[BUG] Can't author trivial functions

[JohelEGP]

Title: Can't author trivial functions.

Description:

Currently, Cppfront generates special member functions even when a =defaulted one would do.

When a Cpp2 operator= for a move/copy constructor/assignment or destructor
would perform the same operations as a =defaulted one,
it should be lowered as =defaulted.

Minimal reproducer (https://cpp2.godbolt.org/z/4nozcbdcf):

template<class T> struct union_ {
  union {
    bool nullopt = true;
    T value;
  };
  union_() { }
  union_(const union_&) requires std::is_trivially_copy_constructible_v<T> = default;
  ~union_() requires std::is_trivially_destructible_v<T> = default;
  union_& operator=(const union_&) requires std::is_trivially_copy_assignable_v<T> = default;
  union_(const union_&) { }
  void operator=(const union_&) { }
  ~union_() { }
};
optional: <T> type = {
  has_value: bool = false;
  this: union_<T> = ();
  operator=: (out this) = { }
  // Comment out, pending fix to #323.
  // operator=: (out this, that) requires std::is_trivially_copy_constructible_v<T> = { }
  // operator=: (out this, move that) requires std::is_trivially_move_constructible_v<T> = { }
  // operator=: (inout this, that) requires std::is_trivially_copy_assignable_v<T> = { }
  // operator=: (inout this, move that) requires std::is_trivially_move_assignable_v<T> = { }
  // operator=: (move this) requires std::is_trivially_destructible_v<T> = { }
  private move_construct: (inout this, forward other) = {
    if other.has_value {
      has_value = true;
      this.value = (move other).value;
    }
  }
  operator=: (out this, that) = move_construct(that);
  operator=: (out this, move that) = move_construct(move that);
  private move_assign: (inout this, forward other) = {
    if has_value && other.has_value {
      this.value = (move other).value;
    } else if has_value {
      std::destroy_at(this.value&);
      this.nullopt = true;
    } else if other.has_value {
      has_value = true;
      std::construct_at(this.value&, (move other).value);
    }
  }
  operator=: (inout this, that) = move_assign(that);
  operator=: (inout this, move that) = move_assign(move that);
  operator=: (move this) = { if has_value { std::destroy_at(this.value&); } }
}
test: <T> () = {
  x: optional<T> = ();
  y := x;
  _ = :optional<T> = (move x);
  x = y;
  x = y;
}
main: () = {
  test<i32>();
  test<std::string>();
  static_assert(std::is_trivially_destructible_v<union_<i32>>);
  [[assert Testing: std::is_trivially_destructible_v<optional<i32>>]]
}

Commands:

cppfront main.cpp2
clang++17 -std=c++23 -stdlib=libc++ -lc++abi -pedantic-errors -Wall -Wextra -Wconversion -I . main.cpp

Expected result:

After uncommenting // operator=: (move this) requires std::is_trivially_destructible_v<T> = { },
for the assertion to pass.

Actual result and error:

Cpp2 lowered to Cpp1:

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


#include "cpp2util.h"


template<typename T> class optional;
  

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

template<class T> struct union_ {
  union {
    bool nullopt = true;
    T value;
  };
  union_() { }
  union_(const union_&) requires std::is_trivially_copy_constructible_v<T> = default;
  ~union_() requires std::is_trivially_destructible_v<T> = default;
  union_& operator=(const union_&) requires std::is_trivially_copy_assignable_v<T> = default;
  union_(const union_&) { }
  void operator=(const union_&) { }
  ~union_() { }
};

struct optional_has_value_as_base { bool has_value; };
template<typename T> class optional: public optional_has_value_as_base, public union_<T> {

