diff --git a/toolchain/check/check.cpp b/toolchain/check/check.cpp
index 899ae21eb7b9..b6de302f1077 100644
--- a/toolchain/check/check.cpp
+++ b/toolchain/check/check.cpp
@@ -513,7 +513,8 @@ auto CheckParseTrees(
         continue;
       }
       inputs.push_back({.check_ir_id = unit_info.unit->sem_ir->check_ir_id(),
-                        .filename = unit_info.unit->sem_ir->filename()});
+                        .filename = unit_info.unit->sem_ir->filename(),
+                        .is_lowered = unit_info.unit->is_lowered});
     }
     // TODO: Remove dependence on properties of the first unit here.
     if (auto cpp_domain = InitializeCppDomain(
@@ -534,7 +535,8 @@ auto CheckParseTrees(
       if (auto cpp_domain = InitializeCppDomain(
               unit_info.err_tracker,
               {{.check_ir_id = unit_info.unit->sem_ir->check_ir_id(),
-                .filename = unit_info.unit->sem_ir->filename()}},
+                .filename = unit_info.unit->sem_ir->filename(),
+                .is_lowered = unit_info.unit->is_lowered}},
               fs, unit_info.unit->llvm_context, clang_invocation)) {
         cpp_domains.push_back(std::move(cpp_domain));
         unit_info.cpp_domain = cpp_domains.back().get();
diff --git a/toolchain/check/check.h b/toolchain/check/check.h
index 927c57c831b4..21a460fbbf6f 100644
--- a/toolchain/check/check.h
+++ b/toolchain/check/check.h
@@ -28,6 +28,7 @@ struct Unit {
   // The unit's SemIR, provided as empty and filled in by CheckParseTrees.
   SemIR::File* sem_ir;
   llvm::LLVMContext* llvm_context;
+  bool is_lowered;
   // The total number of files.
   int total_ir_count;
 };
diff --git a/toolchain/check/cpp/domain.h b/toolchain/check/cpp/domain.h
index 1b586f7e4565..a2fd38b2f6bc 100644
--- a/toolchain/check/cpp/domain.h
+++ b/toolchain/check/cpp/domain.h
@@ -26,8 +26,13 @@ namespace Carbon::Check {
 
 // An input Carbon file and its CheckIRId for C++ domain code generation.
 struct CppInputFile {
+  // The ID used to identify this file within SemIR.
   SemIR::CheckIRId check_ir_id;
+  // The Carbon source filename for this input.
   llvm::StringRef filename;
+  // Whether this input IR will be lowered. If not, we don't need to build a
+  // Clang CodeGenerator for it.
+  bool is_lowered;
 };
 
 // A C++ compilation domain, including a live Clang instance that can be used to
diff --git a/toolchain/check/cpp/generate_ast.cpp b/toolchain/check/cpp/generate_ast.cpp
index e46a03b2eb00..8f02c78a3dad 100644
--- a/toolchain/check/cpp/generate_ast.cpp
+++ b/toolchain/check/cpp/generate_ast.cpp
@@ -743,6 +743,10 @@ class GenerateASTAction : public clang::ASTFrontendAction {
     // a single object file, for a faster `carbon build` mode.
     std::vector<std::unique_ptr<clang::ASTConsumer>> consumers;
     for (const auto& input : inputs_) {
+      if (!input.is_lowered) {
+        code_generators_.push_back(nullptr);
+        continue;
+      }
       // TODO: Filter what goes into each code generator. If there are strong
       // external C++ definitions in a Carbon file (for example, in inline C++
       // code), they should be emitted only in that one file.
diff --git a/toolchain/driver/compile_driver.cpp b/toolchain/driver/compile_driver.cpp
index a5f1d3ce3981..881f3d334b4b 100644
--- a/toolchain/driver/compile_driver.cpp
+++ b/toolchain/driver/compile_driver.cpp
@@ -154,6 +154,7 @@ auto CompilationUnit::GetCheckUnit() -> Check::Unit {
           .timings = timings_ ? &*timings_ : nullptr,
           .sem_ir = &*sem_ir_,
           .llvm_context = llvm_context_,
+          .is_lowered = is_lowered(),
           .total_ir_count = total_ir_count_};
 }
 
@@ -175,6 +176,8 @@ auto CompilationUnit::PostCheck() -> void {
 }
 
 auto CompilationUnit::RunLower() -> void {
+  CARBON_CHECK(is_lowered(), "Should not lower this compilation unit");
+
   LogCall("Lower::LowerToLLVM", "lower", [&] {
     Lower::LowerToLLVMOptions options;
     options.llvm_verifier_stream =
@@ -364,27 +367,10 @@ auto CompilationUnit::RunCodeGenHelper() -> bool {
       }
     }
   } else {
-    llvm::SmallString<256> output_filename = llvm::StringRef(output_filename_);
-    if (output_filename.empty()) {
-      if (!source_->is_regular_file()) {
-        // Don't invent file names like `-.o` or `/dev/stdin.o`.
-        // TODO: Consider rephrasing the diagnostic to use the file as the
-        // `Emit` location.
-        CARBON_DIAGNOSTIC(CompileInputNotRegularFile, Error,
-                          "output file name must be specified for input `{0}` "
-                          "that is not a regular file",
-                          std::string);
-        driver_env_->emitter.Emit(CompileInputNotRegularFile, input_filename_);
-        return false;
-      }
-      output_filename = input_filename_;
-      llvm::sys::path::replace_extension(output_filename,
-                                         options_->asm_output ? ".s" : ".o");
-    }
-    CARBON_VLOG("Writing output to: {0}\n", output_filename);
+    CARBON_VLOG("Writing output to: {0}\n", output_filename_);
 
