Don't import a C++ class definition until the class is required to be complete. (#5865)

When importing a class definition, ask Clang to complete it first. This
causes class template specializations to get instantiated as needed when
the type-checking of Carbon requires a C++ class to be complete. It also
allows Clang to implement things like modules-aware definition
visibility checking.

Don't reject importing a class with a virtual base if it's never
required to be complete. Instead, defer diagnosing until the definition
is required.

This also removes the recursion from `MapType`, as mapping a class type
no longer maps its definition.

In order to get diagnostics from instantiation failures, fix a bug that
caused any Clang diagnostics produced after the initial building of the
`ASTUnit` to get discarded. This exposed some duplicate diagnostic
issues in `ImportNameFromCpp` which are fixed here too.

toolchain/check/import_cpp.cpp

@@ -525,6 +525,32 @@ static auto BuildClassDecl(Context& context,
   return {class_decl.class_id, context.types().GetAsTypeInstId(class_decl_id)};
 }
 
+// Imports a record declaration from Clang to Carbon. If successful, returns
+// the new Carbon class declaration `InstId`.
+static auto ImportCXXRecordDecl(Context& context,
+                                clang::CXXRecordDecl* clang_decl)
+    -> SemIR::InstId {
+  auto import_ir_inst_id = AddImportIRInst(context, clang_decl->getLocation());
+
+  auto [class_id, class_inst_id] = BuildClassDecl(
+      context, import_ir_inst_id, GetParentNameScopeId(context, clang_decl),
+      AddIdentifierName(context, clang_decl->getName()));
+
+  // TODO: The caller does the same lookup. Avoid doing it twice.
+  auto clang_decl_id = context.sem_ir().clang_decls().Add(
+      {.decl = clang_decl->getCanonicalDecl(), .inst_id = class_inst_id});
+
+  // Name lookup into the Carbon class looks in the C++ class definition.
+  auto& class_info = context.classes().Get(class_id);
+  class_info.scope_id = context.name_scopes().Add(
+      class_inst_id, SemIR::NameId::None, class_info.parent_scope_id);
+  context.name_scopes()
+      .Get(class_info.scope_id)
+      .set_clang_decl_context_id(clang_decl_id);
+
+  return class_inst_id;
+}
+
 // Determines the Carbon inheritance kind to use for a C++ class definition.
 static auto GetInheritanceKind(clang::CXXRecordDecl* class_def)
     -> SemIR::Class::InheritanceKind {
@@ -558,8 +584,6 @@ static auto GetInheritanceKind(clang::CXXRecordDecl* class_def)
 
 // Checks that the specified finished class definition is valid and builds and
 // returns a corresponding complete type witness instruction.
-// TODO: Remove recursion into mapping field types.
-// NOLINTNEXTLINE(misc-no-recursion)
 static auto ImportClassObjectRepr(Context& context, SemIR::ClassId class_id,
                                   SemIR::ImportIRInstId import_ir_inst_id,
                                   SemIR::TypeInstId class_type_inst_id,
@@ -726,21 +750,14 @@ static auto ImportClassObjectRepr(Context& context, SemIR::ClassId class_id,
 
 // Creates a class definition based on the information in the given Clang
 // declaration, which is assumed to be for a class definition.
-// TODO: Remove recursion into mapping field types.
-// NOLINTNEXTLINE(misc-no-recursion)
 static auto BuildClassDefinition(Context& context,
                                  SemIR::ImportIRInstId import_ir_inst_id,
                                  SemIR::ClassId class_id,
                                  SemIR::TypeInstId class_inst_id,
-                                 SemIR::ClangDeclId clang_decl_id,
                                  clang::CXXRecordDecl* clang_def) -> void {
   auto& class_info = context.classes().Get(class_id);
-  StartClassDefinition(context, class_info, class_inst_id);
-
-  // Name lookup into the Carbon class looks in the C++ class definition.
-  context.name_scopes()
-      .Get(class_info.scope_id)
-      .set_clang_decl_context_id(clang_decl_id);
+  CARBON_CHECK(!class_info.has_definition_started());
+  class_info.definition_id = class_inst_id;
 
   context.inst_block_stack().Push();
 
@@ -757,41 +774,53 @@ static auto BuildClassDefinition(Context& context,
   class_info.body_block_id = context.inst_block_stack().Pop();
 }
 
-// Mark the given `Decl` as failed in `clang_decls`.
-static auto MarkFailedDecl(Context& context, clang::Decl* clang_decl) {
-  context.sem_ir().clang_decls().Add({.decl = clang_decl->getCanonicalDecl(),
-                                      .inst_id = SemIR::ErrorInst::InstId});
-}
-
-// Imports a record declaration from Clang to Carbon. If successful, returns
-// the new Carbon class declaration `InstId`.
-// TODO: Change `clang_decl` to `const &` when lookup is using `clang::DeclID`
-// and we don't need to store the decl for lookup context.
-// TODO: Remove recursion into mapping field types.
-// NOLINTNEXTLINE(misc-no-recursion)
-static auto ImportCXXRecordDecl(Context& context,
-                                clang::CXXRecordDecl* clang_decl)
-    -> SemIR::InstId {
-  clang::CXXRecordDecl* clang_def = clang_decl->getDefinition();
-  if (clang_def) {
-    clang_decl = clang_def;
-  }
-  auto import_ir_inst_id = AddImportIRInst(context, clang_decl->getLocation());
+auto ImportCppClassDefinition(Context& context, SemIR::LocId loc_id,
+                              SemIR::ClassId class_id,
+                              SemIR::ClangDeclId clang_decl_id) -> bool {
+  clang::ASTUnit* ast = context.sem_ir().cpp_ast();
+  CARBON_CHECK(ast);
 
