carbon-lang/toolchain/check/handle_class.cpp

// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

#include "toolchain/base/kind_switch.h"
#include "toolchain/check/context.h"
#include "toolchain/check/convert.h"
#include "toolchain/check/decl_name_stack.h"
#include "toolchain/check/eval.h"
#include "toolchain/check/generic.h"
#include "toolchain/check/handle.h"
#include "toolchain/check/merge.h"
#include "toolchain/check/modifiers.h"
#include "toolchain/check/name_component.h"
#include "toolchain/sem_ir/ids.h"
#include "toolchain/sem_ir/inst.h"
#include "toolchain/sem_ir/typed_insts.h"

namespace Carbon::Check {

// If `type_id` is a class type, get its corresponding `SemIR::Class` object.
// Otherwise returns `nullptr`.
static auto TryGetAsClass(Context& context, SemIR::TypeId type_id)
    -> SemIR::Class* {
  auto class_type = context.types().TryGetAs<SemIR::ClassType>(type_id);
  if (!class_type) {
    return nullptr;
  }
  return &context.classes().Get(class_type->class_id);
}

auto HandleParseNode(Context& context, Parse::ClassIntroducerId node_id)
    -> bool {
  // Create an instruction block to hold the instructions created as part of the
  // class signature, such as generic parameters.
  context.inst_block_stack().Push();
  // Push the bracketing node.
  context.node_stack().Push(node_id);
  // Optional modifiers and the name follow.
  context.decl_introducer_state_stack().Push<Lex::TokenKind::Class>();
  context.decl_name_stack().PushScopeAndStartName();
  // This class is potentially generic.
  StartGenericDecl(context);
  return true;
}

// Tries to merge new_class into prev_class_id. Since new_class won't have a
// definition even if one is upcoming, set is_definition to indicate the planned
// result.
//
// If merging is successful, returns true and may update the previous class.
// Otherwise, returns false. Prints a diagnostic when appropriate.
static auto MergeClassRedecl(Context& context, SemIRLoc new_loc,
                             SemIR::Class& new_class, bool new_is_import,
                             bool new_is_definition, bool new_is_extern,
                             SemIR::ClassId prev_class_id, bool prev_is_extern,
                             SemIR::ImportIRId prev_import_ir_id) -> bool {
  auto& prev_class = context.classes().Get(prev_class_id);
  SemIRLoc prev_loc =
      prev_class.is_defined() ? prev_class.definition_id : prev_class.decl_id;

  // Check the generic parameters match, if they were specified.
  if (!CheckRedeclParamsMatch(context, DeclParams(new_class),
                              DeclParams(prev_class))) {
    return false;
  }

  CheckIsAllowedRedecl(context, Lex::TokenKind::Class, prev_class.name_id,
                       {.loc = new_loc,
                        .is_definition = new_is_definition,
                        .is_extern = new_is_extern},
                       {.loc = prev_loc,
                        .is_definition = prev_class.is_defined(),
                        .is_extern = prev_is_extern},
                       prev_import_ir_id);

  if (new_is_definition && prev_class.is_defined()) {
    // Don't attempt to merge multiple definitions.
    return false;
  }

  // The introducer kind must match the previous declaration.
  // TODO: The rule here is not yet decided. See #3384.
  if (prev_class.inheritance_kind != new_class.inheritance_kind) {
    CARBON_DIAGNOSTIC(ClassRedeclarationDifferentIntroducer, Error,
                      "Class redeclared with different inheritance kind.");
    CARBON_DIAGNOSTIC(ClassRedeclarationDifferentIntroducerPrevious, Note,
                      "Previously declared here.");
    context.emitter()
        .Build(new_loc, ClassRedeclarationDifferentIntroducer)
        .Note(prev_loc, ClassRedeclarationDifferentIntroducerPrevious)
        .Emit();
  }

  if (new_is_definition) {
    prev_class.first_param_node_id = new_class.first_param_node_id;
    prev_class.last_param_node_id = new_class.last_param_node_id;
    prev_class.implicit_param_refs_id = new_class.implicit_param_refs_id;
    prev_class.param_refs_id = new_class.param_refs_id;
    prev_class.definition_id = new_class.definition_id;
    prev_class.scope_id = new_class.scope_id;
    prev_class.body_block_id = new_class.body_block_id;
    prev_class.adapt_id = new_class.adapt_id;
    prev_class.base_id = new_class.base_id;
    prev_class.object_repr_id = new_class.object_repr_id;
  }

  if ((prev_import_ir_id.is_valid() && !new_is_import) ||
      (prev_is_extern && !new_is_extern)) {
    prev_class.decl_id = new_class.decl_id;
    ReplacePrevInstForMerge(
        context, prev_class.parent_scope_id, prev_class.name_id,
        new_is_import ? new_loc.inst_id : new_class.decl_id);
  }
  return true;
}