     std::error_code ec;
-    llvm::raw_fd_ostream output_file(output_filename, ec,
+    llvm::raw_fd_ostream output_file(output_filename_, ec,
                                      llvm::sys::fs::OF_None);
     if (ec) {
       // TODO: Consider rephrasing the diagnostic to use the file as the `Emit`
@@ -392,8 +378,8 @@ auto CompilationUnit::RunCodeGenHelper() -> bool {
       CARBON_DIAGNOSTIC(CompileOutputFileOpenError, Error,
                         "could not open output file `{0}`: {1}", std::string,
                         std::string);
-      driver_env_->emitter.Emit(CompileOutputFileOpenError,
-                                output_filename.str().str(), ec.message());
+      driver_env_->emitter.Emit(CompileOutputFileOpenError, output_filename_,
+                                ec.message());
       return false;
     }
     if (options_->asm_output) {
@@ -647,6 +633,10 @@ auto CompileDriver::Compile(DriverEnv& driver_env) -> DriverResult {
 
   // Lower and optimize.
   for (const auto& unit : units_) {
+    if (!unit->is_lowered()) {
+      continue;
+    }
+
     unit->RunLower();
 
     if (options_->phase != CompileOptions::Phase::Lower) {
@@ -662,28 +652,9 @@ auto CompileDriver::Compile(DriverEnv& driver_env) -> DriverResult {
   CARBON_CHECK(options_->phase == CompileOptions::Phase::CodeGen,
                "CodeGen should be the last stage");
 
-  bool output_last_input_only = options_->output_last_input_only;
-  if (!output_last_input_only && units_.size() > 1 &&
-      !options_->output_filename.empty() && options_->output_filename != "-") {
-    // TODO: Command line structure should change to make this implicit
-    // (passing non-compiling inputs differently), and the warning should be
-    // removed.
-    CARBON_DIAGNOSTIC(
-        CompileMultipleInputsWithOutput, Warning,
-        "only outputting {0} to {1}, skipping output of {2} input "
-        "file{2:s}; pass `--output-last-input-only` to silence this warning",
-        std::string, std::string, Diagnostics::IntAsSelect);
-    driver_env.emitter.Emit(CompileMultipleInputsWithOutput,
-                            units_.back()->input_filename().str(),
-                            options_->output_filename.str(), units_.size() - 1);
-    output_last_input_only = true;
-  }
-
   // Codegen.
-  if (output_last_input_only) {
-    units_.back()->RunCodeGen();
-  } else {
-    for (const auto& unit : units_) {
+  for (const auto& unit : units_) {
+    if (unit->is_lowered()) {
       unit->RunCodeGen();
     }
   }
diff --git a/toolchain/driver/compile_driver.h b/toolchain/driver/compile_driver.h
index f421775683db..cdb1d163b5d6 100644
--- a/toolchain/driver/compile_driver.h
+++ b/toolchain/driver/compile_driver.h
@@ -20,7 +20,10 @@ class MultiUnitCache;
 // Ties together information for a file being compiled.
 class CompilationUnit {
  public:
-  // `driver_env`, `options`, `consumer`, and `target` must be non-null.
+  // `driver_env`, `options`, `consumer`, and `target` must be non-null. If
+  // `output_filename` is empty, no output will be generated for this file. This
+  // is used for inputs that are only used as dependencies of the current
+  // compilation.
   explicit CompilationUnit(SemIR::CheckIRId check_ir_id, int total_ir_count,
                            DriverEnv* driver_env, const CompileOptions* options,
                            Diagnostics::Consumer* consumer,
@@ -67,6 +70,7 @@ class CompilationUnit {
 
   auto input_filename() -> llvm::StringRef { return input_filename_; }
   auto output_filename() -> llvm::StringRef { return output_filename_; }
+  auto is_lowered() -> bool { return !output_filename_.empty(); }
   auto has_include_in_dumps() -> bool {
     return tokens_ && tokens_->has_include_in_dumps();
   }
@@ -236,8 +240,7 @@ class CompileDriver {
   explicit CompileDriver(CompileOptions* options);
 
   // Configure the toolchain to compile all input files and dependencies.
-  // The `map_input` function maps an input file name to an output static
-  // object name.
+  // The `map_input` function maps an input file name to an output file name.
   // Returns `false` on configuration error.
   [[nodiscard]] auto Initialize(
       DriverEnv& driver_env,
diff --git a/toolchain/driver/compile_options.cpp b/toolchain/driver/compile_options.cpp
index 0507e2e13629..fc57f6e510d7 100644
--- a/toolchain/driver/compile_options.cpp
+++ b/toolchain/driver/compile_options.cpp
@@ -374,17 +374,6 @@ Excludes files with the given prefix from dumps.
 )""",
       },
       [&](auto& arg_b) { arg_b.Append(&exclude_dump_file_prefixes); });
-  b.AddFlag(
-      {
-          .name = "output-last-input-only",
-          .help = R"""(
-Only write output for the last input file, ignoring all others.
-
-TODO: This is a temporary workaround and should be removed once separate
-compilation is better implemented.
-)""",
-      },
-      [&](auto& arg_b) { arg_b.Set(&output_last_input_only); });
   b.AddStringOption(
       {
           .name = "sem-ir-crash-dump",
diff --git a/toolchain/driver/compile_options.h b/toolchain/driver/compile_options.h
index b874f8fb5b03..a85ce0520404 100644
--- a/toolchain/driver/compile_options.h
+++ b/toolchain/driver/compile_options.h
@@ -115,7 +115,6 @@ struct CompileOptions {
   Parse::ParseOptions::DumpFormat parse_dump_format;
   bool builtin_sem_ir = false;
   bool prelude_import = true;
-  bool output_last_input_only = false;
   bool include_carbon_core = true;
 
   llvm::SmallVector<llvm::StringRef> exclude_dump_file_prefixes;
diff --git a/toolchain/driver/compile_subcommand.cpp b/toolchain/driver/compile_subcommand.cpp
index f2c65ebcc166..6a913da62db2 100644
--- a/toolchain/driver/compile_subcommand.cpp
+++ b/toolchain/driver/compile_subcommand.cpp
@@ -4,6 +4,10 @@
 
 #include "toolchain/driver/compile_subcommand.h"
 
+#include "llvm/ADT/SmallString.h"
+#include "llvm/Support/Path.h"
+#include "toolchain/diagnostics/emitter.h"
+#include "toolchain/diagnostics/format_providers.h"
 #include "toolchain/driver/compile_driver.h"
 
 namespace Carbon {
@@ -35,12 +39,84 @@ auto CompileSubcommand::Run(DriverEnv& driver_env) -> DriverResult {
     return {.success = false};
   }
 
+  // If we're lowering and have an output file name, we can only support a
+  // single input filename.
+  // TODO: Produce an error in this case rather than skipping all files but the
+  // last, and remove the `--output-last-input-only` flag.
+  if (options_.compile_options.phase >= CompileOptions::Phase::Lower &&
+      options_.compile_options.input_filenames.size() > 1 &&
+      !options_.output_last_input_only &&
+      !options_.compile_options.output_filename.empty() &&
+      options_.compile_options.output_filename != "-") {
+    CARBON_DIAGNOSTIC(
+        CompileMultipleInputsWithOutput, Warning,
+        "only outputting {0} to {1}, skipping output of {2} input "
+        "file{2:s}; pass `--output-last-input-only` to silence this "
+        "warning",
+        std::string, std::string, Diagnostics::IntAsSelect);
+    driver_env.emitter.Emit(
+        CompileMultipleInputsWithOutput,
+        options_.compile_options.input_filenames.back().str(),
+        options_.compile_options.output_filename.str(),
+        options_.compile_options.input_filenames.size() - 1);
+  }
+
+  llvm::StringSet<> input_filenames(llvm::from_range,
+                                    options_.compile_options.input_filenames);
+
   auto compile_driver = CompileDriver(&options_.compile_options);
 
-  if (!compile_driver.Initialize(
-          driver_env, [&](llvm::StringRef) -> std::string {
-            return options_.compile_options.output_filename.str();
-          })) {
+  bool init_success = true;
+  auto get_output_filename =
+      [&](llvm::StringRef input_filename) -> std::string {
+    // We only generate output for inputs specified on the command line,
+    // not for inputs discovered through imports.
+    if (!input_filenames.contains(input_filename)) {
+      return "";
+    }
+
+    // If the output filename is "-", that's used for all inputs.
+    if (options_.compile_options.output_filename == "-") {
+      return "-";
+    }
+
+    // If single output filename was specified, it's used for the final
+    // input filename only.
+    if (!options_.compile_options.output_filename.empty()) {
+      if (input_filename == options_.compile_options.input_filenames.back()) {
+        return options_.compile_options.output_filename.str();
+      }
+      return "";
+    }
+
+    // Otherwise, generate an output filename for each explicitly-specified
+    // input file.
+    bool is_regular_file = true;
+    if (input_filename == "-") {
+      // TODO: If we would produce textual output, using "-" as the default
+      // output filename here would be reasonable and useful.
+      is_regular_file = false;
+    } else if (auto status = driver_env.fs->status(input_filename);
+               status && status->isOther()) {
+      is_regular_file = false;
+    }
+    if (!is_regular_file) {
+      CARBON_DIAGNOSTIC(CompileInputNotRegularFile, Error,
+                        "output file name must be specified for input "
+                        "`{0}` that is not a regular file",
+                        std::string);
+      driver_env.emitter.Emit(CompileInputNotRegularFile, input_filename.str());
+      init_success = false;
+      return "";
+    }
+    llvm::SmallString<256> output_filename = input_filename;
+    llvm::sys::path::replace_extension(
+        output_filename, options_.compile_options.asm_output ? ".s" : ".o");
+    return output_filename.str().str();
+  };
+
+  if (!compile_driver.Initialize(driver_env, get_output_filename) ||
+      !init_success) {
     return {.success = false};
   }
 
diff --git a/toolchain/driver/compile_subcommand.h b/toolchain/driver/compile_subcommand.h
index 47a64ba13353..8aa57568484d 100644
--- a/toolchain/driver/compile_subcommand.h
+++ b/toolchain/driver/compile_subcommand.h
@@ -16,10 +16,22 @@ namespace Carbon {
 struct CompileSubcommandOptions {
   auto Build(CommandLine::CommandBuilder& b) -> void {
     compile_options.BuildForCompileSubcommand(b, &codegen_options);
+    b.AddFlag(
+        {
+            .name = "output-last-input-only",
+            .help = R"""(
+Only write output for the last input file, ignoring all others.
+
+TODO: This is a temporary workaround and should be removed once separate
+compilation is better implemented.
+)""",
+        },
+        [&](auto& arg_b) { arg_b.Set(&output_last_input_only); });
   }
 
   CodegenOptions codegen_options;
   CompileOptions compile_options;
+  bool output_last_input_only = false;
 };
 
 // Implements the compile subcommand of the driver.
diff --git a/toolchain/driver/testdata/stdin.carbon b/toolchain/driver/testdata/stdin.carbon
index 27df3370f956..e53f7f351ab3 100644
--- a/toolchain/driver/testdata/stdin.carbon
+++ b/toolchain/driver/testdata/stdin.carbon
@@ -6,7 +6,7 @@
 // dump flags.
 // TODO: Align lex/parse with check/lower.
 //
-// ARGS: compile - --phase=lower --no-prelude-import --target=x86_64-linux-gnu --dump-tokens --dump-parse-tree --parse-dump-format=yaml-postorder --dump-sem-ir --dump-raw-sem-ir --dump-llvm-ir
+// ARGS: compile - --output=- --phase=lower --no-prelude-import --target=x86_64-linux-gnu --dump-tokens --dump-parse-tree --parse-dump-format=yaml-postorder --dump-sem-ir --dump-raw-sem-ir --dump-llvm-ir
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
diff --git a/toolchain/lower/testdata/function/generic/call_different_specific.carbon b/toolchain/lower/testdata/function/generic/call_different_specific.carbon
index dc454daec477..b96f762c0ad5 100644
--- a/toolchain/lower/testdata/function/generic/call_different_specific.carbon
+++ b/toolchain/lower/testdata/function/generic/call_different_specific.carbon
@@ -48,7 +48,7 @@ fn M(ptr_i32: i32*, ptr_f64: f64*) {
 // CHECK:STDOUT: ; ModuleID = 'call_different_specific.carbon'
 // CHECK:STDOUT: source_filename = "call_different_specific.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: %type.58 = type {}
+// CHECK:STDOUT: %type.0 = type {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: ; Function Attrs: nounwind
 // CHECK:STDOUT: define void @_CM.Main(ptr %ptr_i32, ptr %ptr_f64) #0 !dbg !4 {
@@ -73,7 +73,7 @@ fn M(ptr_i32: i32*, ptr_f64: f64*) {
 // CHECK:STDOUT: ; Function Attrs: nounwind
 // CHECK:STDOUT: define linkonce_odr void @_CF.Main.58016a73bff04416(ptr %x) #0 !dbg !27 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %A.call = call %type.58 @_CA.Main.ee509b43cf4eb3c5(%type.58 zeroinitializer), !dbg !30
+// CHECK:STDOUT:   %A.call = call %type.0 @_CA.Main.ee509b43cf4eb3c5(%type.0 zeroinitializer), !dbg !30
 // CHECK:STDOUT:   call void @_CB.Main.58016a73bff04416(ptr %x), !dbg !31
 // CHECK:STDOUT:   ret void, !dbg !32
 // CHECK:STDOUT: }
@@ -81,15 +81,15 @@ fn M(ptr_i32: i32*, ptr_f64: f64*) {
 // CHECK:STDOUT: ; Function Attrs: nounwind
 // CHECK:STDOUT: define linkonce_odr void @_CF.Main.fa1f0912a5bbe922(ptr %x) #0 !dbg !33 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %A.call = call %type.58 @_CA.Main.ee509b43cf4eb3c5(%type.58 zeroinitializer), !dbg !36
+// CHECK:STDOUT:   %A.call = call %type.0 @_CA.Main.ee509b43cf4eb3c5(%type.0 zeroinitializer), !dbg !36
 // CHECK:STDOUT:   call void @_CB.Main.fa1f0912a5bbe922(ptr %x), !dbg !37
 // CHECK:STDOUT:   ret void, !dbg !38
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: ; Function Attrs: nounwind
-// CHECK:STDOUT: define linkonce_odr %type.58 @_CA.Main.ee509b43cf4eb3c5(%type.58 %x) #0 !dbg !39 {
+// CHECK:STDOUT: define linkonce_odr %type.0 @_CA.Main.ee509b43cf4eb3c5(%type.0 %x) #0 !dbg !39 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   ret %type.58 %x, !dbg !44
+// CHECK:STDOUT:   ret %type.0 %x, !dbg !44
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: ; Function Attrs: nounwind
diff --git a/toolchain/lower/testdata/interop/cpp/operators.carbon b/toolchain/lower/testdata/interop/cpp/operators.carbon
index 610f3dad578a..b199ae8608d4 100644
--- a/toolchain/lower/testdata/interop/cpp/operators.carbon
+++ b/toolchain/lower/testdata/interop/cpp/operators.carbon
@@ -290,7 +290,7 @@ fn Driver(x: Cpp.A, y: Cpp.A) {
 // 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: %struct.A.62 = type { i32 }
+// CHECK:STDOUT: %struct.A.4 = type { i32 }
 // CHECK:STDOUT:
 // CHECK:STDOUT: $_Zlt1AS_ = comdat any
 // CHECK:STDOUT:
@@ -313,8 +313,8 @@ fn Driver(x: Cpp.A, y: Cpp.A) {
 // CHECK:STDOUT:   %x.addr = alloca ptr, align 8
 // CHECK:STDOUT:   %y.addr = alloca ptr, align 8
 // CHECK:STDOUT:   %return.addr = alloca ptr, align 8
-// CHECK:STDOUT:   %agg.tmp = alloca %struct.A.62, align 4
-// CHECK:STDOUT:   %agg.tmp1 = alloca %struct.A.62, align 4