-  auto [class_id, class_inst_id] = BuildClassDecl(
-      context, import_ir_inst_id, GetParentNameScopeId(context, clang_decl),
-      AddIdentifierName(context, clang_decl->getName()));
+  auto* clang_decl = cast<clang::CXXRecordDecl>(
+      context.sem_ir().clang_decls().Get(clang_decl_id).decl);
+  auto class_inst_id = context.types().GetAsTypeInstId(
+      context.classes().Get(class_id).first_owning_decl_id);
 
-  // TODO: The caller does the same lookup. Avoid doing it twice.
-  auto clang_decl_id = context.sem_ir().clang_decls().Add(
-      {.decl = clang_decl->getCanonicalDecl(), .inst_id = class_inst_id});
+  // TODO: Map loc_id into a clang location and use it for diagnostics if
+  // instantiation fails, instead of annotating the diagnostic with another
+  // location.
+  clang::SourceLocation loc = clang_decl->getLocation();
+  Diagnostics::AnnotationScope annotate_diagnostics(
+      &context.emitter(), [&](auto& builder) {
+        CARBON_DIAGNOSTIC(InCppTypeCompletion, Note,
+                          "while completing C++ class type {0}", SemIR::TypeId);
+        builder.Note(loc_id, InCppTypeCompletion,
+                     context.classes().Get(class_id).self_type_id);
+      });
 
-  if (clang_def) {
-    BuildClassDefinition(context, import_ir_inst_id, class_id, class_inst_id,
-                         clang_decl_id, clang_def);
+  // Ask Clang whether the type is complete. This triggers template
+  // instantiation if necessary.
+  clang::DiagnosticErrorTrap trap(ast->getDiagnostics());
+  if (!ast->getSema().isCompleteType(
+          loc, context.ast_context().getRecordType(clang_decl))) {
+    // Type is incomplete. Nothing more to do, but tell the caller if we
+    // produced an error.
+    return !trap.hasErrorOccurred();
   }
 
-  return class_inst_id;
+  clang::CXXRecordDecl* clang_def = clang_decl->getDefinition();
+  CARBON_CHECK(clang_def, "Complete type has no definition");
+
+  auto import_ir_inst_id =
+      context.insts().GetCanonicalLocId(class_inst_id).import_ir_inst_id();
+  BuildClassDefinition(context, import_ir_inst_id, class_id, class_inst_id,
+                       clang_def);
+  return true;
+}
+
+// Mark the given `Decl` as failed in `clang_decls`.
+static auto MarkFailedDecl(Context& context, clang::Decl* clang_decl) {
+  context.sem_ir().clang_decls().Add({.decl = clang_decl->getCanonicalDecl(),
+                                      .inst_id = SemIR::ErrorInst::InstId});
 }
 
 // Creates an integer type of the given size.
@@ -837,9 +866,6 @@ static auto MapBuiltinType(Context& context, clang::QualType qual_type,
 }
 
 // Maps a C++ record type to a Carbon type.
-// TODO: Support more record types.
-// TODO: Remove recursion mapping fields of class types.
-// NOLINTNEXTLINE(misc-no-recursion)
 static auto MapRecordType(Context& context, const clang::RecordType& type)
     -> TypeExpr {
   auto* record_decl = clang::dyn_cast<clang::CXXRecordDecl>(type.getDecl());
@@ -862,8 +888,6 @@ static auto MapRecordType(Context& context, const clang::RecordType& type)
 // Maps a C++ type that is not a wrapper type such as a pointer to a Carbon
 // type.
 // TODO: Support more types.
-// TODO: Remove recursion mapping fields of class types.
-// NOLINTNEXTLINE(misc-no-recursion)
 static auto MapNonWrapperType(Context& context, clang::QualType type)
     -> TypeExpr {
   if (const auto* builtin_type = type->getAs<clang::BuiltinType>()) {
@@ -928,8 +952,6 @@ static auto MapPointerType(Context& context, SemIR::LocId loc_id,
 // Maps a C++ type to a Carbon type. `type` should not be canonicalized because
 // we check for pointer nullability and nullability will be lost by
 // canonicalization.
-// TODO: Remove recursion mapping fields of class types.
-// NOLINTNEXTLINE(misc-no-recursion)
 static auto MapType(Context& context, SemIR::LocId loc_id, clang::QualType type)
     -> TypeExpr {
   // Unwrap any type modifiers and wrappers.
@@ -1318,7 +1340,6 @@ static auto AddDependentUnimportedTypeDecls(const Context& context,
 
   if (const auto* record_type = type->getAs<clang::RecordType>()) {
     AddDependentDecl(context, record_type->getDecl(), decls);
-    // TODO: Also import bases and fields if the class is defined.
   }
 }
 

toolchain/check/import_cpp.h

@@ -30,6 +30,13 @@ auto ImportNameFromCpp(Context& context, SemIR::LocId loc_id,
                        SemIR::NameScopeId scope_id, SemIR::NameId name_id)
     -> SemIR::ScopeLookupResult;
 