  public: explicit optional();
  // Comment out, pending fix to #323.
  // operator=: (out this, that) requires std::is_trivially_copy_constructible_v<T> = { }
  // operator=: (out this, move that) requires std::is_trivially_move_constructible_v<T> = { }
  // operator=: (inout this, that) requires std::is_trivially_copy_assignable_v<T> = { }
  // operator=: (inout this, move that) requires std::is_trivially_move_assignable_v<T> = { }
  // operator=: (move this) requires std::is_trivially_destructible_v<T> = { }
  private: auto move_construct(auto&& other) -> void;
    

  public: optional(optional const& that);
  public: optional(optional&& that) noexcept;
  private: auto move_assign(auto&& other) -> void;
    

  public: auto operator=(optional const& that) -> optional& ;
  public: auto operator=(optional&& that) noexcept -> optional& ;
  public: ~optional() noexcept;
};
template<typename T> auto test() -> void;
  

auto main() -> int;
  

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


  template <typename T> optional<T>::optional()
                            : optional_has_value_as_base{ false }
                            , union_<T>{  }
  {}

  template <typename T> auto optional<T>::move_construct(auto&& other) -> void{
    if (other.has_value) {
      has_value = true;
      (*this).value = (std::move(CPP2_FORWARD(other))).value;
    }
  }
  template <typename T> optional<T>::optional(optional const& that)
                                : optional_has_value_as_base{ that.has_value }
                                , union_<T>{ that }
   { move_construct(that);  }
  template <typename T> optional<T>::optional(optional&& that) noexcept
                                     : optional_has_value_as_base{ std::move(that).has_value }
                                     , union_<T>{ std::move(that) }
   { move_construct(std::move(std::move(that)));  }
  template <typename T> auto optional<T>::move_assign(auto&& other) -> void{
    if (has_value && other.has_value) {
      (*this).value = (std::move(other)).value;
    }else {if (has_value) {
      std::destroy_at(&(*this).value);
      (*this).nullopt = true;
    }else {if (other.has_value) {
      has_value = true;
      std::construct_at(&(*this).value, (std::move(CPP2_FORWARD(other))).value);
    }}}
  }
  template <typename T> auto optional<T>::operator=(optional const& that) -> optional&  { 
                                  has_value = that.has_value;
                                  union_<T>::operator= ( that );
  move_assign(that);
                                  return *this;
   }
  template <typename T> auto optional<T>::operator=(optional&& that) noexcept -> optional&  { 
                                       has_value = std::move(that).has_value;
                                       union_<T>::operator= ( std::move(that) );
  move_assign(std::move(std::move(that)));
                                       return *this;
   }
  template <typename T> optional<T>::~optional() noexcept{if (has_value) {std::destroy_at(&(*this).value); }}

template<typename T> auto test() -> void{
  optional<T> x {}; 
  auto y {x}; 
  (void) optional<T>{std::move(x)};
  x = y;
  x = std::move(y);
}
auto main() -> int{
  test<cpp2::i32>();
  test<std::string>();
  static_assert(std::is_trivially_destructible_v<union_<cpp2::i32>>);
  cpp2::Testing.expects(std::is_trivially_destructible_v<optional<cpp2::i32>>, "");
}

Output:

Testing violation
libc++abi: terminating

[JohelEGP]

As an opt-out of emitted Cpp1 =default in case triviality is undesirable,
how about supporting an explicit empty statement with the placeholder _;?
The alternative is for everyone to use their own "do nothing" statement.

Then this Cpp2:

operator=: (out this) = { }          // 1
operator=: (out this) = _;           // 2
while do_stuff().and_continue { _; } // 3

Can lower to this Cpp1:

t() = default;                      // 1
t() { }                             // 2
while (do_stuff().and_continue) { } // 3

[realgdman]

I like idea, but why not other way around? = { } emits { }, and = _ emits = default, because it is already close to meaning "insert what fits here and I don't care what exactly it is"

[JohelEGP]

while do_stuff().and_continue { _; } // 3 is silly.
{ } is already allowed.

[JohelEGP]

Or add a @trivial metafunction that lowers to = default.