// Adds the name to name lookup. If there's a conflict, tries to merge. May
// update class_decl and class_info when merging.
static auto MergeOrAddName(Context& context, Parse::AnyClassDeclId node_id,
                           const DeclNameStack::NameContext& name_context,
                           SemIR::InstId class_decl_id,
                           SemIR::ClassDecl& class_decl,
                           SemIR::Class& class_info, bool is_definition,
                           bool is_extern, SemIR::AccessKind access_kind)
    -> void {
  auto prev_id = context.decl_name_stack().LookupOrAddName(
      name_context, class_decl_id, access_kind);
  if (!prev_id.is_valid()) {
    return;
  }

  auto prev_class_id = SemIR::ClassId::Invalid;
  auto prev_import_ir_id = SemIR::ImportIRId::Invalid;
  auto prev = context.insts().Get(prev_id);
  CARBON_KIND_SWITCH(prev) {
    case CARBON_KIND(SemIR::ClassDecl class_decl): {
      prev_class_id = class_decl.class_id;
      break;
    }
    case CARBON_KIND(SemIR::ImportRefLoaded import_ref): {
      auto import_ir_inst =
          context.import_ir_insts().Get(import_ref.import_ir_inst_id);

      // Verify the decl so that things like aliases are name conflicts.
      const auto* import_ir =
          context.import_irs().Get(import_ir_inst.ir_id).sem_ir;
      if (!import_ir->insts().Is<SemIR::ClassDecl>(import_ir_inst.inst_id)) {
        break;
      }

      // Use the constant value to get the ID.
      auto decl_value = context.insts().Get(
          context.constant_values().GetConstantInstId(prev_id));
      if (auto class_type = decl_value.TryAs<SemIR::ClassType>()) {
        prev_class_id = class_type->class_id;
        prev_import_ir_id = import_ir_inst.ir_id;
      } else if (auto generic_class_type =
                     context.types().TryGetAs<SemIR::GenericClassType>(
                         decl_value.type_id())) {
        prev_class_id = generic_class_type->class_id;
        prev_import_ir_id = import_ir_inst.ir_id;
      }
      break;
    }
    default:
      break;
  }

  if (!prev_class_id.is_valid()) {
    // This is a redeclaration of something other than a class.
    context.DiagnoseDuplicateName(class_decl_id, prev_id);
    return;
  }

  // TODO: Fix prev_is_extern logic.
  if (MergeClassRedecl(context, node_id, class_info,
                       /*new_is_import=*/false, is_definition, is_extern,
                       prev_class_id, /*prev_is_extern=*/false,
                       prev_import_ir_id)) {
    // When merging, use the existing entity rather than adding a new one.
    class_decl.class_id = prev_class_id;
    class_decl.type_id = prev.type_id();
    // TODO: Validate that the redeclaration doesn't set an access modifier.
  }
}

static auto BuildClassDecl(Context& context, Parse::AnyClassDeclId node_id,
                           bool is_definition)
    -> std::tuple<SemIR::ClassId, SemIR::InstId> {
  auto name = PopNameComponent(context);
  auto name_context = context.decl_name_stack().FinishName(name);
  context.node_stack()
      .PopAndDiscardSoloNodeId<Parse::NodeKind::ClassIntroducer>();

  // Process modifiers.
  auto [_, parent_scope_inst] =
      context.name_scopes().GetInstIfValid(name_context.parent_scope_id);
  auto introducer =
      context.decl_introducer_state_stack().Pop<Lex::TokenKind::Class>();
  CheckAccessModifiersOnDecl(context, introducer, parent_scope_inst);
  LimitModifiersOnDecl(context, introducer,
                       KeywordModifierSet::Class | KeywordModifierSet::Access |
                           KeywordModifierSet::Extern);
  RestrictExternModifierOnDecl(context, introducer, parent_scope_inst,
                               is_definition);

  bool is_extern = introducer.modifier_set.HasAnyOf(KeywordModifierSet::Extern);
  auto inheritance_kind =
      introducer.modifier_set.HasAnyOf(KeywordModifierSet::Abstract)
          ? SemIR::Class::Abstract
      : introducer.modifier_set.HasAnyOf(KeywordModifierSet::Base)
          ? SemIR::Class::Base
          : SemIR::Class::Final;

  auto decl_block_id = context.inst_block_stack().Pop();