+// CHECK:STDOUT:   %agg.tmp = alloca %struct.A.4, align 4
+// CHECK:STDOUT:   %agg.tmp1 = alloca %struct.A.4, align 4
 // CHECK:STDOUT:   store ptr %x, ptr %x.addr, align 8, !tbaa !21
 // CHECK:STDOUT:   store ptr %y, ptr %y.addr, align 8, !tbaa !21
 // CHECK:STDOUT:   store ptr %return, ptr %return.addr, align 8, !tbaa !24
@@ -323,9 +323,9 @@ fn Driver(x: Cpp.A, y: Cpp.A) {
 // CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 4 %agg.tmp, ptr align 4 %1, i64 4, i1 false), !tbaa.struct !26
 // CHECK:STDOUT:   %2 = load ptr, ptr %y.addr, align 8, !tbaa !21
 // CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 4 %agg.tmp1, ptr align 4 %2, i64 4, i1 false), !tbaa.struct !26
-// CHECK:STDOUT:   %coerce.dive = getelementptr inbounds nuw %struct.A.62, ptr %agg.tmp, i32 0, i32 0
+// CHECK:STDOUT:   %coerce.dive = getelementptr inbounds nuw %struct.A.4, ptr %agg.tmp, i32 0, i32 0
 // CHECK:STDOUT:   %3 = load i32, ptr %coerce.dive, align 4
-// CHECK:STDOUT:   %coerce.dive2 = getelementptr inbounds nuw %struct.A.62, ptr %agg.tmp1, i32 0, i32 0
+// CHECK:STDOUT:   %coerce.dive2 = getelementptr inbounds nuw %struct.A.4, ptr %agg.tmp1, i32 0, i32 0
 // CHECK:STDOUT:   %4 = load i32, ptr %coerce.dive2, align 4
 // CHECK:STDOUT:   %call = call noundef zeroext i1 @_Zlt1AS_(i32 %3, i32 %4)
 // CHECK:STDOUT:   %storedv = zext i1 %call to i8
@@ -339,8 +339,8 @@ fn Driver(x: Cpp.A, y: Cpp.A) {
 // CHECK:STDOUT:   %x.addr = alloca ptr, align 8
 // CHECK:STDOUT:   %y.addr = alloca ptr, align 8
 // CHECK:STDOUT:   %return.addr = alloca ptr, align 8
-// CHECK:STDOUT:   %agg.tmp = alloca %struct.A.62, align 4
-// CHECK:STDOUT:   %agg.tmp1 = alloca %struct.A.62, align 4
+// CHECK:STDOUT:   %agg.tmp = alloca %struct.A.4, align 4
+// CHECK:STDOUT:   %agg.tmp1 = alloca %struct.A.4, align 4
 // CHECK:STDOUT:   store ptr %x, ptr %x.addr, align 8, !tbaa !21
 // CHECK:STDOUT:   store ptr %y, ptr %y.addr, align 8, !tbaa !21
 // CHECK:STDOUT:   store ptr %return, ptr %return.addr, align 8, !tbaa !24
@@ -349,9 +349,9 @@ fn Driver(x: Cpp.A, y: Cpp.A) {
 // CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 4 %agg.tmp, ptr align 4 %1, i64 4, i1 false), !tbaa.struct !26
 // CHECK:STDOUT:   %2 = load ptr, ptr %y.addr, align 8, !tbaa !21
 // CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 4 %agg.tmp1, ptr align 4 %2, i64 4, i1 false), !tbaa.struct !26
-// CHECK:STDOUT:   %coerce.dive = getelementptr inbounds nuw %struct.A.62, ptr %agg.tmp, i32 0, i32 0
+// CHECK:STDOUT:   %coerce.dive = getelementptr inbounds nuw %struct.A.4, ptr %agg.tmp, i32 0, i32 0
 // CHECK:STDOUT:   %3 = load i32, ptr %coerce.dive, align 4
-// CHECK:STDOUT:   %coerce.dive2 = getelementptr inbounds nuw %struct.A.62, ptr %agg.tmp1, i32 0, i32 0
+// CHECK:STDOUT:   %coerce.dive2 = getelementptr inbounds nuw %struct.A.4, ptr %agg.tmp1, i32 0, i32 0
 // CHECK:STDOUT:   %4 = load i32, ptr %coerce.dive2, align 4
 // CHECK:STDOUT:   %call = call noundef zeroext i1 @_Zle1AS_(i32 %3, i32 %4)
 // CHECK:STDOUT:   %storedv = zext i1 %call to i8
@@ -365,8 +365,8 @@ fn Driver(x: Cpp.A, y: Cpp.A) {
 // CHECK:STDOUT:   %x.addr = alloca ptr, align 8
 // CHECK:STDOUT:   %y.addr = alloca ptr, align 8
 // CHECK:STDOUT:   %return.addr = alloca ptr, align 8
-// CHECK:STDOUT:   %agg.tmp = alloca %struct.A.62, align 4
-// CHECK:STDOUT:   %agg.tmp1 = alloca %struct.A.62, align 4
+// CHECK:STDOUT:   %agg.tmp = alloca %struct.A.4, align 4
+// CHECK:STDOUT:   %agg.tmp1 = alloca %struct.A.4, align 4
 // CHECK:STDOUT:   store ptr %x, ptr %x.addr, align 8, !tbaa !21
 // CHECK:STDOUT:   store ptr %y, ptr %y.addr, align 8, !tbaa !21
 // CHECK:STDOUT:   store ptr %return, ptr %return.addr, align 8, !tbaa !24
@@ -375,9 +375,9 @@ fn Driver(x: Cpp.A, y: Cpp.A) {
 // CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 4 %agg.tmp, ptr align 4 %1, i64 4, i1 false), !tbaa.struct !26
 // CHECK:STDOUT:   %2 = load ptr, ptr %y.addr, align 8, !tbaa !21
 // CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 4 %agg.tmp1, ptr align 4 %2, i64 4, i1 false), !tbaa.struct !26
