diff --git a/common/command_line.cpp b/common/command_line.cpp
index ce8f07e1b7af..443194c92e30 100644
--- a/common/command_line.cpp
+++ b/common/command_line.cpp
@@ -108,13 +108,15 @@ class MetaPrinter {
   //
   // This adds meta subcommands or options to print both help and version
   // information for the command.
-  void RegisterWithCommand(const Command& command, CommandBuilder& builder);
+  auto RegisterWithCommand(const Command& command, CommandBuilder& builder)
+      -> void;
 
-  void PrintHelp(const Command& command) const;
-  void PrintHelpForSubcommandName(const Command& command,
-                                  llvm::StringRef subcommand_name) const;
-  void PrintVersion(const Command& command) const;
-  void PrintSubcommands(const Command& command) const;
+  auto PrintHelp(const Command& command) const -> void;
+  auto PrintHelpForSubcommandName(const Command& command,
+                                  llvm::StringRef subcommand_name) const
+      -> void;
+  auto PrintVersion(const Command& command) const -> void;
+  auto PrintSubcommands(const Command& command) const -> void;
 
  private:
   // The indent is calibrated to allow a short and long option after a two
@@ -193,33 +195,37 @@ Prints the version of this command.
   };
 
   // A general helper for rendering a text block.
-  void PrintTextBlock(llvm::StringRef indent, llvm::StringRef text) const;
+  auto PrintTextBlock(llvm::StringRef indent, llvm::StringRef text) const
+      -> void;
 
   // Helpers for version and build information printing.
-  void PrintRawVersion(const Command& command, llvm::StringRef indent) const;
-  void PrintRawBuildInfo(const Command& command, llvm::StringRef indent) const;
+  auto PrintRawVersion(const Command& command, llvm::StringRef indent) const
+      -> void;
+  auto PrintRawBuildInfo(const Command& command, llvm::StringRef indent) const
+      -> void;
 
   // Helpers for printing components of help and usage output for arguments,
   // including options and positional arguments.
-  void PrintArgValueUsage(const Arg& arg) const;
-  void PrintOptionUsage(const Arg& option) const;
-  void PrintOptionShortName(const Arg& arg) const;
-  void PrintArgShortValues(const Arg& arg) const;
-  void PrintArgLongValues(const Arg& arg, llvm::StringRef indent) const;
-  void PrintArgHelp(const Arg& arg, llvm::StringRef indent) const;
+  auto PrintArgValueUsage(const Arg& arg) const -> void;
+  auto PrintOptionUsage(const Arg& option) const -> void;
+  auto PrintOptionShortName(const Arg& arg) const -> void;
+  auto PrintArgShortValues(const Arg& arg) const -> void;
+  auto PrintArgLongValues(const Arg& arg, llvm::StringRef indent) const -> void;
+  auto PrintArgHelp(const Arg& arg, llvm::StringRef indent) const -> void;
 
   // Helpers for printing command usage summaries.
-  void PrintRawUsageCommandAndOptions(
+  auto PrintRawUsageCommandAndOptions(
       const Command& command,
-      int max_option_width = MaxLeafOptionUsageWidth) const;
-  void PrintRawUsage(const Command& command, llvm::StringRef indent) const;
-  void PrintUsage(const Command& command) const;
+      int max_option_width = MaxLeafOptionUsageWidth) const -> void;
+  auto PrintRawUsage(const Command& command, llvm::StringRef indent) const
+      -> void;
+  auto PrintUsage(const Command& command) const -> void;
 
   // Helpers to print various sections of `PrintHelp` that only occur within
   // that output.
-  void PrintHelpSubcommands(const Command& command) const;
-  void PrintHelpPositionalArgs(const Command& command) const;
-  void PrintHelpOptions(const Command& command) const;
+  auto PrintHelpSubcommands(const Command& command) const -> void;
+  auto PrintHelpPositionalArgs(const Command& command) const -> void;
+  auto PrintHelpOptions(const Command& command) const -> void;
 
   llvm::raw_ostream* out_;
 
@@ -231,8 +237,8 @@ Prints the version of this command.
   llvm::StringRef help_subcommand_;
 };
 
-void MetaPrinter::RegisterWithCommand(const Command& command,
-                                      CommandBuilder& builder) {
+auto MetaPrinter::RegisterWithCommand(const Command& command,
+                                      CommandBuilder& builder) -> void {
   bool is_subcommand = command.parent;
   bool has_subcommands = !command.subcommands.empty();
 
@@ -285,7 +291,7 @@ void MetaPrinter::RegisterWithCommand(const Command& command,
   }
 }
 
-void MetaPrinter::PrintHelp(const Command& command) const {
+auto MetaPrinter::PrintHelp(const Command& command) const -> void {
   // TODO: begin using the short setting to customize the output.
   (void)short_help_;
 
@@ -320,8 +326,8 @@ void MetaPrinter::PrintHelp(const Command& command) const {
   *out_ << "\n";
 }
 
-void MetaPrinter::PrintHelpForSubcommandName(
-    const Command& command, llvm::StringRef subcommand_name) const {
+auto MetaPrinter::PrintHelpForSubcommandName(
+    const Command& command, llvm::StringRef subcommand_name) const -> void {
   for (const auto& subcommand : command.subcommands) {
     if (subcommand->info.name == subcommand_name) {
       PrintHelp(*subcommand);
@@ -335,7 +341,7 @@ void MetaPrinter::PrintHelpForSubcommandName(
         << "'\n";
 }
 
-void MetaPrinter::PrintVersion(const Command& command) const {
+auto MetaPrinter::PrintVersion(const Command& command) const -> void {
   CARBON_CHECK(
       !command.info.version.empty(),
       "Printing should not be enabled without a version string configured.");
@@ -347,21 +353,21 @@ void MetaPrinter::PrintVersion(const Command& command) const {
   }
 }
 
-void MetaPrinter::PrintSubcommands(const Command& command) const {
+auto MetaPrinter::PrintSubcommands(const Command& command) const -> void {
   PrintListOfAlternatives(*out_, llvm::ArrayRef(command.subcommands),
                           [](const std::unique_ptr<Command>& subcommand) {
                             return subcommand->info.name;
                           });
 }
 
-void MetaPrinter::PrintRawVersion(const Command& command,
-                                  llvm::StringRef indent) const {
+auto MetaPrinter::PrintRawVersion(const Command& command,
+                                  llvm::StringRef indent) const -> void {
   // Newlines are trimmed from the version string an a closing newline added but
   // no other formatting is performed.
   *out_ << indent << command.info.version.trim('\n') << "\n";
 }
-void MetaPrinter::PrintRawBuildInfo(const Command& command,
-                                    llvm::StringRef indent) const {
+auto MetaPrinter::PrintRawBuildInfo(const Command& command,
+                                    llvm::StringRef indent) const -> void {
   // Print the build info line-by-line without any wrapping in case it
   // contains line-oriented formatted text, but drop leading and trailing blank
   // lines.
@@ -372,8 +378,8 @@ void MetaPrinter::PrintRawBuildInfo(const Command& command,
   }
 }
 
-void MetaPrinter::PrintTextBlock(llvm::StringRef indent,
-                                 llvm::StringRef text) const {
+auto MetaPrinter::PrintTextBlock(llvm::StringRef indent,
+                                 llvm::StringRef text) const -> void {
   // Strip leading and trailing newlines to make it easy to use multiline raw
   // string literals that will naturally have those.
   text = text.trim('\n');
@@ -448,7 +454,7 @@ void MetaPrinter::PrintTextBlock(llvm::StringRef indent,
   }
 }
 
