carbon-lang/toolchain/check/member_access.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/check/member_access.h"

#include <optional>

#include "llvm/ADT/STLExtras.h"
#include "toolchain/base/kind_switch.h"
#include "toolchain/check/context.h"
#include "toolchain/check/convert.h"
#include "toolchain/check/import_ref.h"
#include "toolchain/diagnostics/diagnostic_emitter.h"
#include "toolchain/sem_ir/generic.h"
#include "toolchain/sem_ir/ids.h"
#include "toolchain/sem_ir/inst.h"
#include "toolchain/sem_ir/name_scope.h"
#include "toolchain/sem_ir/typed_insts.h"

namespace Carbon::Check {

// Returns the lookup scope corresponding to base_id, or nullopt if not a scope.
// On invalid scopes, prints a diagnostic and still returns the scope.
static auto GetAsLookupScope(Context& context, SemIR::LocId loc_id,
                             SemIR::ConstantId base_const_id)
    -> std::optional<LookupScope> {
  auto base_id = context.constant_values().GetInstId(base_const_id);
  auto base = context.insts().Get(base_id);
  if (auto base_as_namespace = base.TryAs<SemIR::Namespace>()) {
    return LookupScope{.name_scope_id = base_as_namespace->name_scope_id,
                       .specific_id = SemIR::SpecificId::Invalid};
  }
  // TODO: Consider refactoring the near-identical class and interface support
  // below.
  if (auto base_as_class = base.TryAs<SemIR::ClassType>()) {
    context.TryToDefineType(
        context.GetTypeIdForTypeConstant(base_const_id), [&] {
          CARBON_DIAGNOSTIC(QualifiedExprInIncompleteClassScope, Error,
                            "Member access into incomplete class `{0}`.",
                            std::string);
          return context.emitter().Build(
              loc_id, QualifiedExprInIncompleteClassScope,
              context.sem_ir().StringifyType(base_const_id));
        });
    auto& class_info = context.classes().Get(base_as_class->class_id);
    return LookupScope{.name_scope_id = class_info.scope_id,
                       .specific_id = base_as_class->specific_id};
  }
  if (auto base_as_interface = base.TryAs<SemIR::InterfaceType>()) {
    context.TryToDefineType(
        context.GetTypeIdForTypeConstant(base_const_id), [&] {
          CARBON_DIAGNOSTIC(QualifiedExprInUndefinedInterfaceScope, Error,
                            "Member access into undefined interface `{0}`.",
                            std::string);
          return context.emitter().Build(
              loc_id, QualifiedExprInUndefinedInterfaceScope,
              context.sem_ir().StringifyType(base_const_id));
        });
    auto& interface_info =
        context.interfaces().Get(base_as_interface->interface_id);
    return LookupScope{.name_scope_id = interface_info.scope_id,
                       .specific_id = base_as_interface->specific_id};
  }
  // TODO: Per the design, if `base_id` is any kind of type, then lookup should
  // treat it as a name scope, even if it doesn't have members. For example,
  // `(i32*).X` should fail because there's no name `X` in `i32*`, not because
  // there's no name `X` in `type`.
  return std::nullopt;
}

// Returns the index of the specified class element within the class's
// representation.
static auto GetClassElementIndex(Context& context, SemIR::InstId element_id)
    -> SemIR::ElementIndex {
  auto element_inst = context.insts().Get(element_id);
  if (auto field = element_inst.TryAs<SemIR::FieldDecl>()) {
    return field->index;
  }
  if (auto base = element_inst.TryAs<SemIR::BaseDecl>()) {
    return base->index;
  }
  CARBON_FATAL("Unexpected value {0} in class element name", element_inst);
}

// Returns whether `function_id` is an instance method, that is, whether it has
// an implicit `self` parameter.
static auto IsInstanceMethod(const SemIR::File& sem_ir,
                             SemIR::FunctionId function_id) -> bool {
  const auto& function = sem_ir.functions().Get(function_id);
  for (auto param_id :
       sem_ir.inst_blocks().GetOrEmpty(function.implicit_param_refs_id)) {
    auto param =
        SemIR::Function::GetParamFromParamRefId(sem_ir, param_id).second;
    if (param.name_id == SemIR::NameId::SelfValue) {
      return true;
    }
  }

  return false;
}

// Returns the FunctionId of the current function if it exists.
static auto GetCurrentFunction(Context& context)
    -> std::optional<SemIR::FunctionId> {
  if (context.return_scope_stack().empty()) {
    return std::nullopt;
  }

  return context.insts()
      .GetAs<SemIR::FunctionDecl>(context.return_scope_stack().back().decl_id)
      .function_id;
}