+// Given a class declaration that was imported from C++, attempt to import a
+// corresponding class definition. Returns true if nothing went wrong (whether
+// or not a definition could be imported), false if a diagnostic was produced.
+auto ImportCppClassDefinition(Context& context, SemIR::LocId loc_id,
+                              SemIR::ClassId class_id,
+                              SemIR::ClangDeclId clang_decl_id) -> bool;
+
 }  // namespace Carbon::Check
 
 #endif  // CARBON_TOOLCHAIN_CHECK_IMPORT_CPP_H_

toolchain/check/testdata/interop/cpp/class/base.carbon

@@ -158,6 +158,19 @@ fn ConvertB(p: Cpp.C*) -> Cpp.B* {
 struct A { int a; };
 struct B : virtual A {};
 
+void UseB(B * _Nonnull p);
+
+// --- use_virtual_inheritance_incomplete.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp library "virtual_inheritance.h";
+
+fn Convert(p: Cpp.B*) {
+  // OK, doesn't require `Cpp.B` to be a complete type.
+  Cpp.UseB(p);
+}
+
 // --- fail_todo_use_virtual_inheritance.carbon
 
 library "[[@TEST_NAME]]";
@@ -166,11 +179,11 @@ library "[[@TEST_NAME]]";
 // CHECK:STDERR: ./virtual_inheritance.h:3: error: semantics TODO: `class with virtual bases` [SemanticsTodo]
 import Cpp library "virtual_inheritance.h";
 
-// CHECK:STDERR: fail_todo_use_virtual_inheritance.carbon:[[@LINE+4]]:15: note: in `Cpp` name lookup for `B` [InCppNameLookup]
-// CHECK:STDERR: fn Convert(p: Cpp.B*) -> Cpp.A* {
-// CHECK:STDERR:               ^~~~~
-// CHECK:STDERR:
 fn Convert(p: Cpp.B*) -> Cpp.A* {
+  // CHECK:STDERR: fail_todo_use_virtual_inheritance.carbon:[[@LINE+11]]:3: note: while completing C++ class type `Cpp.B` [InCppTypeCompletion]
+  // CHECK:STDERR:   return p;
+  // CHECK:STDERR:   ^~~~~~~~~
+  // CHECK:STDERR:
   // CHECK:STDERR: fail_todo_use_virtual_inheritance.carbon:[[@LINE+7]]:3: error: cannot implicitly convert expression of type `Cpp.B*` to `Cpp.A*` [ConversionFailure]
   // CHECK:STDERR:   return p;
   // CHECK:STDERR:   ^~~~~~~~~
@@ -221,9 +234,9 @@ class V {
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %Derived: type = class_type @Derived [concrete]
-// CHECK:STDOUT:   %Base: type = class_type @Base [concrete]
 // CHECK:STDOUT:   %ptr.ddb: type = ptr_type %Derived [concrete]
 // CHECK:STDOUT:   %pattern_type.5c8: type = pattern_type %ptr.ddb [concrete]
+// CHECK:STDOUT:   %Base: type = class_type @Base [concrete]
 // CHECK:STDOUT:   %ptr.fb2: type = ptr_type %Base [concrete]
 // CHECK:STDOUT:   %pattern_type.72a: type = pattern_type %ptr.fb2 [concrete]
 // CHECK:STDOUT:   %ConvertPtr.type: type = fn_type @ConvertPtr [concrete]
@@ -345,9 +358,9 @@ class V {
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %Derived: type = class_type @Derived [concrete]
-// CHECK:STDOUT:   %Base: type = class_type @Base [concrete]
 // CHECK:STDOUT:   %int_32: Core.IntLiteral = int_value 32 [concrete]
 // CHECK:STDOUT:   %i32: type = class_type @Int, @Int(%int_32) [concrete]
+// CHECK:STDOUT:   %Base: type = class_type @Base [concrete]
 // CHECK:STDOUT:   %Base.elem: type = unbound_element_type %Base, %i32 [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -389,8 +402,8 @@ class V {
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %Derived: type = class_type @Derived [concrete]
-// CHECK:STDOUT:   %Base: type = class_type @Base [concrete]
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %Base: type = class_type @Base [concrete]
 // CHECK:STDOUT:   %Base.f.type: type = fn_type @Base.f [concrete]
 // CHECK:STDOUT:   %Base.f: %Base.f.type = struct_value () [concrete]
 // CHECK:STDOUT:   %Base.g.type: type = fn_type @Base.g [concrete]