-void MetaPrinter::PrintArgValueUsage(const Arg& arg) const {
+auto MetaPrinter::PrintArgValueUsage(const Arg& arg) const -> void {
   if (!arg.info.value_name.empty()) {
     *out_ << arg.info.value_name;
     return;
@@ -465,7 +471,7 @@ void MetaPrinter::PrintArgValueUsage(const Arg& arg) const {
   *out_ << "...";
 }
 
-void MetaPrinter::PrintOptionUsage(const Arg& option) const {
+auto MetaPrinter::PrintOptionUsage(const Arg& option) const -> void {
   if (option.kind == Arg::Kind::Flag) {
     *out_ << "--" << (option.default_flag ? "no-" : "") << option.info.name;
     return;
@@ -480,12 +486,12 @@ void MetaPrinter::PrintOptionUsage(const Arg& option) const {
   }
 }
 
-void MetaPrinter::PrintOptionShortName(const Arg& arg) const {
+auto MetaPrinter::PrintOptionShortName(const Arg& arg) const -> void {
   CARBON_CHECK(!arg.info.short_name.empty(), "No short name to use.");
   *out_ << "-" << arg.info.short_name;
 }
 
-void MetaPrinter::PrintArgShortValues(const Arg& arg) const {
+auto MetaPrinter::PrintArgShortValues(const Arg& arg) const -> void {
   CARBON_CHECK(
       arg.kind == Arg::Kind::OneOf,
       "Only one-of arguments have interesting value snippets to print.");
@@ -494,8 +500,8 @@ void MetaPrinter::PrintArgShortValues(const Arg& arg) const {
     *out_ << sep << value_string;
   }
 }
-void MetaPrinter::PrintArgLongValues(const Arg& arg,
-                                     llvm::StringRef indent) const {
+auto MetaPrinter::PrintArgLongValues(const Arg& arg,
+                                     llvm::StringRef indent) const -> void {
   *out_ << indent << "Possible values:\n";
   // TODO: It would be good to add help text for each value and then print it
   // here.
@@ -509,7 +515,8 @@ void MetaPrinter::PrintArgLongValues(const Arg& arg,
   }
 }
 
-void MetaPrinter::PrintArgHelp(const Arg& arg, llvm::StringRef indent) const {
+auto MetaPrinter::PrintArgHelp(const Arg& arg, llvm::StringRef indent) const
+    -> void {
   // Print out the main help text.
   PrintTextBlock(indent, arg.info.help);
 
@@ -540,8 +547,9 @@ void MetaPrinter::PrintArgHelp(const Arg& arg, llvm::StringRef indent) const {
   }
 }
 
-void MetaPrinter::PrintRawUsageCommandAndOptions(const Command& command,
-                                                 int max_option_width) const {
+auto MetaPrinter::PrintRawUsageCommandAndOptions(const Command& command,
+                                                 int max_option_width) const
+    -> void {
   // Recursively print parent usage first with a compressed width.
   if (command.parent) {
     PrintRawUsageCommandAndOptions(*command.parent, MaxParentOptionUsageWidth);
@@ -597,8 +605,8 @@ void MetaPrinter::PrintRawUsageCommandAndOptions(const Command& command,
   }
 }
 
-void MetaPrinter::PrintRawUsage(const Command& command,
-                                llvm::StringRef indent) const {
+auto MetaPrinter::PrintRawUsage(const Command& command,
+                                llvm::StringRef indent) const -> void {
   if (!command.info.usage.empty()) {
     PrintTextBlock(indent, command.info.usage);
     return;
@@ -644,7 +652,7 @@ void MetaPrinter::PrintRawUsage(const Command& command,
   }
 }
 
-void MetaPrinter::PrintUsage(const Command& command) const {
+auto MetaPrinter::PrintUsage(const Command& command) const -> void {
   if (!command.parent) {
     *out_ << "Usage:\n";
   } else {
@@ -653,7 +661,7 @@ void MetaPrinter::PrintUsage(const Command& command) const {
   PrintRawUsage(command, "  ");
 }
 
-void MetaPrinter::PrintHelpSubcommands(const Command& command) const {
+auto MetaPrinter::PrintHelpSubcommands(const Command& command) const -> void {
   bool first_subcommand = true;
   for (const auto& subcommand : command.subcommands) {
     if (subcommand->is_help_hidden) {
@@ -673,7 +681,8 @@ void MetaPrinter::PrintHelpSubcommands(const Command& command) const {
   }
 }
 
-void MetaPrinter::PrintHelpPositionalArgs(const Command& command) const {
+auto MetaPrinter::PrintHelpPositionalArgs(const Command& command) const
+    -> void {
   bool first_positional_arg = true;
   for (const auto& positional_arg : command.positional_args) {
     if (positional_arg->is_help_hidden) {
@@ -694,7 +703,7 @@ void MetaPrinter::PrintHelpPositionalArgs(const Command& command) const {
   }
 }
 
-void MetaPrinter::PrintHelpOptions(const Command& command) const {
+auto MetaPrinter::PrintHelpOptions(const Command& command) const -> void {
   bool first_option = true;
   for (const auto& option : command.options) {
     if (option->is_help_hidden) {
@@ -729,7 +738,7 @@ class Parser {
  public:
   // `out` must not be null.
   explicit Parser(llvm::raw_ostream* out, CommandInfo command_info,
-                  llvm::function_ref<void(CommandBuilder&)> build);
+                  llvm::function_ref<auto(CommandBuilder&)->void> build);
 
   auto Parse(llvm::ArrayRef<llvm::StringRef> unparsed_args)
       -> ErrorOr<ParseResult>;
@@ -750,9 +759,9 @@ class Parser {
   // character's numeric value makes for a convenient map.
   using ShortOptionTableT = std::array<OptionMapT::mapped_type*, 128>;
 
-  void PopulateMaps(const Command& command);
+  auto PopulateMaps(const Command& command) -> void;
 
-  void SetOptionDefault(const Arg& option);
+  auto SetOptionDefault(const Arg& option) -> void;
 
   auto ParseNegatedFlag(const Arg& flag, std::optional<llvm::StringRef> value)
       -> ErrorOr<Success>;
@@ -801,7 +810,7 @@ class Parser {
   ActionT arg_meta_action_;
 };
 
-void Parser::PopulateMaps(const Command& command) {
+auto Parser::PopulateMaps(const Command& command) -> void {
   option_map_.clear();
   for (const auto& option : command.options) {
     option_map_.insert({option->info.name, {option.get(), false}});
@@ -825,7 +834,7 @@ void Parser::PopulateMaps(const Command& command) {
   }
 }
 
-void Parser::SetOptionDefault(const Arg& option) {
+auto Parser::SetOptionDefault(const Arg& option) -> void {
   CARBON_CHECK(option.has_default, "No default value available!");
   switch (option.kind) {
     case Arg::Kind::Flag:
@@ -1235,7 +1244,7 @@ auto Parser::ParsePositionalSuffix(
 }
 
 Parser::Parser(llvm::raw_ostream* out, CommandInfo command_info,
-               llvm::function_ref<void(CommandBuilder&)> build)
+               llvm::function_ref<auto(CommandBuilder&)->void> build)
     : meta_printer_(out), root_command_(command_info) {
   // Run the command building lambda on a builder for the root command.
   CommandBuilder builder(&root_command_, &meta_printer_);
@@ -1305,48 +1314,50 @@ auto Parser::Parse(llvm::ArrayRef<llvm::StringRef> unparsed_args)
   return FinalizeParse();
 }
 
