Implement support for copying C++ classes. (#6434)

When performing impl lookup for `Core.Copy` for a C++ class type, look
for a copy constructor. If we find one, synthesize an impl witness that
calls the constructor.

This adds initial support for impl lookup to delegate to the C++ interop
logic for queries involving C++ types. For now, we don't implement the
rules from #6166 that compare a synthesized type structure for the C++
impl against the best Carbon type structure, but the framework for
building that support is established here.

Currently there is no caching of the lookup here, and we build unique
`ImplWitnessTable`s for each lookup, which leads to each impl lookup
producing a distinct facet value. This results in some errors in generic
contexts; this will be addressed in follow-up changes. This PR aims only
to support the non-generic case.

---------

Co-authored-by: Dana Jansens <danakj@orodu.net>
Co-authored-by: Carbon Infra Bot <carbon-external-infra@google.com>

toolchain/check/BUILD

@@ -24,6 +24,7 @@ cc_library(
         "cpp/access.cpp",
         "cpp/call.cpp",
         "cpp/custom_type_mapping.cpp",
+        "cpp/impl_lookup.cpp",
         "cpp/import.cpp",
         "cpp/location.cpp",
         "cpp/macros.cpp",
@@ -77,6 +78,7 @@ cc_library(
         "cpp/access.h",
         "cpp/call.h",
         "cpp/custom_type_mapping.h",
+        "cpp/impl_lookup.h",
         "cpp/import.h",
         "cpp/location.h",
         "cpp/macros.h",

toolchain/check/cpp/impl_lookup.cpp

@@ -0,0 +1,190 @@
+// 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/cpp/impl_lookup.h"
+
+#include "clang/Sema/Sema.h"
+#include "toolchain/base/kind_switch.h"
+#include "toolchain/check/cpp/import.h"
+#include "toolchain/check/cpp/location.h"
+#include "toolchain/check/cpp/overload_resolution.h"
+#include "toolchain/check/impl.h"
+#include "toolchain/check/impl_lookup.h"
+#include "toolchain/check/import_ref.h"
+#include "toolchain/check/inst.h"
+#include "toolchain/check/type.h"
+#include "toolchain/sem_ir/ids.h"
+#include "toolchain/sem_ir/typed_insts.h"
+
+namespace Carbon::Check {
+
+// If the given type is a C++ class type, returns the corresponding class
+// declaration. Otherwise returns nullptr.
+// TODO: Handle qualified types.
+static auto TypeAsClassDecl(Context& context, SemIR::TypeId type_id)
+    -> clang::CXXRecordDecl* {
+  auto class_type = context.types().TryGetAs<SemIR::ClassType>(type_id);
+  if (!class_type) {
+    // Not a class.
+    return nullptr;
+  }
+
+  SemIR::NameScopeId class_scope_id =
+      context.classes().Get(class_type->class_id).scope_id;
+  if (!class_scope_id.has_value()) {
+    return nullptr;
+  }
+
+  const auto& scope = context.name_scopes().Get(class_scope_id);
+  auto decl_id = scope.clang_decl_context_id();
+  if (!decl_id.has_value()) {
+    return nullptr;
+  }
+
+  return dyn_cast<clang::CXXRecordDecl>(
+      context.clang_decls().Get(decl_id).key.decl);
+}
+
+// Builds a witness that the given type implements the given interface,
+// populating it with the specified set of values. Returns a corresponding
+// lookup result. Produces a diagnostic and returns `None` if the specified
+// values aren't suitable for the interface.
+static auto BuildWitness(Context& context, SemIR::LocId loc_id,
+                         SemIR::TypeId self_type_id,
+                         SemIR::SpecificInterface specific_interface,
+                         llvm::ArrayRef<SemIR::InstId> values)
+    -> SemIR::InstId {
+  const auto& interface =
+      context.interfaces().Get(specific_interface.interface_id);
+  auto assoc_entities =
+      context.inst_blocks().GetOrEmpty(interface.associated_entities_id);
+  if (assoc_entities.size() != values.size()) {
+    context.TODO(loc_id, ("Unsupported definition of interface " +
+                          context.names().GetFormatted(interface.name_id))
+                             .str());
+    return SemIR::ErrorInst::InstId;
+  }
+
+  // Prepare an empty witness table.
+  auto witness_table_id =
+      context.inst_blocks().AddUninitialized(assoc_entities.size());
+  auto witness_table = context.inst_blocks().GetMutable(witness_table_id);
+  for (auto& witness_value_id : witness_table) {
+    witness_value_id = SemIR::InstId::ImplWitnessTablePlaceholder;
+  }
+
+  // Build a witness. We use an `ImplWitness` with an `impl_id` of `None` to
+  // represent a synthesized witness.
+  // TODO: Stop using `ImplWitnessTable` here and add a distinct instruction
+  // that doesn't contain an `InstId` and supports deduplication.
+  auto witness_table_inst_id = AddInst<SemIR::ImplWitnessTable>(
+      context, loc_id,
+      {.elements_id = witness_table_id, .impl_id = SemIR::ImplId::None});
+  auto witness_id = AddInst<SemIR::ImplWitness>(
+      context, loc_id,
+      {.type_id = GetSingletonType(context, SemIR::WitnessType::TypeInstId),
+       .witness_table_id = witness_table_inst_id,
+       .specific_id = SemIR::SpecificId::None});
+
+  // Fill in the witness table.
+  for (const auto& [assoc_entity_id, value_id, witness_value_id] :
+       llvm::zip_equal(assoc_entities, values, witness_table)) {
+    LoadImportRef(context, assoc_entity_id);
+    auto decl_id =
+        context.constant_values().GetInstId(SemIR::GetConstantValueInSpecific(
+            context.sem_ir(), specific_interface.specific_id, assoc_entity_id));
+    CARBON_CHECK(decl_id.has_value(), "Non-constant associated entity");
+    auto decl = context.insts().Get(decl_id);
+    CARBON_KIND_SWITCH(decl) {
+      case CARBON_KIND(SemIR::StructValue struct_value): {
+        if (struct_value.type_id == SemIR::ErrorInst::TypeId) {
+          return SemIR::ErrorInst::InstId;
+        }
+        witness_value_id = CheckAssociatedFunctionImplementation(
+            context,
+            context.types().GetAs<SemIR::FunctionType>(struct_value.type_id),
+            value_id, self_type_id, witness_id,
+            /*defer_thunk_definition=*/false);
+        break;
+      }
+      case SemIR::AssociatedConstantDecl::Kind: {
+        context.TODO(loc_id,
+                     "Associated constant in interface with synthesized impl");
+        return SemIR::ErrorInst::InstId;
+      }
+      default:
+        CARBON_CHECK(decl_id == SemIR::ErrorInst::InstId,
+                     "Unexpected kind of associated entity {0}", decl);
+        return SemIR::ErrorInst::InstId;
+    }
+  }
+
+  return witness_id;
+}
+
+static auto LookupCopyImpl(Context& context, SemIR::LocId loc_id,
+                           SemIR::TypeId self_type_id,
+                           SemIR::SpecificInterface specific_interface)
+    -> SemIR::InstId {
+  auto* class_decl = TypeAsClassDecl(context, self_type_id);
+  if (!class_decl) {
+    // TODO: Should we also provide a `Copy` implementation for enumerations?
+    return SemIR::InstId::None;
+  }
+
+  auto* ctor = context.clang_sema().LookupCopyingConstructor(
+      class_decl, clang::Qualifiers::Const);
+  if (!ctor) {
+    // TODO: If the impl lookup failure is an error, we should produce a
+    // diagnostic explaining why the class is not copyable.
+    return SemIR::InstId::None;
+  }
+
+  auto ctor_id =
+      context.clang_sema().DiagnoseUseOfOverloadedDecl(
+          ctor, GetCppLocation(context, loc_id))
+          ? SemIR::ErrorInst::InstId
+          : ImportCppFunctionDecl(context, loc_id, ctor, /*num_params=*/1);
+  if (auto ctor_decl =
+          context.insts().TryGetAsWithId<SemIR::FunctionDecl>(ctor_id)) {
+    CheckCppOverloadAccess(context, loc_id,
+                           clang::DeclAccessPair::make(ctor, ctor->getAccess()),
+                           ctor_decl->inst_id);
+  } else {
+    CARBON_CHECK(ctor_id == SemIR::ErrorInst::InstId);
+    return SemIR::ErrorInst::InstId;
+  }
+  return BuildWitness(context, loc_id, self_type_id, specific_interface,
+                      {ctor_id});
+}
+
+auto LookupCppImpl(Context& context, SemIR::LocId loc_id,
+                   SemIR::TypeId self_type_id,
+                   SemIR::SpecificInterface specific_interface,
+                   const TypeStructure* best_impl_type_structure,
+                   SemIR::LocId best_impl_loc_id) -> SemIR::InstId {
+  // Determine whether this is an interface that we have special knowledge of.
+  auto& interface = context.interfaces().Get(specific_interface.interface_id);
+  if (!context.name_scopes().IsCorePackage(interface.parent_scope_id)) {
+    return SemIR::InstId::None;
+  }
+  if (!interface.name_id.AsIdentifierId().has_value()) {
+    return SemIR::InstId::None;
+  }
+
+  if (context.identifiers().Get(interface.name_id.AsIdentifierId()) == "Copy") {
+    return LookupCopyImpl(context, loc_id, self_type_id, specific_interface);
+  }
+
+  // TODO: Handle other interfaces.
+
+  // TODO: Infer a C++ type structure and check whether it's less strict than
+  // the best Carbon type structure.
+  static_cast<void>(best_impl_type_structure);
+  static_cast<void>(best_impl_loc_id);
+
+  return SemIR::InstId::None;
+}
+
+}  // namespace Carbon::Check

toolchain/check/cpp/impl_lookup.h

@@ -0,0 +1,44 @@
+// 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
+
+#ifndef CARBON_TOOLCHAIN_CHECK_CPP_IMPL_LOOKUP_H_
+#define CARBON_TOOLCHAIN_CHECK_CPP_IMPL_LOOKUP_H_
+
+#include "toolchain/check/context.h"
+#include "toolchain/check/impl_lookup.h"
+#include "toolchain/check/type_structure.h"
+#include "toolchain/sem_ir/ids.h"
+#include "toolchain/sem_ir/specific_interface.h"
+
+namespace Carbon::Check {
+
+// Performs lookup for an impl witness for a query involving C++ types. Returns
+// a witness value, or `None` if a synthesized C++ witness should not be used.
+//
+// If `interface` is an interface for which we can synthesize a witness based on
+// C++ operator overloads or special member functions, performs the suitable C++
+// lookup to determine if this interface should be considered implemented for
+// the specified type, and if so, synthesizes and returns a suitable witness.
+//
+// `best_impl_type_structure` provides the type structure of the best-matching
+// impl declaration. If this is better than every viable C++ candidate, a "none"
+// result will be returned. If this is worse than the best viable C++ candidate
+// according to C++ rules, a witness for the C++ candidate will be returned.
+// Otherwise, it is at least as good as the best viable C++ candidate, but there
+// is some C++ candidate that has a better type structure, in which case the
+// result is ambiguous and we diagnose an error. This parameter can be null if
+// there is no usable impl for this query.
+//
+// `best_impl_loc_id` gives the location of the impl corresponding to the best
+// type structure, and can be `None` if `best_impl_type_structure` is null. This
+// parameter is used only for ambiguity diagnostics.
+auto LookupCppImpl(Context& context, SemIR::LocId loc_id,
+                   SemIR::TypeId self_type_id,
+                   SemIR::SpecificInterface specific_interface,
+                   const TypeStructure* best_impl_type_structure,
+                   SemIR::LocId best_impl_loc_id) -> SemIR::InstId;
+
+}  // namespace Carbon::Check
+
+#endif  // CARBON_TOOLCHAIN_CHECK_CPP_IMPL_LOOKUP_H_

toolchain/check/cpp/import.cpp

@@ -40,6 +40,7 @@
 #include "toolchain/check/convert.h"
 #include "toolchain/check/cpp/access.h"
 #include "toolchain/check/cpp/custom_type_mapping.h"
+#include "toolchain/check/cpp/location.h"
 #include "toolchain/check/cpp/macros.h"
 #include "toolchain/check/cpp/thunk.h"
 #include "toolchain/check/diagnostic_helpers.h"
@@ -1809,6 +1810,11 @@ static auto ImportFunctionDecl(Context& context, SemIR::LocId loc_id,
         function_info.SetHasCppThunk(thunk_function_decl_id);
       }
     }
+  } else {
+    // Inform Clang that the function has been referenced. This will trigger
+    // instantiation if needed.
+    context.clang_sema().MarkFunctionReferenced(GetCppLocation(context, loc_id),
+                                                clang_decl);
   }
 
   return function_info.first_owning_decl_id;

toolchain/check/cpp/operators.cpp

@@ -25,7 +25,7 @@ static auto GetClangOperatorKind(Context& context, SemIR::LocId loc_id,
     -> std::optional<clang::OverloadedOperatorKind> {
   // Unary operators.
   if (interface_name == "Destroy" || interface_name == "As" ||
-      interface_name == "ImplicitAs") {
+      interface_name == "ImplicitAs" || interface_name == "Copy") {
     // TODO: Support destructors and conversions.
     return std::nullopt;
   }
@@ -280,17 +280,37 @@ auto IsCppOperatorMethodDecl(clang::Decl* decl) -> bool {
   return clang_method_decl && clang_method_decl->isOverloadedOperator();
 }
 
-auto IsCppOperatorMethod(Context& context, SemIR::InstId inst_id) -> bool {
+static auto GetAsCppFunctionDecl(Context& context, SemIR::InstId inst_id)
+    -> clang::FunctionDecl* {
   auto function_type = context.types().TryGetAs<SemIR::FunctionType>(
       context.insts().Get(inst_id).type_id());
   if (!function_type) {
-    return false;
+    return nullptr;
   }
   SemIR::ClangDeclId clang_decl_id =
       context.functions().Get(function_type->function_id).clang_decl_id;
-  return clang_decl_id.has_value() &&
-         IsCppOperatorMethodDecl(
-             context.clang_decls().Get(clang_decl_id).key.decl);
+  return clang_decl_id.has_value()
+             ? dyn_cast<clang::FunctionDecl>(
+                   context.clang_decls().Get(clang_decl_id).key.decl)
+             : nullptr;
+}
+
+auto IsCppOperatorMethod(Context& context, SemIR::InstId inst_id) -> bool {
+  auto* function_decl = GetAsCppFunctionDecl(context, inst_id);
+  return function_decl && IsCppOperatorMethodDecl(function_decl);
+}
+
+auto IsCppConstructorOrNonMethodOperator(Context& context,
+                                         SemIR::InstId inst_id) -> bool {
+  auto* function_decl = GetAsCppFunctionDecl(context, inst_id);
+  if (!function_decl) {
+    return false;
+  }
+  if (isa<clang::CXXConstructorDecl>(function_decl)) {
+    return true;
+  }
+  return !isa<clang::CXXMethodDecl>(function_decl) &&
+         function_decl->isOverloadedOperator();
 }
 
 }  // namespace Carbon::Check

toolchain/check/cpp/operators.h

@@ -25,6 +25,12 @@ auto IsCppOperatorMethodDecl(clang::Decl* decl) -> bool;
 // than as the first argument.
 auto IsCppOperatorMethod(Context& context, SemIR::InstId inst_id) -> bool;
 
+// Returns whether the specified instruction refers to a C++ constructor or
+// non-operator method. If so, when mapping from a Carbon interface to a C++
+// call, we pass a `self` parameter as the first argument instead.
+auto IsCppConstructorOrNonMethodOperator(Context& context,
+                                         SemIR::InstId inst_id) -> bool;
+
 }  // namespace Carbon::Check
 
 #endif  // CARBON_TOOLCHAIN_CHECK_CPP_OPERATORS_H_

toolchain/check/cpp/overload_resolution.cpp

@@ -162,7 +162,6 @@ auto PerformCppOverloadResolution(Context& context, SemIR::LocId loc_id,
     case clang::OverloadingResult::OR_Success: {
       CARBON_CHECK(best_viable_fn->Function);
       CARBON_CHECK(!best_viable_fn->RewriteKind);
-      sema.MarkFunctionReferenced(loc, best_viable_fn->Function);
       SemIR::InstId result_id = ImportCppFunctionDecl(
           context, loc_id, best_viable_fn->Function, arg_exprs.size());
       if (auto fn_decl =

toolchain/check/impl.cpp

@@ -38,26 +38,25 @@ static auto NoteAssociatedFunction(Context& context, DiagnosticBuilder& builder,
                function.name_id);
 }
 
-// Checks that `impl_function_id` is a valid implementation of the function
-// described in the interface as `interface_function_id`. Returns the value to
-// put into the corresponding slot in the witness table, which can be
-// `BuiltinErrorInst` if the function is not usable.
-static auto CheckAssociatedFunctionImplementation(
+auto CheckAssociatedFunctionImplementation(
     Context& context, SemIR::FunctionType interface_function_type,
     SemIR::InstId impl_decl_id, SemIR::TypeId self_type_id,
-    SemIR::InstId witness_inst_id) -> SemIR::InstId {
+    SemIR::InstId witness_inst_id, bool defer_thunk_definition)
+    -> SemIR::InstId {
   auto impl_function_decl =
       context.insts().TryGetAs<SemIR::FunctionDecl>(impl_decl_id);
   if (!impl_function_decl) {
-    CARBON_DIAGNOSTIC(ImplFunctionWithNonFunction, Error,
-                      "associated function {0} implemented by non-function",
-                      SemIR::NameId);
-    auto builder = context.emitter().Build(
-        impl_decl_id, ImplFunctionWithNonFunction,
-        context.functions().Get(interface_function_type.function_id).name_id);
-    NoteAssociatedFunction(context, builder,
-                           interface_function_type.function_id);
-    builder.Emit();
+    if (impl_decl_id != SemIR::ErrorInst::InstId) {
+      CARBON_DIAGNOSTIC(ImplFunctionWithNonFunction, Error,
+                        "associated function {0} implemented by non-function",
+                        SemIR::NameId);
+      auto builder = context.emitter().Build(
+          impl_decl_id, ImplFunctionWithNonFunction,
+          context.functions().Get(interface_function_type.function_id).name_id);
+      NoteAssociatedFunction(context, builder,
+                             interface_function_type.function_id);
+      builder.Emit();
+    }
 
     return SemIR::ErrorInst::InstId;
   }
@@ -80,7 +79,8 @@ static auto CheckAssociatedFunctionImplementation(
           impl_enclosing_specific_id, self_type_id, witness_inst_id);
 
   return BuildThunk(context, interface_function_type.function_id,
-                    interface_function_specific_id, impl_decl_id);
+                    interface_function_specific_id, impl_decl_id,
+                    defer_thunk_definition);
 }
 
 // Builds an initial witness from the rewrites in the facet type, if any.
@@ -212,7 +212,7 @@ auto FinishImplWitness(Context& context, SemIR::ImplId impl_id) -> void {
           used_decl_ids.push_back(lookup_result.target_inst_id());
           witness_value = CheckAssociatedFunctionImplementation(
               context, *fn_type, lookup_result.target_inst_id(), self_type_id,
-              impl.witness_id);
+              impl.witness_id, /*defer_thunk_definition=*/true);
         } else {
           CARBON_DIAGNOSTIC(
               ImplMissingFunction, Error,

toolchain/check/impl.h

@@ -35,6 +35,16 @@ auto AssignImplIdInWitness(Context& context, SemIR::ImplId impl_id,
 // being concrete.
 auto IsImplEffectivelyFinal(Context& context, const SemIR::Impl& impl) -> bool;
 
+// Checks that `impl_function_id` is a valid implementation of the function
+// described in the interface as `interface_function_id`. Returns the value to
+// put into the corresponding slot in the witness table, which can be
+// `ErrorInst::InstId` if the function is not usable.
+auto CheckAssociatedFunctionImplementation(
+    Context& context, SemIR::FunctionType interface_function_type,
+    SemIR::InstId impl_decl_id, SemIR::TypeId self_type_id,
+    SemIR::InstId witness_inst_id, bool defer_thunk_definition)
+    -> SemIR::InstId;
+
 // Checks that the constraint specified for the impl is valid and identified.
 // Returns the interface that the impl implements. On error, issues a diagnostic
 // and returns `None`.

toolchain/check/impl_lookup.cpp

@@ -10,6 +10,7 @@
 #include <variant>
 
 #include "toolchain/base/kind_switch.h"
+#include "toolchain/check/cpp/impl_lookup.h"
 #include "toolchain/check/deduce.h"
 #include "toolchain/check/diagnostic_helpers.h"
 #include "toolchain/check/eval.h"
@@ -46,9 +47,23 @@ static auto FindAssociatedImportIRs(
     if (!decl_id.has_value()) {
       return;
     }
-    if (auto ir_id = GetCanonicalImportIRInst(context, decl_id).ir_id();
-        ir_id.has_value()) {
-      result.push_back(ir_id);
+
+    auto import_ir_inst = GetCanonicalImportIRInst(context, decl_id);
+    const auto* sem_ir = &context.sem_ir();
+    if (import_ir_inst.ir_id().has_value()) {
+      sem_ir = context.import_irs().Get(import_ir_inst.ir_id()).sem_ir;
+    }
+
+    // For an instruction imported from C++, `GetCanonicalImportIRInst` returns
+    // the final Carbon import instruction, so go one extra step to check for a
+    // C++ import.
+    if (auto import_ir_inst_id =
+            sem_ir->insts().GetImportSource(import_ir_inst.inst_id());
+        import_ir_inst_id.has_value()) {
+      result.push_back(
+          sem_ir->import_ir_insts().Get(import_ir_inst_id).ir_id());
+    } else if (import_ir_inst.ir_id().has_value()) {
+      result.push_back(import_ir_inst.ir_id());
     }
   };
 
@@ -788,16 +803,29 @@ struct CandidateImpl {
   TypeStructure type_structure;
 };
 
+struct CandidateImpls {
+  llvm::SmallVector<CandidateImpl> impls;
+  bool consider_cpp_candidates = false;
+};
+
 // Returns the list of candidates impls for lookup to select from.
 static auto CollectCandidateImplsForQuery(
     Context& context, bool final_only, SemIR::ConstantId query_self_const_id,
     const TypeStructure& query_type_structure,
-    SemIR::SpecificInterface& query_specific_interface)
-    -> llvm::SmallVector<CandidateImpl> {
+    SemIR::SpecificInterface& query_specific_interface) -> CandidateImpls {
+  CandidateImpls candidates;
+
   auto import_irs = FindAssociatedImportIRs(context, query_self_const_id,
                                             query_specific_interface);
 
   for (auto import_ir_id : import_irs) {
+    // If `Cpp` is an associated package, then we'll instead look for C++
+    // operator overloads for certain well-known interfaces.
+    if (import_ir_id == SemIR::ImportIRId::Cpp) {
+      candidates.consider_cpp_candidates = true;
+      continue;
+    }
+
     // Instead of importing all impls, only import ones that are in some way
     // connected to this query.
     ImportImplFilter filter(context, import_ir_id, query_specific_interface);
@@ -811,7 +839,6 @@ static auto CollectCandidateImplsForQuery(
     }
   }
 
-  llvm::SmallVector<CandidateImpl> candidate_impls;
   for (auto [id, impl] : context.impls().enumerate()) {
     CARBON_CHECK(impl.witness_id.has_value());
 
@@ -854,7 +881,7 @@ static auto CollectCandidateImplsForQuery(
       continue;
     }
 
-    candidate_impls.push_back(
+    candidates.impls.push_back(
         {id, impl.definition_id, std::move(*type_structure)});
   }
 
@@ -866,9 +893,29 @@ static auto CollectCandidateImplsForQuery(
   // TODO: Allow Carbon code to provide a priority ordering explicitly. For
   // now they have all the same priority, so the priority is the order in
   // which they are found in code.
-  llvm::stable_sort(candidate_impls, compare);
+  llvm::stable_sort(candidates.impls, compare);
 