// Returns the highest allowed access. For example, if this returns `Protected`
// then only `Public` and `Protected` accesses are allowed--not `Private`.
static auto GetHighestAllowedAccess(Context& context, SemIRLoc loc,
                                    SemIR::ConstantId name_scope_const_id)
    -> SemIR::AccessKind {
  // TODO: Maybe use LookupUnqualifiedName for `Self` to support things like
  // `var x: Self.ParentProtectedType`?
  auto current_function = GetCurrentFunction(context);
  // If `current_function` is a `nullopt` then we're accessing from a global
  // variable.
  if (!current_function) {
    return SemIR::AccessKind::Public;
  }

  auto scope_id = context.functions().Get(*current_function).parent_scope_id;
  if (!scope_id.is_valid()) {
    return SemIR::AccessKind::Public;
  }
  auto scope = context.name_scopes().Get(scope_id);

  // Lookup the inst for `Self` in the parent scope of the current function.
  auto [self_type_inst_id, _] = context.LookupNameInExactScope(
      loc, SemIR::NameId::SelfType, scope_id, scope);
  if (!self_type_inst_id.is_valid()) {
    return SemIR::AccessKind::Public;
  }

  // TODO: Support other types for `Self`.
  auto self_class_type =
      context.insts().TryGetAs<SemIR::ClassType>(self_type_inst_id);
  if (!self_class_type) {
    return SemIR::AccessKind::Public;
  }

  auto self_class_info = context.classes().Get(self_class_type->class_id);

  // TODO: Support other types.
  if (auto class_type = context.insts().TryGetAs<SemIR::ClassType>(
          context.constant_values().GetInstId(name_scope_const_id))) {
    auto class_info = context.classes().Get(class_type->class_id);

    if (self_class_info.self_type_id == class_info.self_type_id) {
      return SemIR::AccessKind::Private;
    }

    // If the `type_id` of `Self` does not match with the one we're currently
    // accessing, try checking if this class is of the parent type of `Self`.
    if (auto base_decl = context.insts().TryGetAsIfValid<SemIR::BaseDecl>(
            self_class_info.base_id)) {
      if (base_decl->base_type_id == class_info.self_type_id) {
        return SemIR::AccessKind::Protected;
      }
    } else if (auto adapt_decl =
                   context.insts().TryGetAsIfValid<SemIR::AdaptDecl>(
                       self_class_info.adapt_id)) {
      if (adapt_decl->adapted_type_id == class_info.self_type_id) {
        return SemIR::AccessKind::Protected;
      }
    }
  }

  return SemIR::AccessKind::Public;
}

// Returns whether `scope` is a scope for which impl lookup should be performed
// if we find an associated entity.
static auto ScopeNeedsImplLookup(Context& context, LookupScope scope) -> bool {
  auto [_, inst] = context.name_scopes().GetInstIfValid(scope.name_scope_id);
  if (!inst) {
    return false;
  }

  if (inst->Is<SemIR::InterfaceDecl>()) {
    // Don't perform impl lookup if an associated entity is named as a member of
    // a facet type.
    return false;
  }
  if (inst->Is<SemIR::Namespace>()) {
    // Don't perform impl lookup if an associated entity is named as a namespace
    // member.
    // TODO: This case is not yet listed in the design.
    return false;
  }
  // Any other kind of scope is assumed to be a type that implements the
  // interface containing the associated entity, and impl lookup is performed.
  return true;
}

// Given a type and an interface, searches for an impl that describes how that
// type implements that interface, and returns the corresponding witness.
// Returns an invalid InstId if no matching impl is found.
static auto LookupInterfaceWitness(Context& context,
                                   SemIR::ConstantId type_const_id,
                                   SemIR::TypeId interface_type_id)
    -> SemIR::InstId {
  // TODO: Add a better impl lookup system. At the very least, we should only be
  // considering impls that are for the same interface we're querying. We can
  // also skip impls that mention any types that aren't part of our impl query.
  for (const auto& impl : context.impls().array_ref()) {
    if (!context.constant_values().AreEqualAcrossDeclarations(
            context.types().GetConstantId(impl.self_id), type_const_id)) {
      continue;
    }
    if (!context.types().AreEqualAcrossDeclarations(impl.constraint_id,
                                                    interface_type_id)) {
      // TODO: An impl of a constraint type should be treated as implementing
      // the constraint's interfaces.
      continue;
    }
    if (!impl.witness_id.is_valid()) {
      // TODO: Diagnose if the impl isn't defined yet?
      return SemIR::InstId::Invalid;
    }
    LoadImportRef(context, impl.witness_id);
    return impl.witness_id;
  }
  return SemIR::InstId::Invalid;
}