-// CHECK:STDOUT:   %coerce.dive = getelementptr inbounds nuw %struct.A.62, ptr %agg.tmp, i32 0, i32 0
+// CHECK:STDOUT:   %coerce.dive = getelementptr inbounds nuw %struct.A.4, ptr %agg.tmp, i32 0, i32 0
 // CHECK:STDOUT:   %3 = load i32, ptr %coerce.dive, align 4
-// CHECK:STDOUT:   %coerce.dive2 = getelementptr inbounds nuw %struct.A.62, ptr %agg.tmp1, i32 0, i32 0
+// CHECK:STDOUT:   %coerce.dive2 = getelementptr inbounds nuw %struct.A.4, ptr %agg.tmp1, i32 0, i32 0
 // CHECK:STDOUT:   %4 = load i32, ptr %coerce.dive2, align 4
 // CHECK:STDOUT:   %call = call noundef zeroext i1 @_Zgt1AS_(i32 %3, i32 %4)
 // CHECK:STDOUT:   %storedv = zext i1 %call to i8
@@ -391,8 +391,8 @@ fn Driver(x: Cpp.A, y: Cpp.A) {
 // CHECK:STDOUT:   %x.addr = alloca ptr, align 8
 // CHECK:STDOUT:   %y.addr = alloca ptr, align 8
 // CHECK:STDOUT:   %return.addr = alloca ptr, align 8
-// CHECK:STDOUT:   %agg.tmp = alloca %struct.A.62, align 4
-// CHECK:STDOUT:   %agg.tmp1 = alloca %struct.A.62, align 4
+// CHECK:STDOUT:   %agg.tmp = alloca %struct.A.4, align 4
+// CHECK:STDOUT:   %agg.tmp1 = alloca %struct.A.4, align 4
 // CHECK:STDOUT:   store ptr %x, ptr %x.addr, align 8, !tbaa !21
 // CHECK:STDOUT:   store ptr %y, ptr %y.addr, align 8, !tbaa !21
 // CHECK:STDOUT:   store ptr %return, ptr %return.addr, align 8, !tbaa !24
@@ -401,9 +401,9 @@ fn Driver(x: Cpp.A, y: Cpp.A) {
 // CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 4 %agg.tmp, ptr align 4 %1, i64 4, i1 false), !tbaa.struct !26
 // CHECK:STDOUT:   %2 = load ptr, ptr %y.addr, align 8, !tbaa !21
 // CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 4 %agg.tmp1, ptr align 4 %2, i64 4, i1 false), !tbaa.struct !26
-// CHECK:STDOUT:   %coerce.dive = getelementptr inbounds nuw %struct.A.62, ptr %agg.tmp, i32 0, i32 0
+// CHECK:STDOUT:   %coerce.dive = getelementptr inbounds nuw %struct.A.4, ptr %agg.tmp, i32 0, i32 0
 // CHECK:STDOUT:   %3 = load i32, ptr %coerce.dive, align 4
-// CHECK:STDOUT:   %coerce.dive2 = getelementptr inbounds nuw %struct.A.62, ptr %agg.tmp1, i32 0, i32 0
+// CHECK:STDOUT:   %coerce.dive2 = getelementptr inbounds nuw %struct.A.4, ptr %agg.tmp1, i32 0, i32 0
 // CHECK:STDOUT:   %4 = load i32, ptr %coerce.dive2, align 4
 // CHECK:STDOUT:   %call = call noundef zeroext i1 @_Zge1AS_(i32 %3, i32 %4)
 // CHECK:STDOUT:   %storedv = zext i1 %call to i8
@@ -467,15 +467,15 @@ fn Driver(x: Cpp.A, y: Cpp.A) {
 // CHECK:STDOUT: ; Function Attrs: mustprogress nounwind uwtable
 // CHECK:STDOUT: define linkonce_odr dso_local noundef zeroext i1 @_Zlt1AS_(i32 %x.coerce, i32 %y.coerce) #4 comdat {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %x = alloca %struct.A.62, align 4
-// CHECK:STDOUT:   %y = alloca %struct.A.62, align 4
-// CHECK:STDOUT:   %coerce.dive = getelementptr inbounds nuw %struct.A.62, ptr %x, i32 0, i32 0
+// CHECK:STDOUT:   %x = alloca %struct.A.4, align 4
+// CHECK:STDOUT:   %y = alloca %struct.A.4, align 4
+// CHECK:STDOUT:   %coerce.dive = getelementptr inbounds nuw %struct.A.4, ptr %x, i32 0, i32 0
 // CHECK:STDOUT:   store i32 %x.coerce, ptr %coerce.dive, align 4
-// CHECK:STDOUT:   %coerce.dive1 = getelementptr inbounds nuw %struct.A.62, ptr %y, i32 0, i32 0
+// CHECK:STDOUT:   %coerce.dive1 = getelementptr inbounds nuw %struct.A.4, ptr %y, i32 0, i32 0
 // CHECK:STDOUT:   store i32 %y.coerce, ptr %coerce.dive1, align 4
-// CHECK:STDOUT:   %value = getelementptr inbounds nuw %struct.A.62, ptr %x, i32 0, i32 0
+// CHECK:STDOUT:   %value = getelementptr inbounds nuw %struct.A.4, ptr %x, i32 0, i32 0
 // CHECK:STDOUT:   %0 = load i32, ptr %value, align 4, !tbaa !61
-// CHECK:STDOUT:   %value2 = getelementptr inbounds nuw %struct.A.62, ptr %y, i32 0, i32 0
+// CHECK:STDOUT:   %value2 = getelementptr inbounds nuw %struct.A.4, ptr %y, i32 0, i32 0
 // CHECK:STDOUT:   %1 = load i32, ptr %value2, align 4, !tbaa !61
 // CHECK:STDOUT:   %cmp = icmp slt i32 %0, %1
 // CHECK:STDOUT:   ret i1 %cmp
@@ -487,19 +487,19 @@ fn Driver(x: Cpp.A, y: Cpp.A) {
 // CHECK:STDOUT: ; Function Attrs: mustprogress nounwind uwtable
