Repository: carbon-language/carbon-lang
File: toolchain/check/import_cpp.cpp (C++)

Code before:
    context.TODO(loc_id, "Unsupported: Non-global function");
    return SemIR::ErrorInst::SingletonInstId;
  }
  if (clang_decl->getTemplatedKind() != clang::FunctionDecl::TK_NonTemplate) {
    context.TODO(loc_id, "Unsupported: Template function");
    return SemIR::ErrorInst::SingletonInstId;
  }
  if (!clang_decl->param_empty()) {
    context.TODO(loc_id, "Unsupported: Function with parameters");
    return SemIR::ErrorInst::SingletonInstId;
  }

  if (!clang_decl->getReturnType()->isVoidType() &&
      clang_decl->getReturnType().getAsString() != "int") {
    context.TODO(loc_id, "Unsupported: Function return type");
    return SemIR::ErrorInst::SingletonInstId;
  }

  auto function_decl = SemIR::FunctionDecl{
      SemIR::TypeId::None, SemIR::FunctionId::None, SemIR::InstBlockId::Empty};
  auto decl_id = AddPlaceholderInst(
      context, SemIR::LocIdAndInst(Parse::NodeId::None, function_decl));

  auto return_slot_pattern_id = SemIR::InstId::None;
  if (clang_decl->getReturnType().getAsString() == "int") {
    auto size_id = context.ints().Add(32);
    auto type_id = MakeIntType(context, Parse::NodeId::None,
                               SemIR::IntKind::Signed, size_id);
    auto type_inst_id = MakeIntTypeLiteral(context, Parse::NodeId::None,
                                           SemIR::IntKind::Signed, size_id);

    return_slot_pattern_id = AddInstInNoBlock(
        context, SemIR::LocIdAndInst::NoLoc(SemIR::ReturnSlotPattern(
                     {.type_id = type_id, .type_inst_id = type_inst_id})));
    auto param_pattern_id = AddInstInNoBlock(
        context, SemIR::LocIdAndInst::NoLoc(SemIR::OutParamPattern(
                     {.type_id = type_id,
                      .subpattern_id = return_slot_pattern_id,
                      .index = SemIR::CallParamIndex::None})));
    return_slot_pattern_id = param_pattern_id;
  }

  auto function_info = SemIR::Function{
      {.name_id = name_id,
       .parent_scope_id = scope_id,
       .generic_id = SemIR::GenericId::None,
       .first_param_node_id = Parse::NodeId::None,
       .last_param_node_id = Parse::NodeId::None,
       .pattern_block_id = SemIR::InstBlockId::Empty,
       .implicit_param_patterns_id = SemIR::InstBlockId::Empty,
       .param_patterns_id = SemIR::InstBlockId::Empty,

Review comment (suggestion):
Given that `int` is platform-dependent, would it be worth diagnosing if `int` isn't 32-bit inside the AST?

Note, I do think we want to only support 32-bit here, so a TODO might be fine. Just want to flag the possible error.

Code after:
  clang::QualType ret_type = clang_decl->getReturnType().getCanonicalType();
  if (ret_type->isVoidType()) {
    return SemIR::InstId::None;
  }
  if (const auto* builtin_type = dyn_cast<clang::BuiltinType>(ret_type);
      builtin_type && builtin_type->getKind() == clang::BuiltinType::Int) {
    constexpr int SupportedIntWidth = 32;
    uint64_t int_size = context.ast_context().getTypeSize(ret_type);
    if (int_size != SupportedIntWidth) {
      // TODO: Add tests for this case.
      context.TODO(loc_id,
                   llvm::formatv("Unsupported: return type: {0}, size: {1}",
                                 ret_type.getAsString(), int_size));
      return SemIR::ErrorInst::SingletonInstId;
    }
    IntId size_id = context.ints().Add(int_size);
    // TODO: Fill in a location for the type once available.
    SemIR::TypeId type_id = MakeIntType(context, Parse::NodeId::None,
                                        SemIR::IntKind::Signed, size_id);
    // TODO: Fill in a location for the type once available.
    SemIR::InstId type_inst_id = MakeIntTypeLiteral(
        context, Parse::NodeId::None, SemIR::IntKind::Signed, size_id);

    SemIR::InstId return_slot_pattern_id = AddInstInNoBlock(
        // TODO: Fill in a location for the return type once available.
        context, SemIR::LocIdAndInst::NoLoc(SemIR::ReturnSlotPattern(
                     {.type_id = type_id, .type_inst_id = type_inst_id})));
    SemIR::InstId param_pattern_id = AddInstInNoBlock(
        // TODO: Fill in a location for the return type once available.
        context, SemIR::LocIdAndInst::NoLoc(SemIR::OutParamPattern(
                     {.type_id = type_id,
                      .subpattern_id = return_slot_pattern_id,
                      .index = SemIR::CallParamIndex::None})));
    return param_pattern_id;
  }
  context.TODO(loc_id, llvm::formatv("Unsupported: return type: {0}",
                                     ret_type.getAsString()));
  return SemIR::ErrorInst::SingletonInstId;
}

// Imports a function declaration from Clang to Carbon. If successful, returns
// the new Carbon function declaration `InstId`.
static auto ImportFunctionDecl(Context& context, SemIR::LocId loc_id,
                               SemIR::NameScopeId scope_id,
                               SemIR::NameId name_id,
                               const clang::FunctionDecl* clang_decl)
    -> SemIR::InstId {
  if (clang_decl->isVariadic()) {
    context.TODO(loc_id, "Unsupported: Variadic function");
    return SemIR::ErrorInst::SingletonInstId;
  }