toolchain/check/testdata/interop/cpp/class/class.carbon

@@ -439,10 +439,10 @@ fn MyF(bar: Cpp.Bar*);
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %Bar: type = class_type @Bar [concrete]
 // CHECK:STDOUT:   %ptr.f68: type = ptr_type %Bar [concrete]
-// CHECK:STDOUT:   %Bar.elem: type = unbound_element_type %Bar, %ptr.f68 [concrete]
 // CHECK:STDOUT:   %pattern_type: type = pattern_type %ptr.f68 [concrete]
 // CHECK:STDOUT:   %MyF.type: type = fn_type @MyF [concrete]
 // CHECK:STDOUT:   %MyF: %MyF.type = struct_value () [concrete]
+// CHECK:STDOUT:   %Bar.elem: type = unbound_element_type %Bar, %ptr.f68 [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: imports {

toolchain/check/testdata/interop/cpp/class/field.carbon

@@ -165,10 +165,10 @@ fn G(s: Cpp.Union) -> i32 {
 // CHECK:STDOUT:   %Struct: type = class_type @Struct [concrete]
 // CHECK:STDOUT:   %int_32: Core.IntLiteral = int_value 32 [concrete]
 // CHECK:STDOUT:   %i32: type = class_type @Int, @Int(%int_32) [concrete]
+// CHECK:STDOUT:   %tuple.type.189: type = tuple_type (%i32, %i32, %i32) [concrete]
 // CHECK:STDOUT:   %Struct.elem.86b: type = unbound_element_type %Struct, %i32 [concrete]
 // CHECK:STDOUT:   %ptr.235: type = ptr_type %i32 [concrete]
 // CHECK:STDOUT:   %Struct.elem.765: type = unbound_element_type %Struct, %ptr.235 [concrete]
-// CHECK:STDOUT:   %tuple.type.189: type = tuple_type (%i32, %i32, %i32) [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: imports {

toolchain/check/testdata/interop/cpp/class/struct.carbon

@@ -451,10 +451,10 @@ fn MyF(bar: Cpp.Bar*);
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %Bar: type = class_type @Bar [concrete]
 // CHECK:STDOUT:   %ptr.f68: type = ptr_type %Bar [concrete]
-// CHECK:STDOUT:   %Bar.elem: type = unbound_element_type %Bar, %ptr.f68 [concrete]
 // CHECK:STDOUT:   %pattern_type: type = pattern_type %ptr.f68 [concrete]
 // CHECK:STDOUT:   %MyF.type: type = fn_type @MyF [concrete]
 // CHECK:STDOUT:   %MyF: %MyF.type = struct_value () [concrete]
+// CHECK:STDOUT:   %Bar.elem: type = unbound_element_type %Bar, %ptr.f68 [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: imports {