 // CHECK:STDOUT: define linkonce_odr dso_local noundef zeroext i1 @_Zle1AS_(i32 %x.coerce, i32 %y.coerce) #4 comdat {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %x = alloca %struct.A.62, align 4
-// CHECK:STDOUT:   %y = alloca %struct.A.62, align 4
-// CHECK:STDOUT:   %agg.tmp = alloca %struct.A.62, align 4
-// CHECK:STDOUT:   %agg.tmp2 = alloca %struct.A.62, align 4
-// CHECK:STDOUT:   %coerce.dive = getelementptr inbounds nuw %struct.A.62, ptr %x, i32 0, i32 0
+// CHECK:STDOUT:   %x = alloca %struct.A.4, align 4
+// CHECK:STDOUT:   %y = alloca %struct.A.4, align 4
+// CHECK:STDOUT:   %agg.tmp = alloca %struct.A.4, align 4
+// CHECK:STDOUT:   %agg.tmp2 = alloca %struct.A.4, align 4
+// CHECK:STDOUT:   %coerce.dive = getelementptr inbounds nuw %struct.A.4, ptr %x, i32 0, i32 0
 // CHECK:STDOUT:   store i32 %x.coerce, ptr %coerce.dive, align 4
-// CHECK:STDOUT:   %coerce.dive1 = getelementptr inbounds nuw %struct.A.62, ptr %y, i32 0, i32 0
+// CHECK:STDOUT:   %coerce.dive1 = getelementptr inbounds nuw %struct.A.4, ptr %y, i32 0, i32 0
 // CHECK:STDOUT:   store i32 %y.coerce, ptr %coerce.dive1, align 4
 // CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 4 %agg.tmp, ptr align 4 %y, i64 4, i1 false), !tbaa.struct !26
 // CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 4 %agg.tmp2, ptr align 4 %x, i64 4, i1 false), !tbaa.struct !26
-// CHECK:STDOUT:   %coerce.dive3 = getelementptr inbounds nuw %struct.A.62, ptr %agg.tmp, i32 0, i32 0
+// CHECK:STDOUT:   %coerce.dive3 = getelementptr inbounds nuw %struct.A.4, ptr %agg.tmp, i32 0, i32 0
 // CHECK:STDOUT:   %0 = load i32, ptr %coerce.dive3, align 4
-// CHECK:STDOUT:   %coerce.dive4 = getelementptr inbounds nuw %struct.A.62, ptr %agg.tmp2, i32 0, i32 0
+// CHECK:STDOUT:   %coerce.dive4 = getelementptr inbounds nuw %struct.A.4, ptr %agg.tmp2, i32 0, i32 0
 // CHECK:STDOUT:   %1 = load i32, ptr %coerce.dive4, align 4
 // CHECK:STDOUT:   %call = call noundef zeroext i1 @_Zlt1AS_(i32 %0, i32 %1)
 // CHECK:STDOUT:   %lnot = xor i1 %call, true
@@ -509,19 +509,19 @@ fn Driver(x: Cpp.A, y: Cpp.A) {
 // CHECK:STDOUT: ; Function Attrs: mustprogress nounwind uwtable
 // CHECK:STDOUT: define linkonce_odr dso_local noundef zeroext i1 @_Zgt1AS_(i32 %x.coerce, i32 %y.coerce) #4 comdat {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %x = alloca %struct.A.62, align 4
-// CHECK:STDOUT:   %y = alloca %struct.A.62, align 4
-// CHECK:STDOUT:   %agg.tmp = alloca %struct.A.62, align 4
-// CHECK:STDOUT:   %agg.tmp2 = alloca %struct.A.62, align 4
-// CHECK:STDOUT:   %coerce.dive = getelementptr inbounds nuw %struct.A.62, ptr %x, i32 0, i32 0
+// CHECK:STDOUT:   %x = alloca %struct.A.4, align 4
+// CHECK:STDOUT:   %y = alloca %struct.A.4, align 4
+// CHECK:STDOUT:   %agg.tmp = alloca %struct.A.4, align 4
+// CHECK:STDOUT:   %agg.tmp2 = alloca %struct.A.4, align 4
+// CHECK:STDOUT:   %coerce.dive = getelementptr inbounds nuw %struct.A.4, ptr %x, i32 0, i32 0
 // CHECK:STDOUT:   store i32 %x.coerce, ptr %coerce.dive, align 4
-// CHECK:STDOUT:   %coerce.dive1 = getelementptr inbounds nuw %struct.A.62, ptr %y, i32 0, i32 0
+// CHECK:STDOUT:   %coerce.dive1 = getelementptr inbounds nuw %struct.A.4, ptr %y, i32 0, i32 0
 // CHECK:STDOUT:   store i32 %y.coerce, ptr %coerce.dive1, align 4
 // CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 4 %agg.tmp, ptr align 4 %y, i64 4, i1 false), !tbaa.struct !26
 // CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 4 %agg.tmp2, ptr align 4 %x, i64 4, i1 false), !tbaa.struct !26
-// CHECK:STDOUT:   %coerce.dive3 = getelementptr inbounds nuw %struct.A.62, ptr %agg.tmp, i32 0, i32 0
+// CHECK:STDOUT:   %coerce.dive3 = getelementptr inbounds nuw %struct.A.4, ptr %agg.tmp, i32 0, i32 0
 // CHECK:STDOUT:   %0 = load i32, ptr %coerce.dive3, align 4
-// CHECK:STDOUT:   %coerce.dive4 = getelementptr inbounds nuw %struct.A.62, ptr %agg.tmp2, i32 0, i32 0
+// CHECK:STDOUT:   %coerce.dive4 = getelementptr inbounds nuw %struct.A.4, ptr %agg.tmp2, i32 0, i32 0
 // CHECK:STDOUT:   %1 = load i32, ptr %coerce.dive4, align 4
 // CHECK:STDOUT:   %call = call noundef zeroext i1 @_Zlt1AS_(i32 %0, i32 %1)
 // CHECK:STDOUT:   ret i1 %call