  // Add the class declaration.
  auto class_decl = SemIR::ClassDecl{.type_id = SemIR::TypeId::TypeType,
                                     .class_id = SemIR::ClassId::Invalid,
                                     .decl_block_id = decl_block_id};
  auto class_decl_id =
      context.AddPlaceholderInst(SemIR::LocIdAndInst(node_id, class_decl));

  // TODO: Store state regarding is_extern.
  SemIR::Class class_info = {
      .name_id = name_context.name_id_for_new_inst(),
      .parent_scope_id = name_context.parent_scope_id_for_new_inst(),
      .generic_id = SemIR::GenericId::Invalid,
      .first_param_node_id = name.first_param_node_id,
      .last_param_node_id = name.last_param_node_id,
      .implicit_param_refs_id = name.implicit_params_id,
      .param_refs_id = name.params_id,
      // `.self_type_id` depends on the ClassType, so is set below.
      .self_type_id = SemIR::TypeId::Invalid,
      .decl_id = class_decl_id,
      .inheritance_kind = inheritance_kind};

  MergeOrAddName(context, node_id, name_context, class_decl_id, class_decl,
                 class_info, is_definition, is_extern,
                 introducer.modifier_set.GetAccessKind());

  // Create a new class if this isn't a valid redeclaration.
  bool is_new_class = !class_decl.class_id.is_valid();
  if (is_new_class) {
    // TODO: If this is an invalid redeclaration of a non-class entity or there
    // was an error in the qualifier, we will have lost track of the class name
    // here. We should keep track of it even if the name is invalid.
    class_info.generic_id = FinishGenericDecl(context, class_decl_id);
    class_decl.class_id = context.classes().Add(class_info);
    if (class_info.is_generic()) {
      class_decl.type_id = context.GetGenericClassType(class_decl.class_id);
    }
  } else {
    FinishGenericRedecl(context, class_decl_id, class_info.generic_id);
  }

  // Write the class ID into the ClassDecl.
  context.ReplaceInstBeforeConstantUse(class_decl_id, class_decl);

  if (is_new_class) {
    // Build the `Self` type using the resulting type constant.
    // TODO: Form this as part of building the definition, not as part of the
    // declaration.
    auto& class_info = context.classes().Get(class_decl.class_id);
    if (class_info.is_generic()) {
      auto instance_id =
          context.generics().GetSelfInstance(class_info.generic_id);
      class_info.self_type_id = context.GetTypeIdForTypeConstant(
          TryEvalInst(context, SemIR::InstId::Invalid,
                      SemIR::ClassType{.type_id = SemIR::TypeId::TypeType,
                                       .class_id = class_decl.class_id,
                                       .instance_id = instance_id}));
    } else {
      class_info.self_type_id = context.GetTypeIdForTypeInst(class_decl_id);
    }
  }

  if (!is_definition && context.IsImplFile() && !is_extern) {
    context.definitions_required().push_back(class_decl_id);
  }

  return {class_decl.class_id, class_decl_id};
}

auto HandleParseNode(Context& context, Parse::ClassDeclId node_id) -> bool {
  BuildClassDecl(context, node_id, /*is_definition=*/false);
  context.decl_name_stack().PopScope();
  return true;
}

auto HandleParseNode(Context& context, Parse::ClassDefinitionStartId node_id)
    -> bool {
  auto [class_id, class_decl_id] =
      BuildClassDecl(context, node_id, /*is_definition=*/true);
  auto& class_info = context.classes().Get(class_id);

  // Track that this declaration is the definition.
  if (!class_info.is_defined()) {
    class_info.definition_id = class_decl_id;
    class_info.scope_id = context.name_scopes().Add(
        class_decl_id, SemIR::NameId::Invalid, class_info.parent_scope_id);
  }

  // Enter the class scope.
  context.scope_stack().Push(
      class_decl_id, class_info.scope_id,
      context.generics().GetSelfInstance(class_info.generic_id));
  StartGenericDefinition(context);

  // Introduce `Self`.
  context.name_scopes().AddRequiredName(
      class_info.scope_id, SemIR::NameId::SelfType,
      context.types().GetInstId(class_info.self_type_id));

  context.inst_block_stack().Push();
  context.node_stack().Push(node_id, class_id);
  context.args_type_info_stack().Push();