-void ArgBuilder::Required(bool is_required) { arg_->is_required = is_required; }
+auto ArgBuilder::Required(bool is_required) -> void {
+  arg_->is_required = is_required;
+}
 
-void ArgBuilder::HelpHidden(bool is_help_hidden) {
+auto ArgBuilder::HelpHidden(bool is_help_hidden) -> void {
   arg_->is_help_hidden = is_help_hidden;
 }
 
 ArgBuilder::ArgBuilder(Arg* arg) : arg_(arg) {}
 
-void FlagBuilder::Default(bool flag_value) {
+auto FlagBuilder::Default(bool flag_value) -> void {
   arg()->has_default = true;
   arg()->default_flag = flag_value;
 }
 
-void FlagBuilder::Set(bool* flag) { arg()->flag_storage = flag; }
+auto FlagBuilder::Set(bool* flag) -> void { arg()->flag_storage = flag; }
 
-void IntegerArgBuilder::Default(int integer_value) {
+auto IntegerArgBuilder::Default(int integer_value) -> void {
   arg()->has_default = true;
   arg()->default_integer = integer_value;
 }
 
-void IntegerArgBuilder::Set(int* integer) {
+auto IntegerArgBuilder::Set(int* integer) -> void {
   arg()->is_append = false;
   arg()->integer_storage = integer;
 }
 
-void IntegerArgBuilder::Append(llvm::SmallVectorImpl<int>* sequence) {
+auto IntegerArgBuilder::Append(llvm::SmallVectorImpl<int>* sequence) -> void {
   arg()->is_append = true;
   arg()->integer_sequence = sequence;
 }
 
-void StringArgBuilder::Default(llvm::StringRef string_value) {
+auto StringArgBuilder::Default(llvm::StringRef string_value) -> void {
   arg()->has_default = true;
   arg()->default_string = string_value;
 }
 
-void StringArgBuilder::Set(llvm::StringRef* string) {
+auto StringArgBuilder::Set(llvm::StringRef* string) -> void {
   arg()->is_append = false;
   arg()->string_storage = string;
 }
 
-void StringArgBuilder::Append(
-    llvm::SmallVectorImpl<llvm::StringRef>* sequence) {
+auto StringArgBuilder::Append(llvm::SmallVectorImpl<llvm::StringRef>* sequence)
+    -> void {
   arg()->is_append = true;
   arg()->string_sequence = sequence;
 }
@@ -1371,8 +1382,9 @@ static auto IsValidName(llvm::StringRef name) -> bool {
   return !name.starts_with("no-");
 }
 
-void CommandBuilder::AddFlag(const ArgInfo& info,
-                             llvm::function_ref<void(FlagBuilder&)> build) {
+auto CommandBuilder::AddFlag(const ArgInfo& info,
+                             llvm::function_ref<auto(FlagBuilder&)->void> build)
+    -> void {
   FlagBuilder builder(AddArgImpl(info, Arg::Kind::Flag));
   // All boolean flags have an implicit default of `false`, although it can be
   // overridden in the build callback.
@@ -1380,56 +1392,64 @@ void CommandBuilder::AddFlag(const ArgInfo& info,
   build(builder);
 }
 
-void CommandBuilder::AddIntegerOption(
-    const ArgInfo& info, llvm::function_ref<void(IntegerArgBuilder&)> build) {
+auto CommandBuilder::AddIntegerOption(
+    const ArgInfo& info,
+    llvm::function_ref<auto(IntegerArgBuilder&)->void> build) -> void {
   IntegerArgBuilder builder(AddArgImpl(info, Arg::Kind::Integer));
   build(builder);
 }
 
-void CommandBuilder::AddStringOption(
-    const ArgInfo& info, llvm::function_ref<void(StringArgBuilder&)> build) {
+auto CommandBuilder::AddStringOption(
+    const ArgInfo& info,
+    llvm::function_ref<auto(StringArgBuilder&)->void> build) -> void {
   StringArgBuilder builder(AddArgImpl(info, Arg::Kind::String));
   build(builder);
 }
 
-void CommandBuilder::AddOneOfOption(
-    const ArgInfo& info, llvm::function_ref<void(OneOfArgBuilder&)> build) {
+auto CommandBuilder::AddOneOfOption(
+    const ArgInfo& info, llvm::function_ref<auto(OneOfArgBuilder&)->void> build)
+    -> void {
   OneOfArgBuilder builder(AddArgImpl(info, Arg::Kind::OneOf));
   build(builder);
 }
 
-void CommandBuilder::AddMetaActionOption(
-    const ArgInfo& info, llvm::function_ref<void(ArgBuilder&)> build) {
+auto CommandBuilder::AddMetaActionOption(
+    const ArgInfo& info, llvm::function_ref<auto(ArgBuilder&)->void> build)
+    -> void {
   ArgBuilder builder(AddArgImpl(info, Arg::Kind::MetaActionOnly));
   build(builder);
 }
 
-void CommandBuilder::AddIntegerPositionalArg(
-    const ArgInfo& info, llvm::function_ref<void(IntegerArgBuilder&)> build) {
+auto CommandBuilder::AddIntegerPositionalArg(
+    const ArgInfo& info,
+    llvm::function_ref<auto(IntegerArgBuilder&)->void> build) -> void {
   AddPositionalArgImpl(info, Arg::Kind::Integer, [build](Arg& arg) {
     IntegerArgBuilder builder(&arg);
     build(builder);
   });
 }
 
-void CommandBuilder::AddStringPositionalArg(
-    const ArgInfo& info, llvm::function_ref<void(StringArgBuilder&)> build) {
+auto CommandBuilder::AddStringPositionalArg(
+    const ArgInfo& info,
+    llvm::function_ref<auto(StringArgBuilder&)->void> build) -> void {
   AddPositionalArgImpl(info, Arg::Kind::String, [build](Arg& arg) {
     StringArgBuilder builder(&arg);
     build(builder);
   });
 }
 
-void CommandBuilder::AddOneOfPositionalArg(
-    const ArgInfo& info, llvm::function_ref<void(OneOfArgBuilder&)> build) {
+auto CommandBuilder::AddOneOfPositionalArg(
+    const ArgInfo& info, llvm::function_ref<auto(OneOfArgBuilder&)->void> build)
+    -> void {
   AddPositionalArgImpl(info, Arg::Kind::OneOf, [build](Arg& arg) {
     OneOfArgBuilder builder(&arg);
     build(builder);
   });
 }
 
-void CommandBuilder::AddSubcommand(
-    const CommandInfo& info, llvm::function_ref<void(CommandBuilder&)> build) {
+auto CommandBuilder::AddSubcommand(
+    const CommandInfo& info,
+    llvm::function_ref<auto(CommandBuilder&)->void> build) -> void {
   CARBON_CHECK(IsValidName(info.name), "Invalid subcommand name: {0}",
                info.name);
   CARBON_CHECK(subcommand_names_.insert(info.name).second,
@@ -1444,11 +1464,11 @@ void CommandBuilder::AddSubcommand(
   builder.Finalize();
 }
 
-void CommandBuilder::HelpHidden(bool is_help_hidden) {
+auto CommandBuilder::HelpHidden(bool is_help_hidden) -> void {
   command_->is_help_hidden = is_help_hidden;
 }
 