// Performs impl lookup for a member name expression. This finds the relevant
// impl witness and extracts the corresponding impl member.
static auto PerformImplLookup(
    Context& context, SemIR::LocId loc_id, SemIR::ConstantId type_const_id,
    SemIR::AssociatedEntityType assoc_type, SemIR::InstId member_id,
    std::optional<Context::BuildDiagnosticFn> missing_impl_diagnoser)
    -> SemIR::InstId {
  auto interface_type =
      context.types().GetAs<SemIR::InterfaceType>(assoc_type.interface_type_id);
  auto& interface = context.interfaces().Get(interface_type.interface_id);
  auto witness_id = LookupInterfaceWitness(context, type_const_id,
                                           assoc_type.interface_type_id);
  if (!witness_id.is_valid()) {
    if (missing_impl_diagnoser) {
      CARBON_DIAGNOSTIC(MissingImplInMemberAccessNote, Note,
                        "Type `{1}` does not implement interface `{0}`.",
                        SemIR::NameId, SemIR::TypeId);
      (*missing_impl_diagnoser)()
          .Note(loc_id, MissingImplInMemberAccessNote, interface.name_id,
                context.GetTypeIdForTypeConstant(type_const_id))
          .Emit();
    } else {
      CARBON_DIAGNOSTIC(MissingImplInMemberAccess, Error,
                        "Cannot access member of interface `{0}` in type `{1}` "
                        "that does not implement that interface.",
                        SemIR::NameId, SemIR::TypeId);
      context.emitter().Emit(loc_id, MissingImplInMemberAccess,
                             interface.name_id,
                             context.GetTypeIdForTypeConstant(type_const_id));
    }
    return SemIR::InstId::BuiltinError;
  }

  auto member_value_id = context.constant_values().GetConstantInstId(member_id);
  if (!member_value_id.is_valid()) {
    if (member_value_id != SemIR::InstId::BuiltinError) {
      context.TODO(member_id, "non-constant associated entity");
    }
    return SemIR::InstId::BuiltinError;
  }

  auto assoc_entity =
      context.insts().TryGetAs<SemIR::AssociatedEntity>(member_value_id);
  if (!assoc_entity) {
    context.TODO(member_id, "unexpected value for associated entity");
    return SemIR::InstId::BuiltinError;
  }

  // TODO: This produces the type of the associated entity with no value for
  // `Self`. The type `Self` might appear in the type of an associated constant,
  // and if so, we'll need to substitute it here somehow.
  auto subst_type_id = SemIR::GetTypeInSpecific(
      context.sem_ir(), interface_type.specific_id, assoc_type.entity_type_id);

  return context.AddInst<SemIR::InterfaceWitnessAccess>(
      loc_id, {.type_id = subst_type_id,
               .witness_id = witness_id,
               .index = assoc_entity->index});
}

// Performs a member name lookup into the specified scope, including performing
// impl lookup if necessary. If the scope is invalid, assume an error has
// already been diagnosed, and return BuiltinError.
static auto LookupMemberNameInScope(Context& context, SemIR::LocId loc_id,
                                    SemIR::InstId /*base_id*/,
                                    SemIR::NameId name_id,
                                    SemIR::ConstantId name_scope_const_id,
                                    LookupScope lookup_scope) -> SemIR::InstId {
  LookupResult result = {.specific_id = SemIR::SpecificId::Invalid,
                         .inst_id = SemIR::InstId::BuiltinError};
  if (lookup_scope.name_scope_id.is_valid()) {
    AccessInfo access_info = {
        .constant_id = name_scope_const_id,
        .highest_allowed_access =
            GetHighestAllowedAccess(context, loc_id, name_scope_const_id),
    };

    result = context.LookupQualifiedName(loc_id, name_id, lookup_scope,
                                         /*required=*/true, access_info);

    if (!result.inst_id.is_valid()) {
      return SemIR::InstId::BuiltinError;
    }
  }

