[BUG] Move/forward on last use doesn't apply to generated code

[JohelEGP]

Title: Move/forward on last use doesn't apply to generated code.

Description:

@union generates set functions with a forward parameter pack.
But the lowered code doesn't forward the pack on its last use.

Minimal reproducer (https://cpp2.godbolt.org/z/E9Eq91nEo):

v: @union @print type = {
  i: int;
}
main: () = { }

Commands:

cppfront main.cpp2
clang++18 -std=c++23 -stdlib=libc++ -lc++abi -pedantic-errors -Wall -Wextra -Wconversion -Werror=unused-result -Werror=unused-value -Werror=unused-parameter -I . main.cpp

Expected result:

Same as when you copy the @printed code to the Cpp2 source (https://cpp2.godbolt.org/z/fa4T55hGE):

    auto v::set_i(

        auto&& ..._args
    ) & -> void
    {
        if (!(is_i()))
        {
            _destroy();
            std::construct_at(reinterpret_cast<int*>(&_storage), CPP2_FORWARD(_args)...);
        }
        else
        {
            *cpp2::assert_not_null(reinterpret_cast<int*>(&_storage)) = int{CPP2_FORWARD(_args)...};
        }
        _discriminator = 0;
    }

Actual result and error:

No implicit forward on last use in the Cpp1 code lowered from generated Cpp2 code.

auto v::set_i(auto&& ..._args) & -> void{if (!(is_i())) {_destroy();std::construct_at(reinterpret_cast<int*>(&_storage), _args...);}else {*cpp2::assert_not_null(reinterpret_cast<int*>(&_storage)) = int{_args...};}_discriminator = 0;}

Cpp2 lowered to Cpp1:

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


#include "cpp2util.h"

#line 1 "/app/example.cpp2"
class v;
#line 2 "/app/example.cpp2"


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

#line 1 "/app/example.cpp2"
class v {
private: std::aligned_storage_t<cpp2::max(sizeof(int))> _storage {}; private: cpp2::i8 _discriminator {-1}; public: [[nodiscard]] auto is_i() const& -> bool;
public: [[nodiscard]] auto i() const& -> int const&;
public: [[nodiscard]] auto i() & -> int&;
public: auto set_i(cpp2::in<int> _value) & -> void;
public: auto set_i(auto&& ..._args) & -> void;
private: auto _destroy() & -> void;
public: ~v() noexcept;
public: explicit v();
public: v(v const& that);

public: v(v&& that) noexcept;
public: auto operator=(v const& that) -> v& ;
public: auto operator=(v&& that) noexcept -> v& ;

#line 3 "/app/example.cpp2"
};
auto main() -> int;

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

#line 1 "/app/example.cpp2"


#line 1 "/app/example.cpp2"
[[nodiscard]] auto v::is_i() const& -> bool { return _discriminator == 0; }
[[nodiscard]] auto v::i() const& -> int const& {
                                           cpp2::Default.expects(is_i(), "");return *cpp2::assert_not_null(reinterpret_cast<int const*>(&_storage)); }
[[nodiscard]] auto v::i() & -> int& {
                                                 cpp2::Default.expects(is_i(), "");return *cpp2::assert_not_null(reinterpret_cast<int*>(&_storage)); }
auto v::set_i(cpp2::in<int> _value) & -> void{if (!(is_i())) {_destroy();std::construct_at(reinterpret_cast<int*>(&_storage), _value);}else {*cpp2::assert_not_null(reinterpret_cast<int*>(&_storage)) = _value;}_discriminator = 0;}
auto v::set_i(auto&& ..._args) & -> void{if (!(is_i())) {_destroy();std::construct_at(reinterpret_cast<int*>(&_storage), _args...);}else {*cpp2::assert_not_null(reinterpret_cast<int*>(&_storage)) = int{_args...};}_discriminator = 0;}
auto v::_destroy() & -> void{
if (_discriminator == 0) {std::destroy_at(reinterpret_cast<int*>(&_storage));}
_discriminator = -1;
}

v::~v() noexcept{_destroy();}
v::v(){}
v::v(v const& that)
        : _storage{  }
        , _discriminator{ -1 }{
if (CPP2_UFCS(is_i)(that)) {set_i(CPP2_UFCS(i)(that));}
}

v::v(v&& that) noexcept
        : _storage{  }
        , _discriminator{ -1 }{
if (CPP2_UFCS(is_i)(std::move(that))) {set_i(CPP2_UFCS(i)(std::move(that)));}
}

auto v::operator=(v const& that) -> v& {
if (CPP2_UFCS(is_i)(that)) {set_i(CPP2_UFCS(i)(that));}
        return *this;
}

auto v::operator=(v&& that) noexcept -> v& {
if (CPP2_UFCS(is_i)(std::move(that))) {set_i(CPP2_UFCS(i)(std::move(that)));}
        return *this;
}
#line 4 "/app/example.cpp2"
auto main() -> int{}

Output:

Step build returned: 0
[1/3] Generating main.cpp
main.cpp2...

v: type =
{
    _storage: std::aligned_storage_t<cpp2::max(sizeof(int))> = ();

    _discriminator: i8 = -1;

    is_i:(in this) -> move bool = _discriminator == 0;

    i:(in this) -> forward int
        pre( is_i() ) = reinterpret_cast<* const int>(_storage&)*;

    i:(inout this) -> forward int
        pre( is_i() ) = reinterpret_cast<* int>(_storage&)*;

    set_i:(
        inout this,
        in _value: int
    ) =
    {
        if !is_i()
        {
            _destroy();
            std::construct_at(reinterpret_cast<* int>(_storage&), _value);
        }
        else
        {
            reinterpret_cast<* int>(_storage&)* = _value;
        }
        _discriminator = 0;
    }

    set_i:(
        inout this,
        forward _args...:
    ) =
    {
        if !is_i()
        {
            _destroy();
            std::construct_at(reinterpret_cast<* int>(_storage&), _args...);
        }
        else
        {
            reinterpret_cast<* int>(_storage&)* = : int = (_args...);
        }
        _discriminator = 0;
    }

    private _destroy:(inout this) =
    {
        if _discriminator == 0
        {
            std::destroy_at(reinterpret_cast<* int>(_storage&));
        }
        _discriminator = -1;
    }

    operator=:(move this) =
    {
        _destroy();
    }

    operator=:(out this) =
    {
    }

    operator=:(
        out this,
        in that
    ) =
    {
        _storage = ();
        _discriminator = -1;
        if that.is_i()
        {
            set_i(that.i());
        }
    }

    operator=:(
        inout this,
        in that
    ) =
    {
        _storage = _;
        _discriminator = _;
        if that.is_i()
        {
            set_i(that.i());
        }
    }
}
 ok (all Cpp2, passes safety checks)

[2/3] Building CXX object CMakeFiles/main.dir/main.cpp.o
[3/3] Linking CXX executable main
Program returned: 0

See also:

Other bugs in this area:

• [BUG] No implicit move for move this, explicit move doesn't parse #857
• [BUG] Last use of copy/move/forward in for _expression_ not exercised #847
• [BUG] Local copy/forward parameter not moved/forwarded #825