-void CommandBuilder::RequiresSubcommand() {
+auto CommandBuilder::RequiresSubcommand() -> void {
   CARBON_CHECK(!command_->subcommands.empty(),
                "Cannot require subcommands unless there are subcommands.");
   CARBON_CHECK(command_->positional_args.empty(),
@@ -1459,7 +1479,7 @@ void CommandBuilder::RequiresSubcommand() {
   command_->kind = Kind::RequiresSubcommand;
 }
 
-void CommandBuilder::Do(ActionT action) {
+auto CommandBuilder::Do(ActionT action) -> void {
   CARBON_CHECK(command_->kind == Kind::Invalid,
                "Already established the kind of this command as: {0}",
                command_->kind);
@@ -1467,7 +1487,7 @@ void CommandBuilder::Do(ActionT action) {
   command_->action = std::move(action);
 }
 
-void CommandBuilder::Meta(ActionT action) {
+auto CommandBuilder::Meta(ActionT action) -> void {
   CARBON_CHECK(command_->kind == Kind::Invalid,
                "Already established the kind of this command as: {0}",
                command_->kind);
@@ -1489,8 +1509,9 @@ auto CommandBuilder::AddArgImpl(const ArgInfo& info, Arg::Kind kind) -> Arg* {
   return arg;
 }
 
-void CommandBuilder::AddPositionalArgImpl(
-    const ArgInfo& info, Arg::Kind kind, llvm::function_ref<void(Arg&)> build) {
+auto CommandBuilder::AddPositionalArgImpl(
+    const ArgInfo& info, Arg::Kind kind,
+    llvm::function_ref<auto(Arg&)->void> build) -> void {
   CARBON_CHECK(IsValidName(info.name), "Invalid argument name: {0}", info.name);
   CARBON_CHECK(
       command_->subcommands.empty(),
@@ -1511,13 +1532,13 @@ void CommandBuilder::AddPositionalArgImpl(
   }
 }
 
-void CommandBuilder::Finalize() {
+auto CommandBuilder::Finalize() -> void {
   meta_printer_->RegisterWithCommand(*command_, *this);
 }
 
 auto Parse(llvm::ArrayRef<llvm::StringRef> unparsed_args,
            llvm::raw_ostream& out, CommandInfo command_info,
-           llvm::function_ref<void(CommandBuilder&)> build)
+           llvm::function_ref<auto(CommandBuilder&)->void> build)
     -> ErrorOr<ParseResult> {
   // Build a parser, which includes building the command description provided by
   // the user.
diff --git a/common/command_line.h b/common/command_line.h
index 62303b339b10..0218165228c1 100644
--- a/common/command_line.h
+++ b/common/command_line.h
@@ -237,7 +237,7 @@ auto operator<<(llvm::raw_ostream& output, ParseResult result)
 
 // Actions are stored in data structures so we use an owning closure to model
 // them.
-using ActionT = std::function<void()>;
+using ActionT = std::function<auto()->void>;
 
 // The core argument info used to render help and other descriptive
 // information. This is used for both options and positional arguments.
@@ -294,17 +294,17 @@ class ArgBuilder {
  public:
   // When marked as required, if an argument is not provided explicitly in the
   // command line the parse will produce an error.
-  void Required(bool is_required);
+  auto Required(bool is_required) -> void;
 
   // An argument can be hidden from the help output.
-  void HelpHidden(bool is_help_hidden);
+  auto HelpHidden(bool is_help_hidden) -> void;
 
   // Sets a meta-action to run when this argument is parsed. This is used to
   // set up arguments like `--help` or `--version` that can be entirely
   // handled during parsing and rather than produce parsed information about
   // the command line, override that for some custom behavior.
   template <typename T>
-  void MetaAction(T action);
+  auto MetaAction(T action) -> void;
 
  protected:
   friend class CommandBuilder;
@@ -323,12 +323,12 @@ class FlagBuilder : public ArgBuilder {
   // Flags can be defaulted to true. However, flags always have *some*
   // default, this merely customizes which value is default. If uncustomized,
   // the default of a flag is false.
-  void Default(bool flag_value);
+  auto Default(bool flag_value) -> void;
 
   // Configures the argument to store a parsed value in the provided storage.
   //
   // This must be called on the builder.
-  void Set(bool* flag);
+  auto Set(bool* flag) -> void;
 
  private:
   using ArgBuilder::ArgBuilder;
@@ -345,7 +345,7 @@ class IntegerArgBuilder : public ArgBuilder {
   // below, this value will be used whenever the argument occurs without an
   // explicit value, but unless the argument is parsed nothing will be
   // appended.
-  void Default(int integer_value);
+  auto Default(int integer_value) -> void;
 
   // Configures the argument to store a parsed value in the provided storage.
   // Each time the argument is parsed, it will write a new value to this
@@ -353,7 +353,7 @@ class IntegerArgBuilder : public ArgBuilder {
   //
   // Exactly one of this method or `Append` below must be configured for the
   // argument.
-  void Set(int* integer);
+  auto Set(int* integer) -> void;
 
   // Configures the argument to append a parsed value to the provided
   // container. Each time the argument is parsed, a new value will be
@@ -361,7 +361,7 @@ class IntegerArgBuilder : public ArgBuilder {
   //
   // Exactly one of this method or `Set` above must be configured for the
   // argument.
-  void Append(llvm::SmallVectorImpl<int>* sequence);
+  auto Append(llvm::SmallVectorImpl<int>* sequence) -> void;
 
  private:
   using ArgBuilder::ArgBuilder;
@@ -378,7 +378,7 @@ class StringArgBuilder : public ArgBuilder {
   // below, this value will be used whenever the argument occurs without an
   // explicit value, but unless the argument is parsed nothing will be
   // appended.
-  void Default(llvm::StringRef string_value);
+  auto Default(llvm::StringRef string_value) -> void;
 
   // Configures the argument to store a parsed value in the provided storage.
   // Each time the argument is parsed, it will write a new value to this
@@ -386,7 +386,7 @@ class StringArgBuilder : public ArgBuilder {
   //
   // Exactly one of this method or `Append` below must be configured for the
   // argument.
-  void Set(llvm::StringRef* string);
+  auto Set(llvm::StringRef* string) -> void;
 
   // Configures the argument to append a parsed value to the provided
   // container. Each time the argument is parsed, a new value will be
@@ -394,7 +394,7 @@ class StringArgBuilder : public ArgBuilder {
   //
   // Exactly one of this method or `Set` above must be configured for the
   // argument.
-  void Append(llvm::SmallVectorImpl<llvm::StringRef>* sequence);
+  auto Append(llvm::SmallVectorImpl<llvm::StringRef>* sequence) -> void;
 
  private:
   using ArgBuilder::ArgBuilder;
@@ -463,7 +463,7 @@ class OneOfArgBuilder : public ArgBuilder {
   // optional, and also will cause that value to be stored into the result
   // even if the argument is not parsed explicitly.
   template <typename T, typename U, size_t N>
-  void SetOneOf(const OneOfValueT<U> (&values)[N], T* result);
+  auto SetOneOf(const OneOfValueT<U> (&values)[N], T* result) -> void;
 