  // TODO: Handle the case where there's control flow in the class body. For
  // example:
  //
  //   class C {
  //     var v: if true then i32 else f64;
  //   }
  //
  // We may need to track a list of instruction blocks here, as we do for a
  // function.
  class_info.body_block_id = context.inst_block_stack().PeekOrAdd();
  return true;
}

// Diagnoses a class-specific declaration appearing outside a class.
static auto DiagnoseClassSpecificDeclOutsideClass(Context& context,
                                                  SemIRLoc loc,
                                                  Lex::TokenKind tok) -> void {
  CARBON_DIAGNOSTIC(ClassSpecificDeclOutsideClass, Error,
                    "`{0}` declaration can only be used in a class.",
                    Lex::TokenKind);
  context.emitter().Emit(loc, ClassSpecificDeclOutsideClass, tok);
}

// Returns the current scope's class declaration, or diagnoses if it isn't a
// class.
static auto GetCurrentScopeAsClassOrDiagnose(Context& context, SemIRLoc loc,
                                             Lex::TokenKind tok)
    -> std::optional<SemIR::ClassDecl> {
  auto class_scope = context.GetCurrentScopeAs<SemIR::ClassDecl>();
  if (!class_scope) {
    DiagnoseClassSpecificDeclOutsideClass(context, loc, tok);
  }
  return class_scope;
}

// Diagnoses a class-specific declaration that is repeated within a class, but
// is not permitted to be repeated.
static auto DiagnoseClassSpecificDeclRepeated(Context& context,
                                              SemIRLoc new_loc,
                                              SemIRLoc prev_loc,
                                              Lex::TokenKind tok) -> void {
  CARBON_DIAGNOSTIC(ClassSpecificDeclRepeated, Error,
                    "Multiple `{0}` declarations in class.{1}", Lex::TokenKind,
                    std::string);
  const llvm::StringRef extra = tok == Lex::TokenKind::Base
                                    ? " Multiple inheritance is not permitted."
                                    : "";
  CARBON_DIAGNOSTIC(ClassSpecificDeclPrevious, Note,
                    "Previous `{0}` declaration is here.", Lex::TokenKind);
  context.emitter()
      .Build(new_loc, ClassSpecificDeclRepeated, tok, extra.str())
      .Note(prev_loc, ClassSpecificDeclPrevious, tok)
      .Emit();
}

auto HandleParseNode(Context& context, Parse::AdaptIntroducerId /*node_id*/)
    -> bool {
  context.decl_introducer_state_stack().Push<Lex::TokenKind::Adapt>();
  return true;
}

auto HandleParseNode(Context& context, Parse::AdaptDeclId node_id) -> bool {
  auto [adapted_type_node, adapted_type_expr_id] =
      context.node_stack().PopExprWithNodeId();

  // Process modifiers. `extend` is permitted, no others are allowed.
  auto introducer =
      context.decl_introducer_state_stack().Pop<Lex::TokenKind::Adapt>();
  LimitModifiersOnDecl(context, introducer, KeywordModifierSet::Extend);

  auto parent_class_decl =
      GetCurrentScopeAsClassOrDiagnose(context, node_id, Lex::TokenKind::Adapt);
  if (!parent_class_decl) {
    return true;
  }

  auto& class_info = context.classes().Get(parent_class_decl->class_id);
  if (class_info.adapt_id.is_valid()) {
    DiagnoseClassSpecificDeclRepeated(context, node_id, class_info.adapt_id,
                                      Lex::TokenKind::Adapt);
    return true;
  }

  auto adapted_type_id = ExprAsType(context, node_id, adapted_type_expr_id);
  adapted_type_id = context.AsCompleteType(adapted_type_id, [&] {
    CARBON_DIAGNOSTIC(IncompleteTypeInAdaptDecl, Error,
                      "Adapted type `{0}` is an incomplete type.",
                      SemIR::TypeId);
    return context.emitter().Build(node_id, IncompleteTypeInAdaptDecl,
                                   adapted_type_id);
  });

  // Build a SemIR representation for the declaration.
  class_info.adapt_id = context.AddInst<SemIR::AdaptDecl>(
      node_id, {.adapted_type_id = adapted_type_id});

