diff --git a/toolchain/check/cpp/impl_lookup.cpp b/toolchain/check/cpp/impl_lookup.cpp
index 872e0f8d450d..6dca304d30e1 100644
--- a/toolchain/check/cpp/impl_lookup.cpp
+++ b/toolchain/check/cpp/impl_lookup.cpp
@@ -61,9 +61,7 @@ struct DeclInfo {
 // Returns SemIR::InstId::None if a C++ function is not found, and
 // SemIR::ErrorInst::InstId if an error occurs.
 static auto GetFunctionId(Context& context, SemIR::LocId loc_id,
-                          DeclInfo decl_info,
-                          const TypeStructure* best_impl_type_structure,
-                          SemIR::LocId best_impl_loc_id) -> SemIR::InstId {
+                          DeclInfo decl_info) -> SemIR::InstId {
   if (!decl_info.decl) {
     // The C++ type is not able to implement the interface.
     return SemIR::InstId::None;
@@ -85,20 +83,13 @@ static auto GetFunctionId(Context& context, SemIR::LocId loc_id,
       context, loc_id, clang::DeclAccessPair::make(cpp_fn, cpp_fn->getAccess()),
       context.insts().GetAsKnownInstId<SemIR::FunctionDecl>(fn_id));
 
-  // 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 fn_id;
 }
 
 static auto BuildCopyWitness(
     Context& context, SemIR::LocId loc_id,
     SemIR::ConstantId query_self_const_id,
-    SemIR::SpecificInterfaceId query_specific_interface_id,
-    const TypeStructure* best_impl_type_structure,
-    SemIR::LocId best_impl_loc_id) -> SemIR::InstId {
+    SemIR::SpecificInterfaceId query_specific_interface_id) -> SemIR::InstId {
   auto& clang_sema = context.clang_sema();
 
   // TODO: This should provide `Copy` for enums and other trivially copyable
@@ -110,8 +101,7 @@ static auto BuildCopyWitness(
   auto decl_info = DeclInfo{.decl = clang_sema.LookupCopyingConstructor(
                                 class_decl, clang::Qualifiers::Const),
                             .signature = {.num_params = 1}};
-  auto fn_id = GetFunctionId(context, loc_id, decl_info,
-                             best_impl_type_structure, best_impl_loc_id);
+  auto fn_id = GetFunctionId(context, loc_id, decl_info);
   if (fn_id == SemIR::ErrorInst::InstId || fn_id == SemIR::InstId::None) {
     return fn_id;
   }
@@ -122,9 +112,7 @@ static auto BuildCopyWitness(
 static auto BuildCppUnsafeDerefWitness(
     Context& context, SemIR::LocId loc_id,
     SemIR::ConstantId query_self_const_id,
-    SemIR::SpecificInterfaceId query_specific_interface_id,
-    const TypeStructure* best_impl_type_structure,
-    SemIR::LocId best_impl_loc_id) -> SemIR::InstId {
+    SemIR::SpecificInterfaceId query_specific_interface_id) -> SemIR::InstId {
   auto& clang_sema = context.clang_sema();
 
   auto* class_decl = TypeAsClassDecl(context, query_self_const_id);
@@ -143,8 +131,7 @@ static auto BuildCppUnsafeDerefWitness(
   }
   auto decl_info =
       DeclInfo{.decl = *candidates.begin(), .signature = {.num_params = 0}};
-  auto fn_id = GetFunctionId(context, loc_id, decl_info,
-                             best_impl_type_structure, best_impl_loc_id);
+  auto fn_id = GetFunctionId(context, loc_id, decl_info);
   if (fn_id == SemIR::ErrorInst::InstId || fn_id == SemIR::InstId::None) {
     return fn_id;
   }
@@ -165,9 +152,7 @@ static auto BuildCppUnsafeDerefWitness(
 static auto BuildDefaultWitness(
     Context& context, SemIR::LocId loc_id,
     SemIR::ConstantId query_self_const_id,
-    SemIR::SpecificInterfaceId query_specific_interface_id,
-    const TypeStructure* best_impl_type_structure,
-    SemIR::LocId best_impl_loc_id) -> SemIR::InstId {
+    SemIR::SpecificInterfaceId query_specific_interface_id) -> SemIR::InstId {
   auto& clang_sema = context.clang_sema();
 
   auto* class_decl = TypeAsClassDecl(context, query_self_const_id);
@@ -183,8 +168,7 @@ static auto BuildDefaultWitness(
   auto decl_info =
       DeclInfo{.decl = clang_sema.LookupDefaultConstructor(class_decl),
                .signature = {.num_params = 0}};
-  auto fn_id = GetFunctionId(context, loc_id, decl_info,
-                             best_impl_type_structure, best_impl_loc_id);
+  auto fn_id = GetFunctionId(context, loc_id, decl_info);
   if (fn_id == SemIR::ErrorInst::InstId || fn_id == SemIR::InstId::None) {
     return fn_id;
   }
@@ -195,9 +179,7 @@ static auto BuildDefaultWitness(
 static auto BuildDestroyWitness(
     Context& context, SemIR::LocId loc_id,
     SemIR::ConstantId query_self_const_id,
-    SemIR::SpecificInterfaceId query_specific_interface_id,
-    const TypeStructure* best_impl_type_structure,
-    SemIR::LocId best_impl_loc_id) -> SemIR::InstId {
+    SemIR::SpecificInterfaceId query_specific_interface_id) -> SemIR::InstId {
   auto& clang_sema = context.clang_sema();
 
   // TODO: This should provide `Destroy` for enums and other trivially
@@ -208,8 +190,7 @@ static auto BuildDestroyWitness(
   }
   auto decl_info = DeclInfo{.decl = clang_sema.LookupDestructor(class_decl),
                             .signature = {.num_params = 0}};
-  auto fn_id = GetFunctionId(context, loc_id, decl_info,
-                             best_impl_type_structure, best_impl_loc_id);
+  auto fn_id = GetFunctionId(context, loc_id, decl_info);
   if (fn_id == SemIR::ErrorInst::InstId || fn_id == SemIR::InstId::None) {
     return fn_id;
   }
@@ -223,23 +204,24 @@ auto LookupCppImpl(Context& context, SemIR::LocId loc_id,
                    SemIR::SpecificInterfaceId query_specific_interface_id,
                    const TypeStructure* best_impl_type_structure,
                    SemIR::LocId best_impl_loc_id) -> SemIR::InstId {
+  // 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);
+
   switch (core_interface) {
     case CoreInterface::Copy:
       return BuildCopyWitness(context, loc_id, query_self_const_id,
-                              query_specific_interface_id,
-                              best_impl_type_structure, best_impl_loc_id);
+                              query_specific_interface_id);
     case CoreInterface::CppUnsafeDeref:
-      return BuildCppUnsafeDerefWitness(
-          context, loc_id, query_self_const_id, query_specific_interface_id,
-          best_impl_type_structure, best_impl_loc_id);
+      return BuildCppUnsafeDerefWitness(context, loc_id, query_self_const_id,
+                                        query_specific_interface_id);
     case CoreInterface::Default:
       return BuildDefaultWitness(context, loc_id, query_self_const_id,
-                                 query_specific_interface_id,
-                                 best_impl_type_structure, best_impl_loc_id);
+                                 query_specific_interface_id);
     case CoreInterface::Destroy:
       return BuildDestroyWitness(context, loc_id, query_self_const_id,
-                                 query_specific_interface_id,
-                                 best_impl_type_structure, best_impl_loc_id);
+                                 query_specific_interface_id);
 
     // IntFitsIn is for Carbon integer types only.
     case CoreInterface::IntFitsIn: