[cpp] List in a variant generates invalid function binding

[Lucky1313]

Given a WIT definition which includes a function returning a variant that includes a list:

package local:test@0.1.0;

interface task {
  variant value {
    a(bool),
    b(list<u8>),
  }
  
  do-thing: func() -> value;
}

world test {
  import task;
}

And generating C++17 bindings using latest master (7f92598):

wit-bingen cpp test.wit

test.cpp:

...snip
extern "C" __attribute__((import_module("local:test/task@0.1.0")))
__attribute__((import_name("do-thing")))
void localX3AtestX2FtaskX400X2E1X2E0X00do_thing(uint8_t *);
local::test::task::Value local::test::task::DoThing()
{
  uintptr_t ret_area[((3*sizeof(void*))+sizeof(uintptr_t)-1)/sizeof(uintptr_t)];
  uint8_t* ptr0 = (uint8_t*)(&ret_area);
  localX3AtestX2FtaskX400X2E1X2E0X00do_thing(ptr0);
  Value variant2;
  switch ((int32_t) (*((uint8_t*) (ptr0 + 0)))) {
    case 0: { 
      variant2.variants = Value::A{(bool((int32_t) (*((uint8_t*) (ptr0 + sizeof(void*))))))};
    } break;
    case 1: { auto len1 = *((size_t*) (ptr0 + (2*sizeof(void*))));

    variant2.variants = Value::B{wit::vector<uint8_t>((uint8_t*)(*((uint8_t**) (ptr0 + sizeof(void*)))), len1)};
  } break;
}
auto result3 = variant2;
return result3;
}
...snip

The bindings are unable to compile, I think due to the copy constructor for the variant that includes the wit::vector being deleted:

/opt/wasi-sdk/bin/clang++ -std=c++20 -fno-exceptions -Wall -Wextra -Wc++-compat -g --target=wasm32-wasip2 -O2 -I/bindings -c /bindings/test.cpp -o test.o
/bindings/test.cpp:44:6: error: call to implicitly-deleted copy constructor of 'Value'
   44 | auto result3 = variant2;
      |      ^         ~~~~~~~~
/bindings/test_cpp.h:11:22: note: copy constructor of 'Value' is implicitly deleted because field 'variants' has a deleted copy constructor
   11 |   std::variant<A, B> variants;
...snip

If the copy from the variant2 to result3 object is removed, and we return the variant2 object directly, the code compiles successfully.

Generated `test.cpp` file

// Generated by `wit-bindgen` 0.43.0. DO NOT EDIT!

// Ensure that the *_component_type.o object is linked in
#ifdef __wasm32__
extern "C" void __component_type_object_force_link_test(void);
__attribute__((used))
void __component_type_object_force_link_test_public_use_in_this_compilation_unit(void) {
  __component_type_object_force_link_test();
}
#endif
#include "test_cpp.h"
#include <cstdlib> // realloc

extern "C" void *cabi_realloc(void *ptr, size_t old_size, size_t align, size_t new_size);

__attribute__((__weak__, __export_name__("cabi_realloc")))
void *cabi_realloc(void *ptr, size_t old_size, size_t align, size_t new_size) {
  (void) old_size;
  if (new_size == 0) return (void*) align;
  void *ret = realloc(ptr, new_size);
  if (!ret) abort();
  return ret;
}


extern "C" __attribute__((import_module("local:test/task@0.1.0")))
__attribute__((import_name("do-thing")))
void localX3AtestX2FtaskX400X2E1X2E0X00do_thing(uint8_t *);
local::test::task::Value local::test::task::DoThing()
{
  uintptr_t ret_area[((3*sizeof(void*))+sizeof(uintptr_t)-1)/sizeof(uintptr_t)];
  uint8_t* ptr0 = (uint8_t*)(&ret_area);
  localX3AtestX2FtaskX400X2E1X2E0X00do_thing(ptr0);
  Value variant2;
  switch ((int32_t) (*((uint8_t*) (ptr0 + 0)))) {
    case 0: { 
      variant2.variants = Value::A{(bool((int32_t) (*((uint8_t*) (ptr0 + sizeof(void*))))))};
    } break;
    case 1: { auto len1 = *((size_t*) (ptr0 + (2*sizeof(void*))));

    variant2.variants = Value::B{wit::vector<uint8_t>((uint8_t*)(*((uint8_t**) (ptr0 + sizeof(void*)))), len1)};
  } break;
}
auto result3 = variant2;
return result3;
}

// Component Adapters

Generated `test_cpp.h` file

// Generated by `wit-bindgen` 0.43.0. DO NOT EDIT!
#ifndef __CPP_GUEST_BINDINGS_TEST_H
#define __CPP_GUEST_BINDINGS_TEST_H
#include <cstdint>
#include <utility>
#include <variant>
#include "wit.h"
namespace local {namespace test {namespace task {struct Value {
  struct A { bool value; };
  struct B { wit::vector<uint8_t> value; };
  std::variant<A, B> variants;
};
Value DoThing();
}}}

#endif

[Lucky1313]

I think this is probably a question for @cpetig or @TartanLlama ?

[TartanLlama]

Looks like a missing std::move in the generated bindings. I can have a look after I've finished with my current set of changes, unless @cpetig wants to!