   // Configures the argument to append a parsed value to the provided
   // container. Each time the argument is parsed, a new value will be
@@ -491,15 +491,15 @@ class OneOfArgBuilder : public ArgBuilder {
   // However, appending one-of arguments cannot use a default. The values must
   // always be explicitly parsed.
   template <typename T, typename U, size_t N>
-  void AppendOneOf(const OneOfValueT<U> (&values)[N],
-                   llvm::SmallVectorImpl<T>* sequence);
+  auto AppendOneOf(const OneOfValueT<U> (&values)[N],
+                   llvm::SmallVectorImpl<T>* sequence) -> void;
 
  private:
   using ArgBuilder::ArgBuilder;
 
   template <typename U, size_t N, typename MatchT, size_t... Indices>
-  void OneOfImpl(const OneOfValueT<U> (&input_values)[N], MatchT match,
-                 std::index_sequence<Indices...> /*indices*/);
+  auto OneOfImpl(const OneOfValueT<U> (&input_values)[N], MatchT match,
+                 std::index_sequence<Indices...> /*indices*/) -> void;
 };
 
 // The extended info for a command, including for a subcommand.
@@ -593,37 +593,45 @@ class CommandBuilder {
  public:
   using Kind = CommandKind;
 
-  void AddFlag(const ArgInfo& info,
-               llvm::function_ref<void(FlagBuilder&)> build);
-  void AddIntegerOption(const ArgInfo& info,
-                        llvm::function_ref<void(IntegerArgBuilder&)> build);
-  void AddStringOption(const ArgInfo& info,
-                       llvm::function_ref<void(StringArgBuilder&)> build);
-  void AddOneOfOption(const ArgInfo& info,
-                      llvm::function_ref<void(OneOfArgBuilder&)> build);
-  void AddMetaActionOption(const ArgInfo& info,
-                           llvm::function_ref<void(ArgBuilder&)> build);
+  auto AddFlag(const ArgInfo& info,
+               llvm::function_ref<auto(FlagBuilder&)->void> build) -> void;
+  auto AddIntegerOption(
+      const ArgInfo& info,
+      llvm::function_ref<auto(IntegerArgBuilder&)->void> build) -> void;
+  auto AddStringOption(const ArgInfo& info,
+                       llvm::function_ref<auto(StringArgBuilder&)->void> build)
+      -> void;
+  auto AddOneOfOption(const ArgInfo& info,
+                      llvm::function_ref<auto(OneOfArgBuilder&)->void> build)
+      -> void;
+  auto AddMetaActionOption(const ArgInfo& info,
+                           llvm::function_ref<auto(ArgBuilder&)->void> build)
+      -> void;
 
-  void AddIntegerPositionalArg(
-      const ArgInfo& info, llvm::function_ref<void(IntegerArgBuilder&)> build);
-  void AddStringPositionalArg(
-      const ArgInfo& info, llvm::function_ref<void(StringArgBuilder&)> build);
-  void AddOneOfPositionalArg(const ArgInfo& info,
-                             llvm::function_ref<void(OneOfArgBuilder&)> build);
+  auto AddIntegerPositionalArg(
+      const ArgInfo& info,
+      llvm::function_ref<auto(IntegerArgBuilder&)->void> build) -> void;
+  auto AddStringPositionalArg(
+      const ArgInfo& info,
+      llvm::function_ref<auto(StringArgBuilder&)->void> build) -> void;
+  auto AddOneOfPositionalArg(
+      const ArgInfo& info,
+      llvm::function_ref<auto(OneOfArgBuilder&)->void> build) -> void;
 
-  void AddSubcommand(const CommandInfo& info,
-                     llvm::function_ref<void(CommandBuilder&)> build);
+  auto AddSubcommand(const CommandInfo& info,
+                     llvm::function_ref<auto(CommandBuilder&)->void> build)
+      -> void;
 
   // Subcommands can be hidden from the help listing of their parents with
   // this setting. Hiding a subcommand doesn't disable its own help, it just
   // removes it from the listing.
-  void HelpHidden(bool is_help_hidden);
+  auto HelpHidden(bool is_help_hidden) -> void;
 
   // Exactly one of these three should be called to select and configure the
   // kind of the built command.
-  void RequiresSubcommand();
-  void Do(ActionT action);
-  void Meta(ActionT meta_action);
+  auto RequiresSubcommand() -> void;
+  auto Do(ActionT action) -> void;
+  auto Meta(ActionT meta_action) -> void;
 
  private:
   friend Parser;
@@ -632,9 +640,9 @@ class CommandBuilder {
   explicit CommandBuilder(Command* command, MetaPrinter* meta_printer);
 
   auto AddArgImpl(const ArgInfo& info, ArgKind kind) -> Arg*;
-  void AddPositionalArgImpl(const ArgInfo& info, ArgKind kind,
-                            llvm::function_ref<void(Arg&)> build);
-  void Finalize();
+  auto AddPositionalArgImpl(const ArgInfo& info, ArgKind kind,
+                            llvm::function_ref<auto(Arg&)->void> build) -> void;
+  auto Finalize() -> void;
 
   Command* command_;
   MetaPrinter* meta_printer_;
@@ -658,7 +666,7 @@ class CommandBuilder {
 // to `out`.
 auto Parse(llvm::ArrayRef<llvm::StringRef> unparsed_args,
            llvm::raw_ostream& out, CommandInfo command_info,
-           llvm::function_ref<void(CommandBuilder&)> build)
+           llvm::function_ref<auto(CommandBuilder&)->void> build)
     -> ErrorOr<ParseResult>;
 
 // Implementation details only below.
@@ -667,8 +675,8 @@ auto Parse(llvm::ArrayRef<llvm::StringRef> unparsed_args,
 struct Arg {
   using Kind = ArgKind;
   using ValueActionT =
-      std::function<bool(const Arg& arg, llvm::StringRef value_string)>;
-  using DefaultActionT = std::function<void(const Arg& arg)>;
+      std::function<auto(const Arg& arg, llvm::StringRef value_string)->bool>;
+  using DefaultActionT = std::function<auto(const Arg& arg)->void>;
 
   explicit Arg(ArgInfo info);
   ~Arg();
@@ -733,7 +741,7 @@ struct Command {
 };
 
 template <typename T>
-void ArgBuilder::MetaAction(T action) {
+auto ArgBuilder::MetaAction(T action) -> void {
   CARBON_CHECK(!arg_->meta_action, "Cannot set a meta action twice!");
   arg_->meta_action = std::move(action);
 }
@@ -757,7 +765,8 @@ auto OneOfArgBuilder::OneOfValue(llvm::StringRef str, T value)
 }
 
 template <typename T, typename U, size_t N>
-void OneOfArgBuilder::SetOneOf(const OneOfValueT<U> (&values)[N], T* result) {
+auto OneOfArgBuilder::SetOneOf(const OneOfValueT<U> (&values)[N], T* result)
+    -> void {
   static_assert(N > 0, "Must include at least one value.");
   arg()->is_append = false;
   OneOfImpl(
@@ -766,8 +775,8 @@ void OneOfArgBuilder::SetOneOf(const OneOfValueT<U> (&values)[N], T* result) {
 }
 
 template <typename T, typename U, size_t N>
-void OneOfArgBuilder::AppendOneOf(const OneOfValueT<U> (&values)[N],
-                                  llvm::SmallVectorImpl<T>* sequence) {
+auto OneOfArgBuilder::AppendOneOf(const OneOfValueT<U> (&values)[N],
+                                  llvm::SmallVectorImpl<T>* sequence) -> void {
   static_assert(N > 0, "Must include at least one value.");
   arg()->is_append = true;
   OneOfImpl(
@@ -787,9 +796,10 @@ void OneOfArgBuilder::AppendOneOf(const OneOfValueT<U> (&values)[N],
 // lambdas that do the type-aware operations and storing those into type-erased
 // function objects.
 template <typename U, size_t N, typename MatchT, size_t... Indices>
-void OneOfArgBuilder::OneOfImpl(const OneOfValueT<U> (&input_values)[N],
+auto OneOfArgBuilder::OneOfImpl(const OneOfValueT<U> (&input_values)[N],
                                 MatchT match,
-                                std::index_sequence<Indices...> /*indices*/) {
+                                std::index_sequence<Indices...> /*indices*/)
+    -> void {
   std::array<llvm::StringRef, N> value_strings = {input_values[Indices].str...};
   std::array<U, N> values = {input_values[Indices].value...};
 
diff --git a/common/command_line_test.cpp b/common/command_line_test.cpp
index 909f5db89010..b2c7b2e57a21 100644
--- a/common/command_line_test.cpp
+++ b/common/command_line_test.cpp
@@ -437,9 +437,9 @@ TEST(ArgParserTest, PositionalAppending) {
   EXPECT_THAT(strings3, ElementsAre(StrEq("e"), StrEq("f")));
 }
 