  // TODO: This duplicates the work that HandleNameAsExpr does. Factor this out.
  auto inst = context.insts().Get(result.inst_id);
  auto type_id = SemIR::GetTypeInSpecific(context.sem_ir(), result.specific_id,
                                          inst.type_id());
  CARBON_CHECK(type_id.is_valid(), "Missing type for member {0}", inst);

  // If the named entity has a constant value that depends on its specific,
  // store the specific too.
  if (result.specific_id.is_valid() &&
      context.constant_values().Get(result.inst_id).is_symbolic()) {
    result.inst_id = context.AddInst<SemIR::SpecificConstant>(
        loc_id, {.type_id = type_id,
                 .inst_id = result.inst_id,
                 .specific_id = result.specific_id});
  }

  // TODO: Use a different kind of instruction that also references the
  // `base_id` so that `SemIR` consumers can find it.
  auto member_id = context.AddInst<SemIR::NameRef>(
      loc_id,
      {.type_id = type_id, .name_id = name_id, .value_id = result.inst_id});

  // If member name lookup finds an associated entity name, and the scope is not
  // a facet type, perform impl lookup.
  //
  // TODO: We need to do this as part of searching extended scopes, because a
  // lookup that finds an associated entity and also finds the corresponding
  // impl member is not supposed to be treated as ambiguous.
  if (auto assoc_type =
          context.types().TryGetAs<SemIR::AssociatedEntityType>(type_id)) {
    if (ScopeNeedsImplLookup(context, lookup_scope)) {
      member_id = PerformImplLookup(context, loc_id, name_scope_const_id,
                                    *assoc_type, member_id, std::nullopt);
    }
  }

  return member_id;
}

// Performs the instance binding step in member access. If the found member is a
// field, forms a class member access. If the found member is an instance
// method, forms a bound method. Otherwise, the member is returned unchanged.
static auto PerformInstanceBinding(Context& context, SemIR::LocId loc_id,
                                   SemIR::InstId base_id,
                                   SemIR::InstId member_id) -> SemIR::InstId {
  auto member_type_id = context.insts().Get(member_id).type_id();
  CARBON_KIND_SWITCH(context.types().GetAsInst(member_type_id)) {
    case CARBON_KIND(SemIR::UnboundElementType unbound_element_type): {
      // Convert the base to the type of the element if necessary.
      base_id = ConvertToValueOrRefOfType(context, loc_id, base_id,
                                          unbound_element_type.class_type_id);

      // Find the specified element, which could be either a field or a base
      // class, and build an element access expression.
      auto element_id = context.constant_values().GetConstantInstId(member_id);
      CARBON_CHECK(element_id.is_valid(),
                   "Non-constant value {0} of unbound element type",
                   context.insts().Get(member_id));
      auto index = GetClassElementIndex(context, element_id);
      auto access_id = context.AddInst<SemIR::ClassElementAccess>(
          loc_id, {.type_id = unbound_element_type.element_type_id,
                   .base_id = base_id,
                   .index = index});
      if (SemIR::GetExprCategory(context.sem_ir(), base_id) ==
              SemIR::ExprCategory::Value &&
          SemIR::GetExprCategory(context.sem_ir(), access_id) !=
              SemIR::ExprCategory::Value) {
        // Class element access on a value expression produces an ephemeral
        // reference if the class's value representation is a pointer to the
        // object representation. Add a value binding in that case so that the
        // expression category of the result matches the expression category of
        // the base.
        access_id = ConvertToValueExpr(context, access_id);
      }
      return access_id;
    }
    case CARBON_KIND(SemIR::FunctionType fn_type): {
      if (IsInstanceMethod(context.sem_ir(), fn_type.function_id)) {
        return context.AddInst<SemIR::BoundMethod>(
            loc_id, {.type_id = context.GetBuiltinType(
                         SemIR::BuiltinInstKind::BoundMethodType),
                     .object_id = base_id,
                     .function_id = member_id});
      }
      [[fallthrough]];
    }
    default:
      // Not an instance member: no instance binding.
      return member_id;
  }
}