-  return candidate_impls;
+  return candidates;
+}
+
+// Given a value whose type `IsFacetTypeOrError`, returns the corresponding
+// type.
+static auto GetFacetAsType(Context& context, SemIR::LocId loc_id,
+                           SemIR::ConstantId facet_or_type_const_id)
+    -> SemIR::TypeId {
+  auto facet_or_type_id =
+      context.constant_values().GetInstId(facet_or_type_const_id);
+  auto type_type_id = context.insts().Get(facet_or_type_id).type_id();
+  CARBON_CHECK(context.types().IsFacetTypeOrError(type_type_id));
+
+  if (context.types().Is<SemIR::FacetType>(type_type_id)) {
+    // It's a facet; access its type.
+    facet_or_type_id = GetOrAddInst<SemIR::FacetAccessType>(
+        context, loc_id,
+        {.type_id = SemIR::TypeType::TypeId,
+         .facet_value_inst_id = facet_or_type_id});
+  }
+  return context.types().GetTypeIdForTypeInstId(facet_or_type_id);
 }
 
 auto EvalLookupSingleImplWitness(Context& context, SemIR::LocId loc_id,
@@ -885,6 +932,35 @@ auto EvalLookupSingleImplWitness(Context& context, SemIR::LocId loc_id,
     return facet_lookup_result;
   }
 
+  // Ensure specifics don't substitute in weird things for the query self.
+  CARBON_CHECK(context.types().IsFacetType(
+      context.insts().Get(eval_query.query_self_inst_id).type_id()));
+  SemIR::ConstantId query_self_const_id =
+      context.constant_values().Get(eval_query.query_self_inst_id);
+
+  // The kind of lookup we're performing, which determines what kind of result
+  // we provide.
+  enum LookupKind {
+    // This is a concrete query, which should either provide a concrete witness
+    // or fail.
+    Concrete,
+    // This query refers to an interface that can be found symbolically within
+    // the facet type of the self value. The lookup will always succeed, but we
+    // are still checking in case a more precise final impl supplies values of
+    // associated constants.
+    FoundInFacet,
+    // This is an impl lookup with a symbolic query.
+    Symbolic,
+  };
+
+  LookupKind kind =
+      QueryIsConcrete(context, query_self_const_id, query_specific_interface)
+          ? Concrete
+      : facet_lookup_result.has_value() ? FoundInFacet
+                                        : Symbolic;
+  CARBON_CHECK(kind != Concrete || !facet_lookup_result.has_value(),
+               "Non-concrete facet lookup value for concrete query");
+
   // If the self type is a facet that provides a witness, then we are in an
   // `interface` or an `impl`. In both cases, we don't want to do any impl
   // lookups. The query will eventually resolve to a concrete witness when it
@@ -897,8 +973,7 @@ auto EvalLookupSingleImplWitness(Context& context, SemIR::LocId loc_id,
   // when the eval block is run, it finds the same `impl`, tries to build a
   // specific from it, which runs the eval block, creating a recursive loop that
   // crashes.
-  bool self_facet_provides_witness = facet_lookup_result.has_value();
-  if (self_facet_provides_witness) {
+  if (kind == FoundInFacet) {
     if (auto bind = context.insts().TryGetAs<SemIR::SymbolicBinding>(
             eval_query.query_self_inst_id)) {
       const auto& entity = context.entity_names().Get(bind->entity_name_id);
@@ -909,24 +984,12 @@ auto EvalLookupSingleImplWitness(Context& context, SemIR::LocId loc_id,
     }
   }
 
-  // Ensure specifics don't substitute in weird things for the query self.
-  CARBON_CHECK(context.types().IsFacetType(
-      context.insts().Get(eval_query.query_self_inst_id).type_id()));
-  SemIR::ConstantId query_self_const_id =
-      context.constant_values().Get(eval_query.query_self_inst_id);
-
   auto query_type_structure = BuildTypeStructure(
       context, context.constant_values().GetInstId(query_self_const_id),
       query_specific_interface);
   if (!query_type_structure) {
     return EvalImplLookupResult::MakeNone();
   }
-  bool query_is_concrete =
-      QueryIsConcrete(context, query_self_const_id, query_specific_interface);
-
-  // If we have a symbolic witness in the self query, then the query can not be
-  // concrete: the query includes a symbolic self value.
-  CARBON_CHECK(!self_facet_provides_witness || !query_is_concrete);
 
   // If the self value is a (symbolic) facet value that has a symbolic witness,
   // then we don't need to do impl lookup, except that we want to find any final
@@ -934,11 +997,11 @@ auto EvalLookupSingleImplWitness(Context& context, SemIR::LocId loc_id,
   // final impls only in that case. Note as in the CHECK above, the query can
   // not be concrete in this case, so only final impls can produce a concrete
   // witness for this query.
-  auto candidate_impls = CollectCandidateImplsForQuery(
-      context, self_facet_provides_witness, query_self_const_id,
-      *query_type_structure, query_specific_interface);
+  auto candidates = CollectCandidateImplsForQuery(
+      context, kind == FoundInFacet, query_self_const_id, *query_type_structure,
+      query_specific_interface);
 
-  for (const auto& candidate : candidate_impls) {
+  for (const auto& candidate : candidates.impls) {
     // In deferred lookup for a symbolic impl witness, while building a
     // specific, there may be no stack yet as this may be the first lookup. If
     // further lookups are started as a result in deduce, they will build the
@@ -951,7 +1014,7 @@ auto EvalLookupSingleImplWitness(Context& context, SemIR::LocId loc_id,
     }
 
     auto result = GetWitnessIdForImpl(
-        context, loc_id, query_is_concrete, query_self_const_id,
+        context, loc_id, kind == Concrete, query_self_const_id,
         query_specific_interface, candidate.impl_id);
     if (result.has_value()) {
       // Record the query which found a final impl witness. It's illegal to
@@ -968,18 +1031,50 @@ auto EvalLookupSingleImplWitness(Context& context, SemIR::LocId loc_id,
              .query = eval_query,
              .impl_witness = result.final_witness()});
       }
+
+      if (kind == Concrete && candidates.consider_cpp_candidates) {
+        // We found a Carbon impl. Also check for a C++ candidate that is a
+        // better match than that impl.
+        auto cpp_witness_id = LookupCppImpl(
+            context, loc_id,
+            GetFacetAsType(context, loc_id, query_self_const_id),
+            query_specific_interface, &candidate.type_structure,
+            SemIR::LocId(
+                context.impls().Get(candidate.impl_id).first_owning_decl_id));
+        if (cpp_witness_id.has_value()) {
+          return EvalImplLookupResult::MakeFinal(cpp_witness_id);
+        }
+      }
+
       return result;
     }
   }
 
-  if (self_facet_provides_witness) {
-    // If we did not find a final impl, but the self value is a facet that
-    // provides a symbolic witness, when we record that an impl will exist for
-    // the specific, but is yet unknown.
-    return EvalImplLookupResult::MakeNonFinal();
-  }
+  // We didn't find a matching impl. Produce a suitable result.
+  switch (kind) {
+    case Concrete:
+      if (candidates.consider_cpp_candidates) {
+        // Look for a matching C++ result, with no Carbon candidate to compare
+        // against.
+        auto cpp_witness_id = LookupCppImpl(
+            context, loc_id,
+            GetFacetAsType(context, loc_id, query_self_const_id),
+            query_specific_interface, nullptr, SemIR::LocId::None);
+        if (cpp_witness_id.has_value()) {
+          return EvalImplLookupResult::MakeFinal(cpp_witness_id);
+        }
+      }
+      return EvalImplLookupResult::MakeNone();
+
+    case FoundInFacet:
+      // We did not find a final impl, but the self value is a facet that
+      // provides a symbolic witness. Record that an impl will exist for the
+      // specific, but is yet unknown.
+      return EvalImplLookupResult::MakeNonFinal();
 