-static auto ParseOneOfOption(llvm::ArrayRef<llvm::StringRef> args,
-                             llvm::raw_ostream& s,
-                             llvm::function_ref<void(OneOfArgBuilder&)> build)
+static auto ParseOneOfOption(
+    llvm::ArrayRef<llvm::StringRef> args, llvm::raw_ostream& s,
+    llvm::function_ref<auto(OneOfArgBuilder&)->void> build)
     -> ErrorOr<ParseResult> {
   return Parse(args, s, TestCommandInfo, [&](auto& b) {
     b.AddOneOfOption({.name = "option"}, build);
diff --git a/common/error.h b/common/error.h
index ec18eb9d1a2e..72486e5821e1 100644
--- a/common/error.h
+++ b/common/error.h
@@ -21,7 +21,7 @@ namespace Carbon {
 // using `ErrorOr<Success>` and `return Success();` if no value needs to be
 // returned.
 struct Success : public Printable<Success> {
-  void Print(llvm::raw_ostream& out) const { out << "Success"; }
+  auto Print(llvm::raw_ostream& out) const -> void { out << "Success"; }
 };
 
 // Tracks an error message.
@@ -50,7 +50,7 @@ class [[nodiscard]] Error : public Printable<Error> {
   }
 
   // Prints the error string.
-  void Print(llvm::raw_ostream& out) const {
+  auto Print(llvm::raw_ostream& out) const -> void {
     if (!location().empty()) {
       out << location() << ": ";
     }
diff --git a/common/hashing_benchmark.cpp b/common/hashing_benchmark.cpp
index b4392fe01f71..9a1b2f1a61ec 100644
--- a/common/hashing_benchmark.cpp
+++ b/common/hashing_benchmark.cpp
@@ -195,7 +195,7 @@ struct LLVMHashBench : HashBenchBase {
 };
 
 template <typename Values, typename Hasher>
-void BM_LatencyHash(benchmark::State& state) {
+auto BM_LatencyHash(benchmark::State& state) -> void {
   uint64_t x = 13;
   Values v;
   Hasher h;
diff --git a/common/map.h b/common/map.h
index 33c261073cec..fae8a2f3f675 100644
--- a/common/map.h
+++ b/common/map.h
@@ -114,7 +114,7 @@ class MapView
 
   // Run the provided callback for every key and value in the map.
   template <typename CallbackT>
-  void ForEach(CallbackT callback)
+  auto ForEach(CallbackT callback) -> void
     requires(std::invocable<CallbackT, KeyT&, ValueT&>);
 
   // This routine is relatively inefficient and only intended for use in
@@ -224,7 +224,7 @@ class MapBase : protected RawHashtable::BaseImpl<InputKeyT, InputValueT,
 
   // Convenience forwarder to the view type.
   template <typename CallbackT>
-  void ForEach(CallbackT callback) const
+  auto ForEach(CallbackT callback) const -> void
     requires(std::invocable<CallbackT, KeyT&, ValueT&>)
   {
     return ViewT(*this).ForEach(callback);
@@ -345,7 +345,7 @@ class MapBase : protected RawHashtable::BaseImpl<InputKeyT, InputValueT,
 
   // Clear all key/value pairs from the map but leave the underlying hashtable
   // allocated and in place.
-  void Clear();
+  auto Clear() -> void;
 
  protected:
   using ImplT::ImplT;
@@ -390,7 +390,7 @@ class Map : public RawHashtable::TableImpl<
 
   // Reset the entire state of the hashtable to as it was when constructed,
   // throwing away any intervening allocations.
-  void Reset();
+  auto Reset() -> void;
 };
 
 template <typename InputKeyT, typename InputValueT, typename InputKeyContextT>
@@ -418,8 +418,8 @@ auto MapView<InputKeyT, InputValueT, InputKeyContextT>::operator[](
 
 template <typename InputKeyT, typename InputValueT, typename InputKeyContextT>
 template <typename CallbackT>
-void MapView<InputKeyT, InputValueT, InputKeyContextT>::ForEach(
-    CallbackT callback)
+auto MapView<InputKeyT, InputValueT, InputKeyContextT>::ForEach(
+    CallbackT callback) -> void
   requires(std::invocable<CallbackT, KeyT&, ValueT&>)
 {
   this->ForEachEntry(
@@ -551,14 +551,14 @@ MapBase<InputKeyT, InputValueT, InputKeyContextT>::Update(
 }
 
 template <typename InputKeyT, typename InputValueT, typename InputKeyContextT>
-void MapBase<InputKeyT, InputValueT, InputKeyContextT>::GrowToAllocSize(
-    ssize_t target_alloc_size, KeyContextT key_context) {
+auto MapBase<InputKeyT, InputValueT, InputKeyContextT>::GrowToAllocSize(
+    ssize_t target_alloc_size, KeyContextT key_context) -> void {
   this->GrowToAllocSizeImpl(target_alloc_size, key_context);
 }
 
 template <typename InputKeyT, typename InputValueT, typename InputKeyContextT>
-void MapBase<InputKeyT, InputValueT, InputKeyContextT>::GrowForInsertCount(
-    ssize_t count, KeyContextT key_context) {
+auto MapBase<InputKeyT, InputValueT, InputKeyContextT>::GrowForInsertCount(
+    ssize_t count, KeyContextT key_context) -> void {
   this->GrowForInsertCountImpl(count, key_context);
 }
 
@@ -570,13 +570,13 @@ auto MapBase<InputKeyT, InputValueT, InputKeyContextT>::Erase(
 }
 
 template <typename InputKeyT, typename InputValueT, typename InputKeyContextT>
-void MapBase<InputKeyT, InputValueT, InputKeyContextT>::Clear() {
+auto MapBase<InputKeyT, InputValueT, InputKeyContextT>::Clear() -> void {
   this->ClearImpl();
 }
 
 template <typename InputKeyT, typename InputValueT, ssize_t SmallSize,
           typename InputKeyContextT>
-void Map<InputKeyT, InputValueT, SmallSize, InputKeyContextT>::Reset() {
+auto Map<InputKeyT, InputValueT, SmallSize, InputKeyContextT>::Reset() -> void {
   this->ResetImpl();
 }
 
diff --git a/common/map_test.cpp b/common/map_test.cpp
index b39bb7cdac1d..b1cc739d3a1b 100644
--- a/common/map_test.cpp
+++ b/common/map_test.cpp
@@ -25,7 +25,7 @@ using ::testing::Pair;
 using ::testing::UnorderedElementsAreArray;
 
 template <typename MapT, typename MatcherRangeT>
-void ExpectMapElementsAre(MapT&& m, MatcherRangeT element_matchers) {
+auto ExpectMapElementsAre(MapT&& m, MatcherRangeT element_matchers) -> void {
   // Now collect the elements into a container.
   using KeyT = typename std::remove_reference<MapT>::type::KeyT;
   using ValueT = typename std::remove_reference<MapT>::type::ValueT;
@@ -41,14 +41,16 @@ void ExpectMapElementsAre(MapT&& m, MatcherRangeT element_matchers) {
 