toolchain/check/testdata/interop/cpp/class/template.carbon

@@ -0,0 +1,217 @@
+// 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/int.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/class/template.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/class/template.carbon
+
+// --- class_template.h
+
+struct Base {};
+
+template<typename T> struct A : Base {
+  void f() const;
+  T n;
+};
+
+using Aint = A<int>;
+using Along = A<long>;
+
+// --- use_class_template.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp library "class_template.h";
+
+fn F(x: Cpp.Aint) -> i32 {
+  // Trigger instantiation through name lookup.
+  //@dump-sem-ir-begin
+  x.f();
+  return x.n;
+  //@dump-sem-ir-end
+}
+
+fn G(p: Cpp.Along*) -> Cpp.Base* {
+  // Trigger instantiation through type completion.
+  //@dump-sem-ir-begin
+  return p;
+  //@dump-sem-ir-end
+}
+
+// --- instantiation_error.h
+
+template<typename T> struct X {
+  using type = typename T::member;
+};
+
+struct Y {
+  using member = int;
+};
+
+using XY = X<Y>;
+using Xint = X<int>;
+
+// --- fail_use_instantiation_error.carbon
+
+library "[[@TEST_NAME]]";
+
+// CHECK:STDERR: fail_use_instantiation_error.carbon:[[@LINE+11]]:1: in import [InImport]
+// CHECK:STDERR: ./instantiation_error.h:3: error: In file included from fail_use_instantiation_error.carbon:[[@LINE+10]]:
+// CHECK:STDERR: ./instantiation_error.h:3:25: error: type 'int' cannot be used prior to '::' because it has no members
+// CHECK:STDERR:     3 |   using type = typename T::member;
+// CHECK:STDERR:       |                         ^
+// CHECK:STDERR:  [CppInteropParseError]
+// CHECK:STDERR: fail_use_instantiation_error.carbon:[[@LINE+5]]:1: in import [InImport]
+// CHECK:STDERR: ./instantiation_error.h:2: note: ./instantiation_error.h:2:29: note: in instantiation of template class 'X<int>' requested here
+// CHECK:STDERR:     2 | template<typename T> struct X {
+// CHECK:STDERR:       |                             ^
+// CHECK:STDERR:  [CppInteropParseNote]
+import Cpp library "instantiation_error.h";
+
+//@dump-sem-ir-begin
+var x: Cpp.XY.r#type = 0;
+
+// CHECK:STDERR: fail_use_instantiation_error.carbon:[[@LINE+4]]:8: note: while completing C++ class type `Cpp.X` [InCppTypeCompletion]
+// CHECK:STDERR: var y: Cpp.Xint.r#type = 0;
+// CHECK:STDERR:        ^~~~~~~~~~~~~
+// CHECK:STDERR:
+var y: Cpp.Xint.r#type = 0;
+//@dump-sem-ir-end
+
+// CHECK:STDOUT: --- use_class_template.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %A.0bedf0.1: type = class_type @A.1 [concrete]
+// CHECK:STDOUT:   %int_32: Core.IntLiteral = int_value 32 [concrete]
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %i32: type = class_type @Int, @Int(%int_32) [concrete]
+// CHECK:STDOUT:   %Base: type = class_type @Base [concrete]
+// CHECK:STDOUT:   %A.elem.c3f: type = unbound_element_type %A.0bedf0.1, %i32 [concrete]
+// CHECK:STDOUT:   %A.f.type: type = fn_type @A.f [concrete]
+// CHECK:STDOUT:   %A.f: %A.f.type = struct_value () [concrete]
+// CHECK:STDOUT:   %A.0bedf0.2: type = class_type @A.2 [concrete]
+// CHECK:STDOUT:   %ptr.270: type = ptr_type %A.0bedf0.2 [concrete]
+// CHECK:STDOUT:   %ptr.fb2: type = ptr_type %Base [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %A.f.decl: %A.f.type = fn_decl @A.f [concrete = constants.%A.f] {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @F(%x.param: %A.0bedf0.1) -> %i32 {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %x.ref.loc9: %A.0bedf0.1 = name_ref x, %x
+// CHECK:STDOUT:   %f.ref: %A.f.type = name_ref f, imports.%A.f.decl [concrete = constants.%A.f]
+// CHECK:STDOUT:   %A.f.bound: <bound method> = bound_method %x.ref.loc9, %f.ref
+// CHECK:STDOUT:   %A.f.call: init %empty_tuple.type = call %A.f.bound(%x.ref.loc9)
+// CHECK:STDOUT:   %x.ref.loc10: %A.0bedf0.1 = name_ref x, %x
+// CHECK:STDOUT:   %n.ref: %A.elem.c3f = name_ref n, @A.1.%.2 [concrete = @A.1.%.2]
+// CHECK:STDOUT:   %.loc10_11.1: ref %i32 = class_element_access %x.ref.loc10, element1
+// CHECK:STDOUT:   %.loc10_11.2: %i32 = bind_value %.loc10_11.1
+// CHECK:STDOUT:   return %.loc10_11.2
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @G(%p.param: %ptr.270) -> %ptr.fb2 {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %p.ref: %ptr.270 = name_ref p, %p
+// CHECK:STDOUT:   %.loc17_11.1: ref %A.0bedf0.2 = deref %p.ref
+// CHECK:STDOUT:   %.loc17_11.2: ref %Base = class_element_access %.loc17_11.1, element0
+// CHECK:STDOUT:   %addr: %ptr.fb2 = addr_of %.loc17_11.2
+// CHECK:STDOUT:   %.loc17_11.3: %ptr.fb2 = converted %p.ref, %addr
+// CHECK:STDOUT:   return %.loc17_11.3
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_use_instantiation_error.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %X.32d59e.1: type = class_type @X.1 [concrete]
+// CHECK:STDOUT:   %int_32: Core.IntLiteral = int_value 32 [concrete]
+// CHECK:STDOUT:   %i32: type = class_type @Int, @Int(%int_32) [concrete]
+// CHECK:STDOUT:   %pattern_type.7ce: type = pattern_type %i32 [concrete]
+// CHECK:STDOUT:   %int_0.5c6: Core.IntLiteral = int_value 0 [concrete]
+// CHECK:STDOUT:   %ImplicitAs.type.205: type = facet_type <@ImplicitAs, @ImplicitAs(%i32)> [concrete]
+// CHECK:STDOUT:   %ImplicitAs.Convert.type.1b6: type = fn_type @ImplicitAs.Convert, @ImplicitAs(%i32) [concrete]
+// CHECK:STDOUT:   %To: Core.IntLiteral = bind_symbolic_name To, 0 [symbolic]
+// CHECK:STDOUT:   %Core.IntLiteral.as.ImplicitAs.impl.Convert.type.0f9: type = fn_type @Core.IntLiteral.as.ImplicitAs.impl.Convert, @Core.IntLiteral.as.ImplicitAs.impl(%To) [symbolic]
+// CHECK:STDOUT:   %Core.IntLiteral.as.ImplicitAs.impl.Convert.f06: %Core.IntLiteral.as.ImplicitAs.impl.Convert.type.0f9 = struct_value () [symbolic]
+// CHECK:STDOUT:   %ImplicitAs.impl_witness.c75: <witness> = impl_witness imports.%ImplicitAs.impl_witness_table.a2f, @Core.IntLiteral.as.ImplicitAs.impl(%int_32) [concrete]
+// CHECK:STDOUT:   %Core.IntLiteral.as.ImplicitAs.impl.Convert.type.035: type = fn_type @Core.IntLiteral.as.ImplicitAs.impl.Convert, @Core.IntLiteral.as.ImplicitAs.impl(%int_32) [concrete]
+// CHECK:STDOUT:   %Core.IntLiteral.as.ImplicitAs.impl.Convert.956: %Core.IntLiteral.as.ImplicitAs.impl.Convert.type.035 = struct_value () [concrete]