-  return EvalImplLookupResult::MakeNone();
+    case Symbolic:
+      return EvalImplLookupResult::MakeNone();
+  }
 }
 
 auto LookupMatchesImpl(Context& context, SemIR::LocId loc_id,

toolchain/check/testdata/interop/cpp/impls/copy.carbon

@@ -0,0 +1,302 @@
+// 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-FILE: toolchain/testing/testdata/min_prelude/convert.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/impls/copy.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/impls/copy.carbon
+
+// --- types.h
+
+class Copyable {
+ public:
+  Copyable(const Copyable&);
+};
+
+class ExplicitCopy {
+ public:
+  explicit ExplicitCopy(const ExplicitCopy&);
+};
+
+class DeletedCopy {
+ public:
+  DeletedCopy(const DeletedCopy&) = delete;
+};
+
+class MoveOnly {
+ public:
+  MoveOnly(MoveOnly&&);
+};
+
+class DeletedMove {
+ public:
+  DeletedMove(DeletedMove&&) = delete;
+};
+
+class NonConstCopy {
+ public:
+  NonConstCopy(NonConstCopy&);
+};
+
+class AmbiguousCopy {
+ public:
+  AmbiguousCopy(const AmbiguousCopy&, int = 0);
+  AmbiguousCopy(const AmbiguousCopy&, char = 0);
+};
+
+class PrivateCopy {
+ private:
+  PrivateCopy(const PrivateCopy&);
+};
+
+class ProtectedCopy {
+ protected:
+  ProtectedCopy(const ProtectedCopy&);
+};
+
+// --- copy_copyable.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp library "types.h";
+
+fn CopyCopyable(c: Cpp.Copyable) -> Cpp.Copyable {
+  //@dump-sem-ir-begin
+  return c;
+  //@dump-sem-ir-end
+}
+
+fn CopyExplicitCopy(c: Cpp.ExplicitCopy) -> Cpp.ExplicitCopy {
+  //@dump-sem-ir-begin
+  return c;
+  //@dump-sem-ir-end
+}
+
+// --- fail_copy_noncopyable.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp library "types.h";
+
+fn CopyDeletedCopy(c: Cpp.DeletedCopy) -> Cpp.DeletedCopy {
+  // CHECK:STDERR: fail_copy_noncopyable.carbon:[[@LINE+8]]:10: error: attempt to use a deleted function [CppInteropParseError]
+  // CHECK:STDERR:    15 |   return c;
+  // CHECK:STDERR:       |          ^
+  // CHECK:STDERR: fail_copy_noncopyable.carbon:[[@LINE-6]]:10: in file included here [InCppInclude]
+  // CHECK:STDERR: ./types.h:14:3: note: 'DeletedCopy' has been explicitly marked deleted here [CppInteropParseNote]
+  // CHECK:STDERR:    14 |   DeletedCopy(const DeletedCopy&) = delete;
+  // CHECK:STDERR:       |   ^
+  // CHECK:STDERR:
+  return c;
+}
+
+fn CopyMoveOnly(c: Cpp.MoveOnly) -> Cpp.MoveOnly {
+  // CHECK:STDERR: fail_copy_noncopyable.carbon:[[@LINE+8]]:10: error: attempt to use a deleted function [CppInteropParseError]
+  // CHECK:STDERR:    27 |   return c;
+  // CHECK:STDERR:       |          ^
+  // CHECK:STDERR: fail_copy_noncopyable.carbon:[[@LINE-18]]:10: in file included here [InCppInclude]
+  // CHECK:STDERR: ./types.h:19:3: note: copy constructor is implicitly deleted because 'MoveOnly' has a user-declared move constructor [CppInteropParseNote]
+  // CHECK:STDERR:    19 |   MoveOnly(MoveOnly&&);
+  // CHECK:STDERR:       |   ^
+  // CHECK:STDERR:
+  return c;
+}
+
+fn CopyDeletedMove(c: Cpp.DeletedMove) -> Cpp.DeletedMove {
+  // CHECK:STDERR: fail_copy_noncopyable.carbon:[[@LINE+8]]:10: error: attempt to use a deleted function [CppInteropParseError]
+  // CHECK:STDERR:    39 |   return c;
+  // CHECK:STDERR:       |          ^
+  // CHECK:STDERR: fail_copy_noncopyable.carbon:[[@LINE-30]]:10: in file included here [InCppInclude]
+  // CHECK:STDERR: ./types.h:24:3: note: copy constructor is implicitly deleted because 'DeletedMove' has a user-declared move constructor [CppInteropParseNote]
+  // CHECK:STDERR:    24 |   DeletedMove(DeletedMove&&) = delete;
+  // CHECK:STDERR:       |   ^
+  // CHECK:STDERR:
+  return c;
+}
+
+fn CopyNonConstCopy(c: Cpp.NonConstCopy) -> Cpp.NonConstCopy {
+  // TODO: List candidates here.
+  // CHECK:STDERR: fail_copy_noncopyable.carbon:[[@LINE+7]]:10: error: cannot copy value of type `Cpp.NonConstCopy` [CopyOfUncopyableType]
+  // CHECK:STDERR:   return c;
+  // CHECK:STDERR:          ^
+  // CHECK:STDERR: fail_copy_noncopyable.carbon:[[@LINE+4]]:10: note: type `Cpp.NonConstCopy` does not implement interface `Core.Copy` [MissingImplInMemberAccessNote]
+  // CHECK:STDERR:   return c;
+  // CHECK:STDERR:          ^
+  // CHECK:STDERR:
+  return c;
+}
+
+fn CopyAmbiguousCopy(c: Cpp.AmbiguousCopy) -> Cpp.AmbiguousCopy {
+  // TODO: List candidates here.
+  // CHECK:STDERR: fail_copy_noncopyable.carbon:[[@LINE+7]]:10: error: cannot copy value of type `Cpp.AmbiguousCopy` [CopyOfUncopyableType]
+  // CHECK:STDERR:   return c;
+  // CHECK:STDERR:          ^
+  // CHECK:STDERR: fail_copy_noncopyable.carbon:[[@LINE+4]]:10: note: type `Cpp.AmbiguousCopy` does not implement interface `Core.Copy` [MissingImplInMemberAccessNote]
+  // CHECK:STDERR:   return c;
+  // CHECK:STDERR:          ^
+  // CHECK:STDERR:
+  return c;
+}
+
+fn CopyPrivateCopy(c: Cpp.PrivateCopy) -> Cpp.PrivateCopy {
+  // TODO: Note is missing location.
+  // CHECK:STDERR: fail_copy_noncopyable.carbon:[[@LINE+5]]:10: error: cannot access private member `PrivateCopy` of type `Cpp.PrivateCopy` [ClassInvalidMemberAccess]
+  // CHECK:STDERR:   return c;
+  // CHECK:STDERR:          ^
+  // CHECK:STDERR: fail_copy_noncopyable.carbon: note: declared here [ClassMemberDeclaration]
+  // CHECK:STDERR:
+  return c;
+}
+
+fn CopyProtectedCopy(c: Cpp.ProtectedCopy) -> Cpp.ProtectedCopy {
+  // TODO: Note is missing location.
+  // CHECK:STDERR: fail_copy_noncopyable.carbon:[[@LINE+5]]:10: error: cannot access protected member `ProtectedCopy` of type `Cpp.ProtectedCopy` [ClassInvalidMemberAccess]
+  // CHECK:STDERR:   return c;
+  // CHECK:STDERR:          ^
+  // CHECK:STDERR: fail_copy_noncopyable.carbon: note: declared here [ClassMemberDeclaration]
+  // CHECK:STDERR:
+  return c;
+}
+
+// --- fail_todo_copy_generically.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp library "types.h";
+
+fn Copy[T:! Core.Copy](c: T) -> T {
+  return c;
+}
+
+fn DoCopy(c: Cpp.Copyable) -> Cpp.Copyable {
+  // TODO: We perform multiple impl lookups for `Cpp.Copyable as Core.Copy`
+  // here, and each one produces a different impl witness, resulting in a
+  // deduction failure.
+  // CHECK:STDERR: fail_todo_copy_generically.carbon:[[@LINE+7]]:10: error: inconsistent deductions for value of generic parameter `T` [DeductionInconsistent]
+  // CHECK:STDERR:   return Copy(c);
+  // CHECK:STDERR:          ^~~~~~~
+  // CHECK:STDERR: fail_todo_copy_generically.carbon:[[@LINE-11]]:1: note: while deducing parameters of generic declared here [DeductionGenericHere]
+  // CHECK:STDERR: fn Copy[T:! Core.Copy](c: T) -> T {
+  // CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  return Copy(c);
+}
+
+class Wrap(T:! Core.Copy) {}
+
+// TODO: This should type-check: each conversion of `Cpp.Copyable` to
+// `Core.Copy` should produce the same value.
+fn EqualWitnesses(p: Wrap(Cpp.Copyable)*) -> Wrap(Cpp.Copyable)* {
+  // CHECK:STDERR: fail_todo_copy_generically.carbon:[[@LINE+7]]:3: error: cannot implicitly convert expression of type `Wrap(Cpp.Copyable as Core.Copy)*` to `Wrap(Cpp.Copyable as Core.Copy)*` [ConversionFailure]