@@ -530,19 +530,19 @@ fn Driver(x: Cpp.A, y: Cpp.A) {
 // CHECK:STDOUT: ; Function Attrs: mustprogress nounwind uwtable
 // CHECK:STDOUT: define linkonce_odr dso_local noundef zeroext i1 @_Zge1AS_(i32 %x.coerce, i32 %y.coerce) #4 comdat {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %x = alloca %struct.A.62, align 4
-// CHECK:STDOUT:   %y = alloca %struct.A.62, align 4
-// CHECK:STDOUT:   %agg.tmp = alloca %struct.A.62, align 4
-// CHECK:STDOUT:   %agg.tmp2 = alloca %struct.A.62, align 4
-// CHECK:STDOUT:   %coerce.dive = getelementptr inbounds nuw %struct.A.62, ptr %x, i32 0, i32 0
+// CHECK:STDOUT:   %x = alloca %struct.A.4, align 4
+// CHECK:STDOUT:   %y = alloca %struct.A.4, align 4
+// CHECK:STDOUT:   %agg.tmp = alloca %struct.A.4, align 4
+// CHECK:STDOUT:   %agg.tmp2 = alloca %struct.A.4, align 4
+// CHECK:STDOUT:   %coerce.dive = getelementptr inbounds nuw %struct.A.4, ptr %x, i32 0, i32 0
 // CHECK:STDOUT:   store i32 %x.coerce, ptr %coerce.dive, align 4
-// CHECK:STDOUT:   %coerce.dive1 = getelementptr inbounds nuw %struct.A.62, ptr %y, i32 0, i32 0
+// CHECK:STDOUT:   %coerce.dive1 = getelementptr inbounds nuw %struct.A.4, ptr %y, i32 0, i32 0
 // CHECK:STDOUT:   store i32 %y.coerce, ptr %coerce.dive1, align 4
 // CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 4 %agg.tmp, ptr align 4 %x, i64 4, i1 false), !tbaa.struct !26
 // CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 4 %agg.tmp2, ptr align 4 %y, i64 4, i1 false), !tbaa.struct !26
-// CHECK:STDOUT:   %coerce.dive3 = getelementptr inbounds nuw %struct.A.62, ptr %agg.tmp, i32 0, i32 0
+// CHECK:STDOUT:   %coerce.dive3 = getelementptr inbounds nuw %struct.A.4, ptr %agg.tmp, i32 0, i32 0
 // CHECK:STDOUT:   %0 = load i32, ptr %coerce.dive3, align 4
-// CHECK:STDOUT:   %coerce.dive4 = getelementptr inbounds nuw %struct.A.62, ptr %agg.tmp2, i32 0, i32 0
+// CHECK:STDOUT:   %coerce.dive4 = getelementptr inbounds nuw %struct.A.4, ptr %agg.tmp2, i32 0, i32 0
 // CHECK:STDOUT:   %1 = load i32, ptr %coerce.dive4, align 4
 // CHECK:STDOUT:   %call = call noundef zeroext i1 @_Zlt1AS_(i32 %0, i32 %1)
 // CHECK:STDOUT:   %lnot = xor i1 %call, true
diff --git a/toolchain/lower/testdata/interop/cpp/std_initializer_list.carbon b/toolchain/lower/testdata/interop/cpp/std_initializer_list.carbon
index dd0d76773517..009c76fd8b01 100644
--- a/toolchain/lower/testdata/interop/cpp/std_initializer_list.carbon
+++ b/toolchain/lower/testdata/interop/cpp/std_initializer_list.carbon
@@ -368,7 +368,7 @@ fn InitNontrivialDtor() {
 // 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: %"class.std::initializer_list.64" = type { ptr, i64 }
+// CHECK:STDOUT: %"class.std::initializer_list.6" = type { ptr, i64 }
 // CHECK:STDOUT:
 // CHECK:STDOUT: $_ZN11vector_likeD2Ev = comdat any
 // CHECK:STDOUT:
@@ -442,7 +442,7 @@ fn InitNontrivialDtor() {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %list.addr = alloca ptr, align 8
 // CHECK:STDOUT:   %return.addr = alloca ptr, align 8
-// CHECK:STDOUT:   %agg.tmp = alloca %"class.std::initializer_list.64", align 8
+// CHECK:STDOUT:   %agg.tmp = alloca %"class.std::initializer_list.6", align 8
 // CHECK:STDOUT:   store ptr %list, ptr %list.addr, align 8, !tbaa !38
 // CHECK:STDOUT:   store ptr %return, ptr %return.addr, align 8, !tbaa !41
 // CHECK:STDOUT:   %0 = load ptr, ptr %return.addr, align 8, !tbaa !41
@@ -521,7 +521,7 @@ fn InitNontrivialDtor() {
 // CHECK:STDOUT: ; Function Attrs: mustprogress nounwind uwtable
 // CHECK:STDOUT: define linkonce_odr dso_local void @_ZN11vector_likeC2ESt16initializer_listIiE(ptr noundef nonnull align 1 dereferenceable(1) %this, ptr %list.coerce0, i64 %list.coerce1) unnamed_addr #2 comdat align 2 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %list = alloca %"class.std::initializer_list.64", align 8
+// CHECK:STDOUT:   %list = alloca %"class.std::initializer_list.6", align 8
 // CHECK:STDOUT:   %this.addr = alloca ptr, align 8
 // CHECK:STDOUT:   %0 = getelementptr inbounds nuw { ptr, i64 }, ptr %list, i32 0, i32 0
 // CHECK:STDOUT:   store ptr %list.coerce0, ptr %0, align 8