+// CHECK:STDOUT:   %ImplicitAs.facet: %ImplicitAs.type.205 = facet_value Core.IntLiteral, (%ImplicitAs.impl_witness.c75) [concrete]
+// CHECK:STDOUT:   %.9c3: type = fn_type_with_self_type %ImplicitAs.Convert.type.1b6, %ImplicitAs.facet [concrete]
+// CHECK:STDOUT:   %Core.IntLiteral.as.ImplicitAs.impl.Convert.bound: <bound method> = bound_method %int_0.5c6, %Core.IntLiteral.as.ImplicitAs.impl.Convert.956 [concrete]
+// CHECK:STDOUT:   %Core.IntLiteral.as.ImplicitAs.impl.Convert.specific_fn: <specific function> = specific_function %Core.IntLiteral.as.ImplicitAs.impl.Convert.956, @Core.IntLiteral.as.ImplicitAs.impl.Convert(%int_32) [concrete]
+// CHECK:STDOUT:   %bound_method: <bound method> = bound_method %int_0.5c6, %Core.IntLiteral.as.ImplicitAs.impl.Convert.specific_fn [concrete]
+// CHECK:STDOUT:   %int_0.6a9: %i32 = int_value 0 [concrete]
+// CHECK:STDOUT:   %X.32d59e.2: type = class_type @X.2 [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Cpp: <namespace> = namespace file.%Cpp.import_cpp, [concrete] {
+// CHECK:STDOUT:     .XY = %X.decl.dec4cb.1
+// CHECK:STDOUT:     .Xint = %X.decl.dec4cb.2
+// CHECK:STDOUT:     import Cpp//...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %X.decl.dec4cb.1: type = class_decl @X.1 [concrete = constants.%X.32d59e.1] {} {}
+// CHECK:STDOUT:   %Core.import_ref.a5b: @Core.IntLiteral.as.ImplicitAs.impl.%Core.IntLiteral.as.ImplicitAs.impl.Convert.type (%Core.IntLiteral.as.ImplicitAs.impl.Convert.type.0f9) = import_ref Core//prelude/parts/int, loc16_39, loaded [symbolic = @Core.IntLiteral.as.ImplicitAs.impl.%Core.IntLiteral.as.ImplicitAs.impl.Convert (constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.f06)]
+// CHECK:STDOUT:   %ImplicitAs.impl_witness_table.a2f = impl_witness_table (%Core.import_ref.a5b), @Core.IntLiteral.as.ImplicitAs.impl [concrete]
+// CHECK:STDOUT:   %X.decl.dec4cb.2: type = class_decl @X.2 [concrete = constants.%X.32d59e.2] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   name_binding_decl {
+// CHECK:STDOUT:     %x.patt: %pattern_type.7ce = binding_pattern x [concrete]
+// CHECK:STDOUT:     %x.var_patt: %pattern_type.7ce = var_pattern %x.patt [concrete]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %x.var: ref %i32 = var %x.var_patt [concrete]
+// CHECK:STDOUT:   %.loc18: type = splice_block %type.ref.loc18 [concrete = constants.%i32] {
+// CHECK:STDOUT:     %Cpp.ref.loc18: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:     %XY.ref: type = name_ref XY, imports.%X.decl.dec4cb.1 [concrete = constants.%X.32d59e.1]
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:     %type.ref.loc18: type = name_ref r#type, %i32.2 [concrete = constants.%i32]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %x: ref %i32 = bind_name x, %x.var [concrete = %x.var]
+// CHECK:STDOUT:   name_binding_decl {
+// CHECK:STDOUT:     %y.patt: %pattern_type.7ce = binding_pattern y [concrete]
+// CHECK:STDOUT:     %y.var_patt: %pattern_type.7ce = var_pattern %y.patt [concrete]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %y.var: ref %i32 = var %y.var_patt [concrete]
+// CHECK:STDOUT:   %.loc24: type = splice_block %type.ref.loc24 [concrete = constants.%i32] {
+// CHECK:STDOUT:     %Cpp.ref.loc24: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:     %Xint.ref: type = name_ref Xint, imports.%X.decl.dec4cb.2 [concrete = constants.%X.32d59e.2]
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:     %type.ref.loc24: type = name_ref r#type, %i32.1 [concrete = constants.%i32]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %y: ref %i32 = bind_name y, %y.var [concrete = %y.var]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @__global_init() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %int_0.loc18: Core.IntLiteral = int_value 0 [concrete = constants.%int_0.5c6]
+// CHECK:STDOUT:   %impl.elem0.loc18: %.9c3 = impl_witness_access constants.%ImplicitAs.impl_witness.c75, element0 [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.956]
+// CHECK:STDOUT:   %bound_method.loc18_1.1: <bound method> = bound_method %int_0.loc18, %impl.elem0.loc18 [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.bound]
+// CHECK:STDOUT:   %specific_fn.loc18: <specific function> = specific_function %impl.elem0.loc18, @Core.IntLiteral.as.ImplicitAs.impl.Convert(constants.%int_32) [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.specific_fn]
+// CHECK:STDOUT:   %bound_method.loc18_1.2: <bound method> = bound_method %int_0.loc18, %specific_fn.loc18 [concrete = constants.%bound_method]
+// CHECK:STDOUT:   %Core.IntLiteral.as.ImplicitAs.impl.Convert.call.loc18: init %i32 = call %bound_method.loc18_1.2(%int_0.loc18) [concrete = constants.%int_0.6a9]
+// CHECK:STDOUT:   %.loc18: init %i32 = converted %int_0.loc18, %Core.IntLiteral.as.ImplicitAs.impl.Convert.call.loc18 [concrete = constants.%int_0.6a9]
+// CHECK:STDOUT:   assign file.%x.var, %.loc18
+// CHECK:STDOUT:   %int_0.loc24: Core.IntLiteral = int_value 0 [concrete = constants.%int_0.5c6]
+// CHECK:STDOUT:   %impl.elem0.loc24: %.9c3 = impl_witness_access constants.%ImplicitAs.impl_witness.c75, element0 [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.956]
+// CHECK:STDOUT:   %bound_method.loc24_1.1: <bound method> = bound_method %int_0.loc24, %impl.elem0.loc24 [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.bound]
+// CHECK:STDOUT:   %specific_fn.loc24: <specific function> = specific_function %impl.elem0.loc24, @Core.IntLiteral.as.ImplicitAs.impl.Convert(constants.%int_32) [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.specific_fn]
+// CHECK:STDOUT:   %bound_method.loc24_1.2: <bound method> = bound_method %int_0.loc24, %specific_fn.loc24 [concrete = constants.%bound_method]
+// CHECK:STDOUT:   %Core.IntLiteral.as.ImplicitAs.impl.Convert.call.loc24: init %i32 = call %bound_method.loc24_1.2(%int_0.loc24) [concrete = constants.%int_0.6a9]
+// CHECK:STDOUT:   %.loc24: init %i32 = converted %int_0.loc24, %Core.IntLiteral.as.ImplicitAs.impl.Convert.call.loc24 [concrete = constants.%int_0.6a9]
+// CHECK:STDOUT:   assign file.%y.var, %.loc24
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT: }
+// CHECK:STDOUT:

toolchain/check/testdata/interop/cpp/class/union.carbon

@@ -361,10 +361,10 @@ fn MyF(bar: Cpp.Bar*);
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %Bar: type = class_type @Bar [concrete]
 // CHECK:STDOUT:   %ptr.f68: type = ptr_type %Bar [concrete]
-// CHECK:STDOUT:   %Bar.elem: type = unbound_element_type %Bar, %ptr.f68 [concrete]
 // CHECK:STDOUT:   %pattern_type: type = pattern_type %ptr.f68 [concrete]
 // CHECK:STDOUT:   %MyF.type: type = fn_type @MyF [concrete]
 // CHECK:STDOUT:   %MyF: %MyF.type = struct_value () [concrete]
+// CHECK:STDOUT:   %Bar.elem: type = unbound_element_type %Bar, %ptr.f68 [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: imports {

toolchain/check/testdata/interop/cpp/function/class.carbon

@@ -440,9 +440,9 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %C: type = class_type @C [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %C.val: %C = struct_value () [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.type.1b3: type = fn_type @T.as.Destroy.impl.Op, @T.as.Destroy.impl(%C) [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.21b: %T.as.Destroy.impl.Op.type.1b3 = struct_value () [concrete]
@@ -485,9 +485,9 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %C: type = class_type @C [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: imports {
@@ -517,9 +517,9 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %C: type = class_type @C [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: imports {
@@ -549,10 +549,10 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %C: type = class_type @C [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %pattern_type.69f: type = pattern_type %C [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %C.val: %C = struct_value () [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.type.9ae: type = fn_type @T.as.Destroy.impl.Op, @T.as.Destroy.impl(%C) [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.dbb: %T.as.Destroy.impl.Op.type.9ae = struct_value () [concrete]
@@ -617,9 +617,9 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %C: type = class_type @C [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %C.val: %C = struct_value () [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.type.b28: type = fn_type @T.as.Destroy.impl.Op, @T.as.Destroy.impl(%C) [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.f48: %T.as.Destroy.impl.Op.type.b28 = struct_value () [concrete]
@@ -667,10 +667,10 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %O: type = class_type @O [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %C: type = class_type @C [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %C.val: %C = struct_value () [concrete]
 // CHECK:STDOUT:   %pattern_type.cff: type = pattern_type %O [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.type.1b8: type = fn_type @T.as.Destroy.impl.Op, @T.as.Destroy.impl(%O) [concrete]
@@ -788,9 +788,9 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %C: type = class_type @C [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %C.val: %C = struct_value () [concrete]
 // CHECK:STDOUT:   %C.bar.type: type = fn_type @C.bar [concrete]
 // CHECK:STDOUT:   %C.bar: %C.bar.type = struct_value () [concrete]