// Validates that the index (required to be an IntLiteral) is valid within the
// tuple size. Returns the index on success, or nullptr on failure.
static auto ValidateTupleIndex(Context& context, SemIR::LocId loc_id,
                               SemIR::Inst operand_inst,
                               SemIR::IntLiteral index_inst, int size)
    -> const llvm::APInt* {
  const auto& index_val = context.ints().Get(index_inst.int_id);
  if (index_val.uge(size)) {
    CARBON_DIAGNOSTIC(
        TupleIndexOutOfBounds, Error,
        "Tuple element index `{0}` is past the end of type `{1}`.", TypedInt,
        SemIR::TypeId);
    context.emitter().Emit(loc_id, TupleIndexOutOfBounds,
                           {.type = index_inst.type_id, .value = index_val},
                           operand_inst.type_id());
    return nullptr;
  }
  return &index_val;
}

auto PerformMemberAccess(Context& context, SemIR::LocId loc_id,
                         SemIR::InstId base_id, SemIR::NameId name_id)
    -> SemIR::InstId {
  // If the base is a name scope, such as a class or namespace, perform lookup
  // into that scope.
  if (auto base_const_id = context.constant_values().Get(base_id);
      base_const_id.is_constant()) {
    if (auto lookup_scope = GetAsLookupScope(context, loc_id, base_const_id)) {
      return LookupMemberNameInScope(context, loc_id, base_id, name_id,
                                     base_const_id, *lookup_scope);
    }
  }

  // If the base isn't a scope, it must have a complete type.
  auto base_type_id = context.insts().Get(base_id).type_id();
  if (!context.TryToCompleteType(base_type_id, [&] {
        CARBON_DIAGNOSTIC(IncompleteTypeInMemberAccess, Error,
                          "Member access into object of incomplete type `{0}`.",
                          SemIR::TypeId);
        return context.emitter().Build(base_id, IncompleteTypeInMemberAccess,
                                       base_type_id);
      })) {
    return SemIR::InstId::BuiltinError;
  }

  // Materialize a temporary for the base expression if necessary.
  base_id = ConvertToValueOrRefExpr(context, base_id);
  base_type_id = context.insts().Get(base_id).type_id();
  auto base_type_const_id = context.types().GetConstantId(base_type_id);

  // Find the scope corresponding to the base type.
  auto lookup_scope = GetAsLookupScope(context, loc_id, base_type_const_id);
  if (!lookup_scope) {
    // The base type is not a name scope. Try some fallback options.
    if (auto struct_type = context.insts().TryGetAs<SemIR::StructType>(
            context.constant_values().GetInstId(base_type_const_id))) {
      // TODO: Do we need to optimize this with a lookup table for O(1)?
      for (auto [i, ref_id] :
           llvm::enumerate(context.inst_blocks().Get(struct_type->fields_id))) {
        auto field = context.insts().GetAs<SemIR::StructTypeField>(ref_id);
        if (name_id == field.name_id) {
          // TODO: Model this as producing a lookup result, and do instance
          // binding separately. Perhaps a struct type should be a name scope.
          return context.AddInst<SemIR::StructAccess>(
              loc_id, {.type_id = field.field_type_id,
                       .struct_id = base_id,
                       .index = SemIR::ElementIndex(i)});
        }
      }
      CARBON_DIAGNOSTIC(QualifiedExprNameNotFound, Error,
                        "Type `{0}` does not have a member `{1}`.",
                        SemIR::TypeId, SemIR::NameId);
      context.emitter().Emit(loc_id, QualifiedExprNameNotFound, base_type_id,
                             name_id);
      return SemIR::InstId::BuiltinError;
    }

    if (base_type_id != SemIR::TypeId::Error) {
      CARBON_DIAGNOSTIC(QualifiedExprUnsupported, Error,
                        "Type `{0}` does not support qualified expressions.",
                        SemIR::TypeId);
      context.emitter().Emit(loc_id, QualifiedExprUnsupported, base_type_id);
    }
    return SemIR::InstId::BuiltinError;
  }

  // Perform lookup into the base type.
  auto member_id = LookupMemberNameInScope(context, loc_id, base_id, name_id,
                                           base_type_const_id, *lookup_scope);