  // Extend the class scope with the adapted type's scope if requested.
  if (introducer.modifier_set.HasAnyOf(KeywordModifierSet::Extend)) {
    auto extended_scope_id = SemIR::NameScopeId::Invalid;
    if (adapted_type_id == SemIR::TypeId::Error) {
      // Recover by not extending any scope. We instead set has_error to true
      // below.
    } else if (auto* adapted_class_info =
                   TryGetAsClass(context, adapted_type_id)) {
      extended_scope_id = adapted_class_info->scope_id;
      CARBON_CHECK(adapted_class_info->scope_id.is_valid())
          << "Complete class should have a scope";
    } else {
      // TODO: Accept any type that has a scope.
      context.TODO(node_id, "extending non-class type");
    }

    auto& class_scope = context.name_scopes().Get(class_info.scope_id);
    if (extended_scope_id.is_valid()) {
      class_scope.extended_scopes.push_back(extended_scope_id);
    } else {
      class_scope.has_error = true;
    }
  }
  return true;
}

auto HandleParseNode(Context& context, Parse::BaseIntroducerId /*node_id*/)
    -> bool {
  context.decl_introducer_state_stack().Push<Lex::TokenKind::Base>();
  return true;
}

auto HandleParseNode(Context& /*context*/, Parse::BaseColonId /*node_id*/)
    -> bool {
  return true;
}

namespace {
// Information gathered about a base type specified in a `base` declaration.
struct BaseInfo {
  // A `BaseInfo` representing an erroneous base.
  static const BaseInfo Error;

  SemIR::TypeId type_id;
  SemIR::NameScopeId scope_id;
};
constexpr BaseInfo BaseInfo::Error = {.type_id = SemIR::TypeId::Error,
                                      .scope_id = SemIR::NameScopeId::Invalid};
}  // namespace

// Diagnoses an attempt to derive from a final type.
static auto DiagnoseBaseIsFinal(Context& context, Parse::NodeId node_id,
                                SemIR::TypeId base_type_id) -> void {
  CARBON_DIAGNOSTIC(BaseIsFinal, Error,
                    "Deriving from final type `{0}`. Base type must be an "
                    "`abstract` or `base` class.",
                    SemIR::TypeId);
  context.emitter().Emit(node_id, BaseIsFinal, base_type_id);
}

// Checks that the specified base type is valid.
static auto CheckBaseType(Context& context, Parse::NodeId node_id,
                          SemIR::InstId base_expr_id) -> BaseInfo {
  auto base_type_id = ExprAsType(context, node_id, base_expr_id);
  base_type_id = context.AsCompleteType(base_type_id, [&] {
    CARBON_DIAGNOSTIC(IncompleteTypeInBaseDecl, Error,
                      "Base `{0}` is an incomplete type.", SemIR::TypeId);
    return context.emitter().Build(node_id, IncompleteTypeInBaseDecl,
                                   base_type_id);
  });

  if (base_type_id == SemIR::TypeId::Error) {
    return BaseInfo::Error;
  }

  auto* base_class_info = TryGetAsClass(context, base_type_id);

  // The base must not be a final class.
  if (!base_class_info) {
    // For now, we treat all types that aren't introduced by a `class`
    // declaration as being final classes.
    // TODO: Once we have a better idea of which types are considered to be
    // classes, produce a better diagnostic for deriving from a non-class type.
    DiagnoseBaseIsFinal(context, node_id, base_type_id);
    return BaseInfo::Error;
  }
  if (base_class_info->inheritance_kind == SemIR::Class::Final) {
    DiagnoseBaseIsFinal(context, node_id, base_type_id);
  }

  CARBON_CHECK(base_class_info->scope_id.is_valid())
      << "Complete class should have a scope";
  return {.type_id = base_type_id, .scope_id = base_class_info->scope_id};
}

auto HandleParseNode(Context& context, Parse::BaseDeclId node_id) -> bool {
  auto [base_type_node_id, base_type_expr_id] =
      context.node_stack().PopExprWithNodeId();

  // Process modifiers. `extend` is required, no others are allowed.
  auto introducer =
      context.decl_introducer_state_stack().Pop<Lex::TokenKind::Base>();
  LimitModifiersOnDecl(context, introducer, KeywordModifierSet::Extend);
  if (!introducer.modifier_set.HasAnyOf(KeywordModifierSet::Extend)) {
    CARBON_DIAGNOSTIC(BaseMissingExtend, Error,
                      "Missing `extend` before `base` declaration in class.");
    context.emitter().Emit(node_id, BaseMissingExtend);
  }

  auto parent_class_decl =
      GetCurrentScopeAsClassOrDiagnose(context, node_id, Lex::TokenKind::Base);
  if (!parent_class_decl) {
    return true;
  }

  auto& class_info = context.classes().Get(parent_class_decl->class_id);
  if (class_info.base_id.is_valid()) {
    DiagnoseClassSpecificDeclRepeated(context, node_id, class_info.base_id,
                                      Lex::TokenKind::Base);
    return true;
  }

  auto base_info = CheckBaseType(context, base_type_node_id, base_type_expr_id);