toolchain/check/testdata/interop/cpp/function/struct.carbon

@@ -432,9 +432,9 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %S: type = class_type @S [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %S.val: %S = struct_value () [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.type.642: type = fn_type @T.as.Destroy.impl.Op, @T.as.Destroy.impl(%S) [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.ab5: %T.as.Destroy.impl.Op.type.642 = struct_value () [concrete]
@@ -477,9 +477,9 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %S: type = class_type @S [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: imports {
@@ -509,9 +509,9 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %S: type = class_type @S [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: imports {
@@ -541,10 +541,10 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %S: type = class_type @S [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %pattern_type.cd8: type = pattern_type %S [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %S.val: %S = struct_value () [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.type.2b5: type = fn_type @T.as.Destroy.impl.Op, @T.as.Destroy.impl(%S) [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.9b3: %T.as.Destroy.impl.Op.type.2b5 = struct_value () [concrete]
@@ -609,9 +609,9 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %S: type = class_type @S [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %S.val: %S = struct_value () [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.type.17f: type = fn_type @T.as.Destroy.impl.Op, @T.as.Destroy.impl(%S) [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.463: %T.as.Destroy.impl.Op.type.17f = struct_value () [concrete]
@@ -659,10 +659,10 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %O: type = class_type @O [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %S: type = class_type @S [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %S.val: %S = struct_value () [concrete]
 // CHECK:STDOUT:   %pattern_type.cff: type = pattern_type %O [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.type.1b8: type = fn_type @T.as.Destroy.impl.Op, @T.as.Destroy.impl(%O) [concrete]
@@ -780,9 +780,9 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %S: type = class_type @S [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %S.val: %S = struct_value () [concrete]
 // CHECK:STDOUT:   %S.bar.type: type = fn_type @S.bar [concrete]
 // CHECK:STDOUT:   %S.bar: %S.bar.type = struct_value () [concrete]

toolchain/check/testdata/interop/cpp/function/union.carbon

@@ -465,9 +465,9 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %U: type = class_type @U [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %U.val: %U = struct_value () [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.type.0b7: type = fn_type @T.as.Destroy.impl.Op, @T.as.Destroy.impl(%U) [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.2fa: %T.as.Destroy.impl.Op.type.0b7 = struct_value () [concrete]
@@ -510,9 +510,9 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %U: type = class_type @U [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: imports {
@@ -542,9 +542,9 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %U: type = class_type @U [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: imports {
@@ -574,10 +574,10 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %U: type = class_type @U [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %pattern_type.eb9: type = pattern_type %U [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %U.val: %U = struct_value () [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.type.2e1: type = fn_type @T.as.Destroy.impl.Op, @T.as.Destroy.impl(%U) [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.d5d: %T.as.Destroy.impl.Op.type.2e1 = struct_value () [concrete]
@@ -642,9 +642,9 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %U: type = class_type @U [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %U.val: %U = struct_value () [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.type.ee1: type = fn_type @T.as.Destroy.impl.Op, @T.as.Destroy.impl(%U) [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.28c: %T.as.Destroy.impl.Op.type.ee1 = struct_value () [concrete]
@@ -692,10 +692,10 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %O: type = class_type @O [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %U: type = class_type @U [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %U.val: %U = struct_value () [concrete]
 // CHECK:STDOUT:   %pattern_type.cff: type = pattern_type %O [concrete]
 // CHECK:STDOUT:   %T.as.Destroy.impl.Op.type.1b8: type = fn_type @T.as.Destroy.impl.Op, @T.as.Destroy.impl(%O) [concrete]
@@ -813,9 +813,9 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %U: type = class_type @U [concrete]
-// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
 // CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %U.val: %U = struct_value () [concrete]
 // CHECK:STDOUT:   %U.bar.type: type = fn_type @U.bar [concrete]
 // CHECK:STDOUT:   %U.bar: %U.bar.type = struct_value () [concrete]

toolchain/check/type_completion.cpp

@@ -7,6 +7,7 @@
 #include "llvm/ADT/SmallVector.h"
 #include "toolchain/base/kind_switch.h"
 #include "toolchain/check/generic.h"
+#include "toolchain/check/import_cpp.h"
 #include "toolchain/check/inst.h"
 #include "toolchain/check/type.h"
 #include "toolchain/diagnostics/format_providers.h"
@@ -274,6 +275,17 @@ auto TypeCompleter::AddNestedIncompleteTypes(SemIR::Inst type_inst) -> bool {
     }
     case CARBON_KIND(SemIR::ClassType inst): {
       auto& class_info = context_->classes().Get(inst.class_id);
+      // If the class was imported from C++, ask Clang to try to complete it.
+      if (!class_info.is_complete() && class_info.scope_id.has_value()) {
+        auto& scope = context_->name_scopes().Get(class_info.scope_id);
+        if (scope.clang_decl_context_id().has_value()) {
+          if (!ImportCppClassDefinition(*context_, loc_id_, inst.class_id,
+                                        scope.clang_decl_context_id())) {
+            // Clang produced a diagnostic. Don't produce one of our own.
+            return false;
+          }
+        }
+      }
       if (!class_info.is_complete()) {
         if (diagnoser_) {
           auto builder = diagnoser_();

toolchain/diagnostics/diagnostic_kind.def

@@ -163,6 +163,7 @@ CARBON_DIAGNOSTIC_KIND(SemanticsTodo)
 // Location context.
 CARBON_DIAGNOSTIC_KIND(InImport)
 CARBON_DIAGNOSTIC_KIND(ResolvingSpecificHere)
+CARBON_DIAGNOSTIC_KIND(InCppTypeCompletion)
 
 // Package/import checking diagnostics.
 CARBON_DIAGNOSTIC_KIND(CppInteropFuzzing)