  // Perform instance binding if we found an instance member.
  member_id = PerformInstanceBinding(context, loc_id, base_id, member_id);

  return member_id;
}

auto PerformCompoundMemberAccess(
    Context& context, SemIR::LocId loc_id, SemIR::InstId base_id,
    SemIR::InstId member_expr_id,
    std::optional<Context::BuildDiagnosticFn> missing_impl_diagnoser)
    -> SemIR::InstId {
  // Materialize a temporary for the base expression if necessary.
  base_id = ConvertToValueOrRefExpr(context, base_id);
  auto base_type_id = context.insts().Get(base_id).type_id();
  auto base_type_const_id = context.types().GetConstantId(base_type_id);

  auto member_id = member_expr_id;
  auto member = context.insts().Get(member_id);

  // If the member expression names an associated entity, impl lookup is always
  // performed using the type of the base expression.
  if (auto assoc_type = context.types().TryGetAs<SemIR::AssociatedEntityType>(
          member.type_id())) {
    member_id =
        PerformImplLookup(context, loc_id, base_type_const_id, *assoc_type,
                          member_id, missing_impl_diagnoser);
  } else if (context.insts().Is<SemIR::TupleType>(
                 context.constant_values().GetInstId(base_type_const_id))) {
    return PerformTupleIndex(context, loc_id, base_id, member_expr_id);
  }

  // Perform instance binding if we found an instance member.
  member_id = PerformInstanceBinding(context, loc_id, base_id, member_id);

  // If we didn't perform impl lookup or instance binding, that's an error
  // because the base expression is not used for anything.
  if (member_id == member_expr_id && member.type_id() != SemIR::TypeId::Error) {
    CARBON_DIAGNOSTIC(CompoundMemberAccessDoesNotUseBase, Error,
                      "Member name of type `{0}` in compound member access is "
                      "not an instance member or an interface member.",
                      SemIR::TypeId);
    context.emitter().Emit(loc_id, CompoundMemberAccessDoesNotUseBase,
                           member.type_id());
  }

  return member_id;
}

auto PerformTupleIndex(Context& context, SemIR::LocId loc_id,
                       SemIR::InstId tuple_inst_id, SemIR::InstId index_inst_id)
    -> SemIR::InstId {
  tuple_inst_id = ConvertToValueOrRefExpr(context, tuple_inst_id);
  auto tuple_inst = context.insts().Get(tuple_inst_id);
  auto tuple_type_id = tuple_inst.type_id();

  auto tuple_type = context.types().TryGetAs<SemIR::TupleType>(tuple_type_id);
  if (!tuple_type) {
    CARBON_DIAGNOSTIC(TupleIndexOnANonTupleType, Error,
                      "Type `{0}` does not support tuple indexing. Only "
                      "tuples can be indexed that way.",
                      SemIR::TypeId);
    context.emitter().Emit(loc_id, TupleIndexOnANonTupleType, tuple_type_id);
    return SemIR::InstId::BuiltinError;
  }

  SemIR::TypeId element_type_id = SemIR::TypeId::Error;
  auto index_node_id = context.insts().GetLocId(index_inst_id);
  index_inst_id = ConvertToValueOfType(
      context, index_node_id, index_inst_id,
      context.GetBuiltinType(SemIR::BuiltinInstKind::IntType));
  auto index_const_id = context.constant_values().Get(index_inst_id);
  if (index_const_id == SemIR::ConstantId::Error) {
    index_inst_id = SemIR::InstId::BuiltinError;
  } else if (!index_const_id.is_template()) {
    // TODO: Decide what to do if the index is a symbolic constant.
    CARBON_DIAGNOSTIC(TupleIndexNotConstant, Error,
                      "Tuple index must be a constant.");
    context.emitter().Emit(loc_id, TupleIndexNotConstant);
    index_inst_id = SemIR::InstId::BuiltinError;
  } else {
    auto index_literal = context.insts().GetAs<SemIR::IntLiteral>(
        context.constant_values().GetInstId(index_const_id));
    auto type_block = context.type_blocks().Get(tuple_type->elements_id);
    if (const auto* index_val = ValidateTupleIndex(
            context, loc_id, tuple_inst, index_literal, type_block.size())) {
      element_type_id = type_block[index_val->getZExtValue()];
    } else {
      index_inst_id = SemIR::InstId::BuiltinError;
    }
  }

  return context.AddInst<SemIR::TupleIndex>(loc_id,
                                            {.type_id = element_type_id,
                                             .tuple_id = tuple_inst_id,
                                             .index_id = index_inst_id});
}

}  // namespace Carbon::Check