  // The `base` value in the class scope has an unbound element type. Instance
  // binding will be performed when it's found by name lookup into an instance.
  auto field_type_id =
      context.GetUnboundElementType(class_info.self_type_id, base_info.type_id);
  class_info.base_id = context.AddInst<SemIR::BaseDecl>(
      node_id,
      {.type_id = field_type_id,
       .base_type_id = base_info.type_id,
       .index = SemIR::ElementIndex(
           context.args_type_info_stack().PeekCurrentBlockContents().size())});

  // Add a corresponding field to the object representation of the class.
  // TODO: Consider whether we want to use `partial T` here.
  // TODO: Should we diagnose if there are already any fields?
  context.args_type_info_stack().AddInstId(
      context.AddInstInNoBlock<SemIR::StructTypeField>(
          node_id, {.name_id = SemIR::NameId::Base,
                    .field_type_id = base_info.type_id}));

  // Bind the name `base` in the class to the base field.
  context.decl_name_stack().AddNameOrDiagnoseDuplicate(
      context.decl_name_stack().MakeUnqualifiedName(node_id,
                                                    SemIR::NameId::Base),
      class_info.base_id, introducer.modifier_set.GetAccessKind());

  // Extend the class scope with the base class.
  if (introducer.modifier_set.HasAnyOf(KeywordModifierSet::Extend)) {
    auto& class_scope = context.name_scopes().Get(class_info.scope_id);
    if (base_info.scope_id.is_valid()) {
      class_scope.extended_scopes.push_back(base_info.scope_id);
    } else {
      class_scope.has_error = true;
    }
  }
  return true;
}

auto HandleParseNode(Context& context, Parse::ClassDefinitionId /*node_id*/)
    -> bool {
  auto fields_id = context.args_type_info_stack().Pop();
  auto class_id =
      context.node_stack().Pop<Parse::NodeKind::ClassDefinitionStart>();
  context.inst_block_stack().Pop();

  // The class type is now fully defined. Compute its object representation.
  auto& class_info = context.classes().Get(class_id);
  if (class_info.adapt_id.is_valid()) {
    class_info.object_repr_id = SemIR::TypeId::Error;
    if (class_info.base_id.is_valid()) {
      CARBON_DIAGNOSTIC(AdaptWithBase, Error,
                        "Adapter cannot have a base class.");
      CARBON_DIAGNOSTIC(AdaptBaseHere, Note, "`base` declaration is here.");
      context.emitter()
          .Build(class_info.adapt_id, AdaptWithBase)
          .Note(class_info.base_id, AdaptBaseHere)
          .Emit();
    } else if (!context.inst_blocks().Get(fields_id).empty()) {
      auto first_field_id = context.inst_blocks().Get(fields_id).front();
      CARBON_DIAGNOSTIC(AdaptWithFields, Error, "Adapter cannot have fields.");
      CARBON_DIAGNOSTIC(AdaptFieldHere, Note,
                        "First field declaration is here.");
      context.emitter()
          .Build(class_info.adapt_id, AdaptWithFields)
          .Note(first_field_id, AdaptFieldHere)
          .Emit();
    } else {
      // The object representation of the adapter is the object representation
      // of the adapted type.
      auto adapted_type_id = context.insts()
                                 .GetAs<SemIR::AdaptDecl>(class_info.adapt_id)
                                 .adapted_type_id;
      // If we adapt an adapter, directly track the non-adapter type we're
      // adapting so that we have constant-time access to it.
      if (auto adapted_class =
              context.types().TryGetAs<SemIR::ClassType>(adapted_type_id)) {
        auto& adapted_class_info =
            context.classes().Get(adapted_class->class_id);
        if (adapted_class_info.adapt_id.is_valid()) {
          adapted_type_id = adapted_class_info.object_repr_id;
        }
      }
      class_info.object_repr_id = adapted_type_id;
    }
  } else {
    class_info.object_repr_id = context.GetStructType(fields_id);
  }

  FinishGenericDefinition(context, class_info.generic_id);

  // The decl_name_stack and scopes are popped by `ProcessNodeIds`.
  return true;
}

}  // namespace Carbon::Check