 // Allow directly using an initializer list.
 template <typename MapT, typename MatcherT>
-void ExpectMapElementsAre(MapT&& m,
-                          std::initializer_list<MatcherT> element_matchers) {
+auto ExpectMapElementsAre(MapT&& m,
+                          std::initializer_list<MatcherT> element_matchers)
+    -> void {
   std::vector<MatcherT> element_matchers_storage = element_matchers;
   ExpectMapElementsAre(m, element_matchers_storage);
 }
 
 template <typename ValueCB, typename RangeT, typename... RangeTs>
-auto MakeKeyValues(ValueCB value_cb, RangeT&& range, RangeTs&&... ranges) {
+auto MakeKeyValues(ValueCB value_cb, RangeT&& range, RangeTs&&... ranges)
+    -> auto {
   using KeyT = typename RangeT::value_type;
   using ValueT = decltype(value_cb(std::declval<KeyT>()));
   std::vector<std::pair<KeyT, ValueT>> elements;
diff --git a/common/raw_hashtable.h b/common/raw_hashtable.h
index 23ff9983fcc8..959c23db8c4b 100644
--- a/common/raw_hashtable.h
+++ b/common/raw_hashtable.h
@@ -691,7 +691,7 @@ class ProbeSequence {
 #endif
   }
 
-  void Next() {
+  auto Next() -> void {
     step_ += GroupSize;
     p_ = (p_ + step_) & mask_;
 #ifndef NDEBUG
diff --git a/common/raw_string_ostream.h b/common/raw_string_ostream.h
index 37b121143c21..210c3ff2391c 100644
--- a/common/raw_string_ostream.h
+++ b/common/raw_string_ostream.h
@@ -51,7 +51,7 @@ class RawStringOstream : public llvm::raw_pwrite_stream {
     str_.append(ptr, size);
   }
 
-  void reserveExtraSpace(uint64_t extra_size) override {
+  auto reserveExtraSpace(uint64_t extra_size) -> void override {
     str_.reserve(str_.size() + extra_size);
   }
 
diff --git a/common/set.h b/common/set.h
index b85027eafde1..f2e325abfb01 100644
--- a/common/set.h
+++ b/common/set.h
@@ -94,7 +94,7 @@ class SetView : RawHashtable::ViewImpl<InputKeyT, void, InputKeyContextT> {
 
   // Run the provided callback for every key in the set.
   template <typename CallbackT>
-  void ForEach(CallbackT callback)
+  auto ForEach(CallbackT callback) -> void
     requires(std::invocable<CallbackT, KeyT&>);
 
   // This routine is relatively inefficient and only intended for use in
@@ -184,7 +184,7 @@ class SetBase
 
   // Convenience forwarder to the view type.
   template <typename CallbackT>
-  void ForEach(CallbackT callback)
+  auto ForEach(CallbackT callback) -> void
     requires(std::invocable<CallbackT, KeyT&>)
   {
     return ViewT(*this).ForEach(callback);
@@ -257,7 +257,7 @@ class SetBase
 
   // Clear all key/value pairs from the set but leave the underlying hashtable
   // allocated and in place.
-  void Clear();
+  auto Clear() -> void;
 
  protected:
   using ImplT::ImplT;
@@ -301,7 +301,7 @@ class Set : public RawHashtable::TableImpl<SetBase<InputKeyT, InputKeyContextT>,
 
   // Reset the entire state of the hashtable to as it was when constructed,
   // throwing away any intervening allocations.
-  void Reset();
+  auto Reset() -> void;
 };
 
 template <typename InputKeyT, typename InputKeyContextT>
@@ -326,7 +326,7 @@ auto SetView<InputKeyT, InputKeyContextT>::Lookup(LookupKeyT lookup_key,
 
 template <typename InputKeyT, typename InputKeyContextT>
 template <typename CallbackT>
-void SetView<InputKeyT, InputKeyContextT>::ForEach(CallbackT callback)
+auto SetView<InputKeyT, InputKeyContextT>::ForEach(CallbackT callback) -> void
   requires(std::invocable<CallbackT, KeyT&>)
 {
   this->ForEachEntry([callback](EntryT& entry) { callback(entry.key()); },
@@ -383,14 +383,14 @@ auto SetBase<InputKeyT, InputKeyContextT>::Insert(LookupKeyT lookup_key,
 }
 
 template <typename InputKeyT, typename InputKeyContextT>
-void SetBase<InputKeyT, InputKeyContextT>::GrowToAllocSize(
-    ssize_t target_alloc_size, KeyContextT key_context) {
+auto SetBase<InputKeyT, InputKeyContextT>::GrowToAllocSize(
+    ssize_t target_alloc_size, KeyContextT key_context) -> void {
   this->GrowToAllocSizeImpl(target_alloc_size, key_context);
 }
 
 template <typename InputKeyT, typename InputKeyContextT>
-void SetBase<InputKeyT, InputKeyContextT>::GrowForInsertCount(
-    ssize_t count, KeyContextT key_context) {
+auto SetBase<InputKeyT, InputKeyContextT>::GrowForInsertCount(
+    ssize_t count, KeyContextT key_context) -> void {
   this->GrowForInsertCountImpl(count, key_context);
 }
 
@@ -403,12 +403,12 @@ auto SetBase<InputKeyT, InputKeyContextT>::Erase(LookupKeyT lookup_key,
 }
 
 template <typename InputKeyT, typename InputKeyContextT>
-void SetBase<InputKeyT, InputKeyContextT>::Clear() {
+auto SetBase<InputKeyT, InputKeyContextT>::Clear() -> void {
   this->ClearImpl();
 }
 
 template <typename InputKeyT, ssize_t SmallSize, typename InputKeyContextT>
-void Set<InputKeyT, SmallSize, InputKeyContextT>::Reset() {
+auto Set<InputKeyT, SmallSize, InputKeyContextT>::Reset() -> void {
   this->ResetImpl();
 }
 
diff --git a/common/set_test.cpp b/common/set_test.cpp
index 1a215898e680..2de94b171cea 100644
--- a/common/set_test.cpp
+++ b/common/set_test.cpp
@@ -21,7 +21,7 @@ using RawHashtable::TestData;
 using ::testing::UnorderedElementsAreArray;
 
 template <typename SetT, typename MatcherRangeT>
-void ExpectSetElementsAre(SetT&& s, MatcherRangeT element_matchers) {
+auto ExpectSetElementsAre(SetT&& s, MatcherRangeT element_matchers) -> void {
   // Collect the elements into a container.
   using KeyT = typename std::remove_reference<SetT>::type::KeyT;
   std::vector<KeyT> entries;
@@ -34,8 +34,9 @@ void ExpectSetElementsAre(SetT&& s, MatcherRangeT element_matchers) {
 