+  // CHECK:STDERR:   return p;
+  // CHECK:STDERR:   ^~~~~~~~~
+  // CHECK:STDERR: fail_todo_copy_generically.carbon:[[@LINE+4]]:3: note: type `Wrap(Cpp.Copyable as Core.Copy)*` does not implement interface `Core.ImplicitAs(Wrap(Cpp.Copyable as Core.Copy)*)` [MissingImplInMemberAccessNote]
+  // CHECK:STDERR:   return p;
+  // CHECK:STDERR:   ^~~~~~~~~
+  // CHECK:STDERR:
+  return p;
+}
+
+// CHECK:STDOUT: --- copy_copyable.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %Copyable: type = class_type @Copyable [concrete]
+// CHECK:STDOUT:   %Copy.type: type = facet_type <@Copy> [concrete]
+// CHECK:STDOUT:   %Copy.Op.type: type = fn_type @Copy.Op [concrete]
+// CHECK:STDOUT:   %Copyable.Copyable.type: type = fn_type @Copyable.Copyable [concrete]
+// CHECK:STDOUT:   %Copyable.Copyable: %Copyable.Copyable.type = struct_value () [concrete]
+// CHECK:STDOUT:   %const.622: type = const_type %Copyable [concrete]
+// CHECK:STDOUT:   %ptr.29d: type = ptr_type %const.622 [concrete]
+// CHECK:STDOUT:   %ptr.e47: type = ptr_type %Copyable [concrete]
+// CHECK:STDOUT:   %Copyable__carbon_thunk.type: type = fn_type @Copyable__carbon_thunk [concrete]
+// CHECK:STDOUT:   %Copyable__carbon_thunk: %Copyable__carbon_thunk.type = struct_value () [concrete]
+// CHECK:STDOUT:   %impl_witness.65e: <witness> = impl_witness @CopyCopyable.%impl_witness_table [concrete]
+// CHECK:STDOUT:   %Copy.facet.26f: %Copy.type = facet_value %Copyable, (%impl_witness.65e) [concrete]
+// CHECK:STDOUT:   %Copyable.Op.type: type = fn_type @Copyable.Op [concrete]
+// CHECK:STDOUT:   %Copyable.Op: %Copyable.Op.type = struct_value () [concrete]
+// CHECK:STDOUT:   %.667: type = fn_type_with_self_type %Copy.Op.type, %Copy.facet.26f [concrete]
+// CHECK:STDOUT:   %ExplicitCopy: type = class_type @ExplicitCopy [concrete]
+// CHECK:STDOUT:   %ExplicitCopy.ExplicitCopy.type: type = fn_type @ExplicitCopy.ExplicitCopy [concrete]
+// CHECK:STDOUT:   %ExplicitCopy.ExplicitCopy: %ExplicitCopy.ExplicitCopy.type = struct_value () [concrete]
+// CHECK:STDOUT:   %const.d8d: type = const_type %ExplicitCopy [concrete]
+// CHECK:STDOUT:   %ptr.093: type = ptr_type %const.d8d [concrete]
+// CHECK:STDOUT:   %ptr.84c: type = ptr_type %ExplicitCopy [concrete]
+// CHECK:STDOUT:   %ExplicitCopy__carbon_thunk.type: type = fn_type @ExplicitCopy__carbon_thunk [concrete]
+// CHECK:STDOUT:   %ExplicitCopy__carbon_thunk: %ExplicitCopy__carbon_thunk.type = struct_value () [concrete]
+// CHECK:STDOUT:   %impl_witness.215: <witness> = impl_witness @CopyExplicitCopy.%impl_witness_table [concrete]
+// CHECK:STDOUT:   %Copy.facet.cfa: %Copy.type = facet_value %ExplicitCopy, (%impl_witness.215) [concrete]
+// CHECK:STDOUT:   %ExplicitCopy.Op.type: type = fn_type @ExplicitCopy.Op [concrete]
+// CHECK:STDOUT:   %ExplicitCopy.Op: %ExplicitCopy.Op.type = struct_value () [concrete]
+// CHECK:STDOUT:   %.a87: type = fn_type_with_self_type %Copy.Op.type, %Copy.facet.cfa [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Copyable.Copyable.decl: %Copyable.Copyable.type = fn_decl @Copyable.Copyable [concrete = constants.%Copyable.Copyable] {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Copyable__carbon_thunk.decl: %Copyable__carbon_thunk.type = fn_decl @Copyable__carbon_thunk [concrete = constants.%Copyable__carbon_thunk] {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %ExplicitCopy.ExplicitCopy.decl: %ExplicitCopy.ExplicitCopy.type = fn_decl @ExplicitCopy.ExplicitCopy [concrete = constants.%ExplicitCopy.ExplicitCopy] {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %ExplicitCopy__carbon_thunk.decl: %ExplicitCopy__carbon_thunk.type = fn_decl @ExplicitCopy__carbon_thunk [concrete = constants.%ExplicitCopy__carbon_thunk] {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @CopyCopyable(%c.param: %Copyable) -> %return.param: %Copyable {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %c.ref: %Copyable = name_ref c, %c
+// CHECK:STDOUT:   %impl_witness_table = impl_witness_table (%Copyable.Op.decl), invalid [concrete]
+// CHECK:STDOUT:   %impl_witness: <witness> = impl_witness %impl_witness_table [concrete = constants.%impl_witness.65e]
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT:   %impl.elem0: %.667 = impl_witness_access constants.%impl_witness.65e, element0 [concrete = constants.%Copyable.Op]
+// CHECK:STDOUT:   %bound_method: <bound method> = bound_method %c.ref, %impl.elem0
+// CHECK:STDOUT:   %.loc8_10.1: ref %Copyable = temporary_storage
+// CHECK:STDOUT:   %Op.ref: %Copyable.Copyable.type = name_ref Op, imports.%Copyable.Copyable.decl [concrete = constants.%Copyable.Copyable]
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT:   %.loc8_10.2: ref %Copyable = value_as_ref %c.ref
+// CHECK:STDOUT:   %addr.loc8_10.1: %ptr.e47 = addr_of %.loc8_10.2
+// CHECK:STDOUT:   %.loc8_10.3: %ptr.29d = as_compatible %addr.loc8_10.1
+// CHECK:STDOUT:   %.loc8_10.4: %ptr.29d = converted %addr.loc8_10.1, %.loc8_10.3
+// CHECK:STDOUT:   %addr.loc8_10.2: %ptr.e47 = addr_of %.loc6_34
+// CHECK:STDOUT:   %Copyable__carbon_thunk.call: init %empty_tuple.type = call imports.%Copyable__carbon_thunk.decl(%.loc8_10.4, %addr.loc8_10.2)
+// CHECK:STDOUT:   %.loc8_10.5: init %Copyable = in_place_init %Copyable__carbon_thunk.call, %.loc6_34
+// CHECK:STDOUT:   return %.loc8_10.5 to %return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @CopyExplicitCopy(%c.param: %ExplicitCopy) -> %return.param: %ExplicitCopy {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %c.ref: %ExplicitCopy = name_ref c, %c
+// CHECK:STDOUT:   %impl_witness_table = impl_witness_table (%ExplicitCopy.Op.decl), invalid [concrete]
+// CHECK:STDOUT:   %impl_witness: <witness> = impl_witness %impl_witness_table [concrete = constants.%impl_witness.215]
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT:   %impl.elem0: %.a87 = impl_witness_access constants.%impl_witness.215, element0 [concrete = constants.%ExplicitCopy.Op]
+// CHECK:STDOUT:   %bound_method: <bound method> = bound_method %c.ref, %impl.elem0
+// CHECK:STDOUT:   %.loc14_10.1: ref %ExplicitCopy = temporary_storage
+// CHECK:STDOUT:   %Op.ref: %ExplicitCopy.ExplicitCopy.type = name_ref Op, imports.%ExplicitCopy.ExplicitCopy.decl [concrete = constants.%ExplicitCopy.ExplicitCopy]
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT:   %.loc14_10.2: ref %ExplicitCopy = value_as_ref %c.ref
+// CHECK:STDOUT:   %addr.loc14_10.1: %ptr.84c = addr_of %.loc14_10.2
+// CHECK:STDOUT:   %.loc14_10.3: %ptr.093 = as_compatible %addr.loc14_10.1
+// CHECK:STDOUT:   %.loc14_10.4: %ptr.093 = converted %addr.loc14_10.1, %.loc14_10.3
+// CHECK:STDOUT:   %addr.loc14_10.2: %ptr.84c = addr_of %.loc12_42
+// CHECK:STDOUT:   %ExplicitCopy__carbon_thunk.call: init %empty_tuple.type = call imports.%ExplicitCopy__carbon_thunk.decl(%.loc14_10.4, %addr.loc14_10.2)
+// CHECK:STDOUT:   %.loc14_10.5: init %ExplicitCopy = in_place_init %ExplicitCopy__carbon_thunk.call, %.loc12_42
+// CHECK:STDOUT:   return %.loc14_10.5 to %return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:

toolchain/check/thunk.cpp

@@ -9,6 +9,7 @@
 #include "toolchain/base/kind_switch.h"
 #include "toolchain/check/call.h"
 #include "toolchain/check/convert.h"
+#include "toolchain/check/cpp/operators.h"
 #include "toolchain/check/deferred_definition_worklist.h"
 #include "toolchain/check/diagnostic_helpers.h"
 #include "toolchain/check/function.h"
@@ -213,67 +214,6 @@ static auto HasDeclaredReturnType(Context& context,
       .return_slot_pattern_id.has_value();
 }
 
-auto BuildThunk(Context& context, SemIR::FunctionId signature_id,
-                SemIR::SpecificId signature_specific_id,
-                SemIR::InstId callee_id) -> SemIR::InstId {
-  auto callee = SemIR::GetCalleeAsFunction(context.sem_ir(), callee_id);
-
-  // Check whether we can use the given function without a thunk.
-  // TODO: For virtual functions, we want different rules for checking `self`.
-  // TODO: This is too strict; for example, we should not compare parameter
-  // names here.
-  if (CheckFunctionTypeMatches(
-          context, context.functions().Get(callee.function_id),
-          context.functions().Get(signature_id), signature_specific_id,
-          /*check_syntax=*/false, /*check_self=*/true, /*diagnose=*/false)) {
-    return callee_id;
-  }
-
-  // From P3763:
-  //   If the function in the interface does not have a return type, the
-  //   program is invalid if the function in the impl specifies a return type.
-  //
-  // Call into the redeclaration checking logic to produce a suitable error.
-  //
-  // TODO: Consider a different rule: always use an explicit return type for the
-  // thunk, and always convert the result of the wrapped call to the return type
-  // of the thunk.
-  if (!HasDeclaredReturnType(context, signature_id) &&
-      HasDeclaredReturnType(context, callee.function_id)) {
-    bool success = CheckFunctionReturnTypeMatches(
-        context, context.functions().Get(callee.function_id),
-        context.functions().Get(signature_id), signature_specific_id);
-    CARBON_CHECK(!success, "Return type unexpectedly matches");
-    return SemIR::ErrorInst::InstId;
-  }
-
-  // Create a scope for the function's parameters and generic parameters.
-  context.scope_stack().PushForDeclName();
-
-  // We can't use the function directly. Build a thunk.
-  // TODO: Check for and diagnose obvious reasons why this will fail, such as
-  // arity mismatch, before trying to build the thunk.
-  auto [function_id, thunk_id] =
-      CloneFunctionDecl(context, SemIR::LocId(callee_id), signature_id,
-                        signature_specific_id, callee.function_id);
-
-  // Track that this function is a thunk.
-  context.functions().Get(function_id).SetThunk(callee_id);
-
-  // Register the thunk to be defined when we reach the end of the enclosing
-  // deferred definition scope, for example an `impl` or `class` definition, as
-  // if the thunk's body were written inline in this location.
-  context.deferred_definition_worklist().SuspendThunkAndPush(
-      context, {
-                   .signature_id = signature_id,
-                   .function_id = function_id,
-                   .decl_id = thunk_id,
-                   .callee_id = callee_id,
-               });
-
-  return thunk_id;
-}
-
 // Build an expression that names the value matched by a pattern.
 static auto BuildPatternRef(Context& context,
                             llvm::ArrayRef<SemIR::InstId> arg_ids,
@@ -303,16 +243,25 @@ auto PerformThunkCall(Context& context, SemIR::LocId loc_id,
                       SemIR::InstId callee_id) -> SemIR::InstId {
   auto& function = context.functions().Get(function_id);
 
+  llvm::SmallVector<SemIR::InstId> args;
+
   // If we have a self parameter, form `self.<callee_id>`.
   if (function.self_param_id.has_value()) {
-    callee_id = PerformCompoundMemberAccess(
-        context, loc_id,
-        BuildPatternRef(context, call_arg_ids, function.self_param_id),
-        callee_id);
+    auto self_arg_id =
+        BuildPatternRef(context, call_arg_ids, function.self_param_id);
+    if (IsCppConstructorOrNonMethodOperator(context, callee_id)) {
+      // When calling a C++ constructor to implement `Copy`, or calling a C++
+      // non-method operator to implement a Carbon operator, the interface has a
+      // `self` parameter but C++ models that parameter as an explicit argument
+      // instead, so add the `self` to the argument list instead in that case.
+      args.push_back(self_arg_id);
+    } else {
+      callee_id =
+          PerformCompoundMemberAccess(context, loc_id, self_arg_id, callee_id);
+    }
   }
 
   // Form an argument list.
-  llvm::SmallVector<SemIR::InstId> args;
   for (auto pattern_id :
        context.inst_blocks().Get(function.param_patterns_id)) {
     args.push_back(BuildPatternRef(context, call_arg_ids, pattern_id));
@@ -423,4 +372,72 @@ auto BuildThunkDefinition(Context& context,
   context.scope_stack().Pop();
 }
 
+auto BuildThunk(Context& context, SemIR::FunctionId signature_id,
+                SemIR::SpecificId signature_specific_id,
+                SemIR::InstId callee_id, bool defer_definition)
+    -> SemIR::InstId {
+  auto callee = SemIR::GetCalleeAsFunction(context.sem_ir(), callee_id);
+
+  // Check whether we can use the given function without a thunk.
+  // TODO: For virtual functions, we want different rules for checking `self`.
+  // TODO: This is too strict; for example, we should not compare parameter
+  // names here.
+  if (CheckFunctionTypeMatches(
+          context, context.functions().Get(callee.function_id),
+          context.functions().Get(signature_id), signature_specific_id,
+          /*check_syntax=*/false, /*check_self=*/true, /*diagnose=*/false)) {
+    return callee_id;
+  }
+
+  // From P3763:
+  //   If the function in the interface does not have a return type, the
+  //   program is invalid if the function in the impl specifies a return type.
+  //
+  // Call into the redeclaration checking logic to produce a suitable error.
+  //
+  // TODO: Consider a different rule: always use an explicit return type for the
+  // thunk, and always convert the result of the wrapped call to the return type
+  // of the thunk.
+  if (!HasDeclaredReturnType(context, signature_id) &&
+      HasDeclaredReturnType(context, callee.function_id)) {
+    bool success = CheckFunctionReturnTypeMatches(
+        context, context.functions().Get(callee.function_id),
+        context.functions().Get(signature_id), signature_specific_id);
+    CARBON_CHECK(!success, "Return type unexpectedly matches");
+    return SemIR::ErrorInst::InstId;
+  }
+
+  // Create a scope for the function's parameters and generic parameters.
+  context.scope_stack().PushForDeclName();
+
+  // We can't use the function directly. Build a thunk.
+  // TODO: Check for and diagnose obvious reasons why this will fail, such as
+  // arity mismatch, before trying to build the thunk.
+  auto [function_id, thunk_id] =
+      CloneFunctionDecl(context, SemIR::LocId(callee_id), signature_id,
+                        signature_specific_id, callee.function_id);
+
+  // Track that this function is a thunk.
+  context.functions().Get(function_id).SetThunk(callee_id);
+
+  if (defer_definition) {
+    // Register the thunk to be defined when we reach the end of the enclosing
+    // deferred definition scope, for example an `impl` or `class` definition,
+    // as if the thunk's body were written inline in this location.
+    context.deferred_definition_worklist().SuspendThunkAndPush(
+        context, {
+                     .signature_id = signature_id,
+                     .function_id = function_id,
+                     .decl_id = thunk_id,
+                     .callee_id = callee_id,
+                 });
+  } else {
+    BuildThunkDefinition(context, signature_id, function_id, thunk_id,
+                         callee_id);
+    context.scope_stack().Pop();
+  }
+
+  return thunk_id;
+}
+
 }  // namespace Carbon::Check

toolchain/check/thunk.h

@@ -16,7 +16,8 @@ namespace Carbon::Check {
 // unchanged if it can be used directly.
 auto BuildThunk(Context& context, SemIR::FunctionId signature_id,
                 SemIR::SpecificId signature_specific_id,
-                SemIR::InstId callee_id) -> SemIR::InstId;
+                SemIR::InstId callee_id, bool defer_definition)
+    -> SemIR::InstId;
 
