1>::type{};

#endif // nsel_CPP17_OR_GREATER

} // namespace std17


#if 0
void swap(ParseError &, ParseError &) noexcept;
#endif

#define STATIC_ASSERT(expr) \
    static_assert( expr, #expr )

int main()
{
    STATIC_ASSERT( std::is_nothrow_move_constructible<ParseError>::value );
    STATIC_ASSERT( std17::is_nothrow_swappable<ParseError>::value );
}

// cl -std:c++17 -EHsc -c effe.cpp
// cl -std:c++14 -EHsc -c effe.cpp

Note: nonstd::expected uses swap() in operator=( expected const & other ).

prompt> cl -std:c++14 -EHsc -c Jina2Cpp-ParseError.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.16.27030.1 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

Jina2Cpp-ParseError.cpp
Jina2Cpp-ParseError.cpp(141): error C2338: std17::is_nothrow_swappable<ParseError>::value

Idem for -std:c++17.

Ok with:

#if 1
void swap(ParseError &, ParseError &) noexcept;
#endif