 // Builds a call to a function that forwards a call argument list built for
 // `function_id` to a call to `callee_id`, for use when building a call from a

toolchain/lower/testdata/interop/cpp/constructor.carbon

@@ -35,6 +35,31 @@ fn F() {
   let c: Cpp.C = Cpp.C.C();
 }
 
+// ============================================================================
+// Copy constructor
+// ============================================================================
+
+// --- copy.h
+
+class Copy {
+ public:
+  Copy();
+  Copy(const Copy&);
+
+ private:
+  int n;
+};
+
+// --- call_copy.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp library "copy.h";
+
+fn Copy(c: Cpp.Copy) -> Cpp.Copy {
+  return c;
+}
+
 // CHECK:STDOUT: ; ModuleID = 'import_default.carbon'
 // CHECK:STDOUT: source_filename = "import_default.carbon"
 // CHECK:STDOUT: target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
@@ -112,3 +137,70 @@ fn F() {
 // CHECK:STDOUT: !9 = !{null}
 // CHECK:STDOUT: !10 = !DILocation(line: 7, column: 18, scope: !7)
 // CHECK:STDOUT: !11 = !DILocation(line: 6, column: 1, scope: !7)
+// CHECK:STDOUT: ; ModuleID = 'call_copy.carbon'
+// CHECK:STDOUT: source_filename = "call_copy.carbon"
+// CHECK:STDOUT: target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+// CHECK:STDOUT: target triple = "x86_64-unknown-linux-gnu"
+// CHECK:STDOUT:
+// CHECK:STDOUT: ; Function Attrs: nounwind
+// CHECK:STDOUT: define void @_CCopy.Main(ptr sret([4 x i8]) %return, ptr %c) #0 !dbg !7 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   %.loc7_10.1.temp = alloca [4 x i8], align 1, !dbg !13
+// CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %.loc7_10.1.temp), !dbg !13
+// CHECK:STDOUT:   call void @_ZN4CopyC1ERKS_.carbon_thunk(ptr %c, ptr %return), !dbg !13
+// CHECK:STDOUT:   ret void, !dbg !14
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: ; Function Attrs: alwaysinline nounwind
+// CHECK:STDOUT: define void @"_COp:thunk.Copy.Cpp"(ptr sret([4 x i8]) %return, ptr %self) #1 !dbg !15 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   call void @_ZN4CopyC1ERKS_.carbon_thunk(ptr %self, ptr %return), !dbg !18
+// CHECK:STDOUT:   ret void, !dbg !18
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: ; Function Attrs: nocallback nofree nosync nounwind willreturn memory(argmem: readwrite)
+// CHECK:STDOUT: declare void @llvm.lifetime.start.p0(ptr captures(none)) #2
+// CHECK:STDOUT:
+// CHECK:STDOUT: ; Function Attrs: alwaysinline mustprogress
+// CHECK:STDOUT: define dso_local void @_ZN4CopyC1ERKS_.carbon_thunk(ptr %0, ptr %return) #3 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   %.addr = alloca ptr, align 8
+// CHECK:STDOUT:   %return.addr = alloca ptr, align 8
+// CHECK:STDOUT:   store ptr %0, ptr %.addr, align 8
+// CHECK:STDOUT:   store ptr %return, ptr %return.addr, align 8
+// CHECK:STDOUT:   %1 = load ptr, ptr %return.addr, align 8
+// CHECK:STDOUT:   %2 = load ptr, ptr %.addr, align 8
+// CHECK:STDOUT:   call void @_ZN4CopyC1ERKS_(ptr nonnull align 4 dereferenceable(4) %1, ptr nonnull align 4 dereferenceable(4) %2)
+// CHECK:STDOUT:   ret void
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: declare void @_ZN4CopyC1ERKS_(ptr nonnull align 4 dereferenceable(4), ptr nonnull align 4 dereferenceable(4)) unnamed_addr #4
+// CHECK:STDOUT:
+// CHECK:STDOUT: attributes #0 = { nounwind }
+// CHECK:STDOUT: attributes #1 = { alwaysinline nounwind }
+// CHECK:STDOUT: attributes #2 = { nocallback nofree nosync nounwind willreturn memory(argmem: readwrite) }
+// CHECK:STDOUT: attributes #3 = { alwaysinline mustprogress "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="0" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
+// CHECK:STDOUT: attributes #4 = { "no-trapping-math"="true" "stack-protector-buffer-size"="0" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
+// CHECK:STDOUT:
+// CHECK:STDOUT: !llvm.module.flags = !{!0, !1, !2, !3, !4}
+// CHECK:STDOUT: !llvm.dbg.cu = !{!5}
+// CHECK:STDOUT:
+// CHECK:STDOUT: !0 = !{i32 7, !"Dwarf Version", i32 5}
+// CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
+// CHECK:STDOUT: !2 = !{i32 1, !"wchar_size", i32 4}
+// CHECK:STDOUT: !3 = !{i32 8, !"PIC Level", i32 0}
+// CHECK:STDOUT: !4 = !{i32 7, !"PIE Level", i32 2}
+// CHECK:STDOUT: !5 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !6, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
+// CHECK:STDOUT: !6 = !DIFile(filename: "call_copy.carbon", directory: "")
+// CHECK:STDOUT: !7 = distinct !DISubprogram(name: "Copy", linkageName: "_CCopy.Main", scope: null, file: !6, line: 6, type: !8, spFlags: DISPFlagDefinition, unit: !5, retainedNodes: !11)
+// CHECK:STDOUT: !8 = !DISubroutineType(types: !9)
+// CHECK:STDOUT: !9 = !{!10, !10}
+// CHECK:STDOUT: !10 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: null, size: 8)
+// CHECK:STDOUT: !11 = !{!12}
+// CHECK:STDOUT: !12 = !DILocalVariable(arg: 1, scope: !7, type: !10)
+// CHECK:STDOUT: !13 = !DILocation(line: 7, column: 10, scope: !7)
+// CHECK:STDOUT: !14 = !DILocation(line: 7, column: 3, scope: !7)
+// CHECK:STDOUT: !15 = distinct !DISubprogram(name: "Op", linkageName: "_COp:thunk.Copy.Cpp", scope: null, file: !6, type: !8, spFlags: DISPFlagDefinition, unit: !5, retainedNodes: !16)
+// CHECK:STDOUT: !16 = !{!17}
+// CHECK:STDOUT: !17 = !DILocalVariable(arg: 1, scope: !15, type: !10)
+// CHECK:STDOUT: !18 = !DILocation(line: 0, scope: !15)

toolchain/sem_ir/inst_fingerprinter.cpp

@@ -264,6 +264,11 @@ struct Worklist {
   }
 
   auto Add(ImplId impl_id) -> void {
+    if (!impl_id.has_value()) {
+      AddInvalid();
+      return;
+    }
+
     const auto& impl = sem_ir->impls().Get(impl_id);
     Add(sem_ir->constant_values().Get(impl.self_id));
     Add(sem_ir->constant_values().Get(impl.constraint_id));

toolchain/sem_ir/type.h

@@ -212,11 +212,18 @@ class TypeStore : public Yaml::Printable<TypeStore> {
   auto TryGetIntTypeInfo(TypeId int_type_id) const
       -> std::optional<IntTypeInfo>;
 
-  // Returns whether `type_id` represents a facet type.
+  // Returns whether `type_id` represents a valid facet type.
   auto IsFacetType(TypeId type_id) const -> bool {
     return type_id == TypeType::TypeId || Is<FacetType>(type_id);
   }
 
+  // Returns whether `type_id` represents any kind of facet type, including the
+  // error instruction, which can be used as a type and so should be treated as
+  // a facet type in some contexts.
+  auto IsFacetTypeOrError(TypeId type_id) const -> bool {
+    return IsFacetType(type_id) || type_id == ErrorInst::TypeId;
+  }
+
   // Returns a list of types that were completed in this file, in the order in
   // which they were completed. Earlier types in this list cannot contain
   // instances of later types.