 // Allow directly using an initializer list.
 template <typename SetT, typename MatcherT>
-void ExpectSetElementsAre(SetT&& s,
-                          std::initializer_list<MatcherT> element_matchers) {
+auto ExpectSetElementsAre(SetT&& s,
+                          std::initializer_list<MatcherT> element_matchers)
+    -> void {
   std::vector<MatcherT> element_matchers_storage = element_matchers;
   ExpectSetElementsAre(s, element_matchers_storage);
 }
diff --git a/common/vlog_test.cpp b/common/vlog_test.cpp
index 3761e24628a6..648a17211d5d 100644
--- a/common/vlog_test.cpp
+++ b/common/vlog_test.cpp
@@ -24,8 +24,8 @@ class VLogger {
     }
   }
 
-  void VLog() { CARBON_VLOG("Test\n"); }
-  void VLogFormatArgs() { CARBON_VLOG("Test {0} {1} {2}\n", 1, 2, 3); }
+  auto VLog() -> void { CARBON_VLOG("Test\n"); }
+  auto VLogFormatArgs() -> void { CARBON_VLOG("Test {0} {1} {2}\n", 1, 2, 3); }
 
   auto TakeStr() -> std::string { return buffer_.TakeStr(); }
 
diff --git a/docs/project/cpp_style_guide.md b/docs/project/cpp_style_guide.md
index 8409ca4379a2..13268f86c201 100644
--- a/docs/project/cpp_style_guide.md
+++ b/docs/project/cpp_style_guide.md
@@ -85,7 +85,8 @@ to pick a consistent option. Where possible,
 [`clang-format`](#suggested-clang-format-contents) should be used to enforce
 these.
 
--   Always use trailing return type syntax for functions and methods.
+-   Always use trailing return type syntax for functions and methods, including
+    `-> void`, for consistency with Carbon syntax.
 -   Place the pointer `*` adjacent to the type: `TypeName* variable_name`.
 -   Only declare one variable at a time (declaring multiple variables requires
     confusing repetition of part of the type).
diff --git a/testing/file_test/autoupdate.h b/testing/file_test/autoupdate.h
index ea9d9917c9fd..9da9ed920061 100644
--- a/testing/file_test/autoupdate.h
+++ b/testing/file_test/autoupdate.h
@@ -42,7 +42,7 @@ class FileTestAutoupdater {
       llvm::StringRef actual_stdout, llvm::StringRef actual_stderr,
       const std::optional<RE2>& default_file_re,
       const llvm::SmallVector<LineNumberReplacement>& line_number_replacements,
-      std::function<void(std::string&)> do_extra_check_replacements)
+      std::function<auto(std::string&)->void> do_extra_check_replacements)
       : file_test_path_(file_test_path),
         test_command_(std::move(test_command)),
         dump_command_(std::move(dump_command)),
@@ -203,7 +203,7 @@ class FileTestAutoupdater {
   const llvm::SmallVector<FileTestLine>& non_check_lines_;
   const std::optional<RE2>& default_file_re_;
   const llvm::SmallVector<LineNumberReplacement>& line_number_replacements_;
-  std::function<void(std::string&)> do_extra_check_replacements_;
+  std::function<auto(std::string&)->void> do_extra_check_replacements_;
 
   // Generated TIP lines, from AddTips.
   llvm::SmallVector<TipLine> tips_;
diff --git a/testing/file_test/file_test_base.h b/testing/file_test/file_test_base.h
index e54e1e1ea855..b1edcf09aac4 100644
--- a/testing/file_test/file_test_base.h
+++ b/testing/file_test/file_test_base.h
@@ -220,9 +220,9 @@ struct FileTestFactory {
 
   // A factory function for tests. The output_mutex is optional; see
   // `FileTestBase::output_mutex_`.
-  std::function<FileTestBase*(llvm::StringRef exe_path,
-                              std::mutex* output_mutex,
-                              llvm::StringRef test_name)>
+  std::function<auto(llvm::StringRef exe_path, std::mutex* output_mutex,
+                     llvm::StringRef test_name)
+                    ->FileTestBase*>
       factory_fn;
 };
 
diff --git a/testing/file_test/file_test_base_test.cpp b/testing/file_test/file_test_base_test.cpp
index 262e3644be20..37d2f5d56b03 100644
--- a/testing/file_test/file_test_base_test.cpp
+++ b/testing/file_test/file_test_base_test.cpp
@@ -273,7 +273,7 @@ auto FileTestBaseTest::Run(
 
   // Choose the test function based on filename.
   auto test_fn =
-      llvm::StringSwitch<std::function<ErrorOr<RunResult>(TestParams&)>>(
+      llvm::StringSwitch<std::function<auto(TestParams&)->ErrorOr<RunResult>>>(
           filename.string())
           .Case("alternating_files.carbon", &TestAlternatingFiles)
           .Case("capture_console_output.carbon", &TestCaptureConsoleOutput)
diff --git a/toolchain/base/pretty_stack_trace_function.h b/toolchain/base/pretty_stack_trace_function.h
index 92eff6199521..8317f0a9c628 100644
--- a/toolchain/base/pretty_stack_trace_function.h
+++ b/toolchain/base/pretty_stack_trace_function.h
@@ -13,14 +13,15 @@ namespace Carbon {
 
 class PrettyStackTraceFunction : public llvm::PrettyStackTraceEntry {
  public:
-  explicit PrettyStackTraceFunction(std::function<void(llvm::raw_ostream&)> fn)
+  explicit PrettyStackTraceFunction(
+      std::function<auto(llvm::raw_ostream&)->void> fn)
       : fn_(std::move(fn)) {}
   ~PrettyStackTraceFunction() override = default;
 
   auto print(llvm::raw_ostream& output) const -> void override { fn_(output); }
 
  private:
-  const std::function<void(llvm::raw_ostream&)> fn_;
+  const std::function<auto(llvm::raw_ostream&)->void> fn_;
 };
 
 }  // namespace Carbon
diff --git a/toolchain/base/yaml.h b/toolchain/base/yaml.h
index 0e4997970366..dfe6d0cbfe4c 100644
--- a/toolchain/base/yaml.h
+++ b/toolchain/base/yaml.h
@@ -47,13 +47,13 @@ class OutputScalar {
           val.print(out, /*isSigned=*/false);
         }) {}
 
-  explicit OutputScalar(std::function<void(llvm::raw_ostream&)> output)
+  explicit OutputScalar(std::function<auto(llvm::raw_ostream&)->void> output)
       : output_(std::move(output)) {}
 
   auto Output(llvm::raw_ostream& out) const -> void { output_(out); }
 
  private:
-  std::function<void(llvm::raw_ostream&)> output_;
+  std::function<auto(llvm::raw_ostream&)->void> output_;
 };
 
 // Adapts a function for outputting YAML as a mapping.
@@ -74,13 +74,13 @@ class OutputMapping {
     llvm::yaml::IO& io_;
   };
 
-  explicit OutputMapping(std::function<void(OutputMapping::Map)> output)
+  explicit OutputMapping(std::function<auto(OutputMapping::Map)->void> output)
       : output_(std::move(output)) {}
 
   auto Output(llvm::yaml::IO& io) -> void { output_(Map(io)); }
 
  private:
-  std::function<void(OutputMapping::Map)> output_;
+  std::function<auto(OutputMapping::Map)->void> output_;
 };
 
 }  // namespace Carbon::Yaml
diff --git a/toolchain/check/deduce.cpp b/toolchain/check/deduce.cpp
index 45396697b71d..703f6bb9c95a 100644
--- a/toolchain/check/deduce.cpp
+++ b/toolchain/check/deduce.cpp
@@ -226,7 +226,7 @@ class DeductionContext {
   auto MakeSpecific() -> SemIR::SpecificId;
 
  private:
-  void NoteInitializingParam(SemIR::InstId param_id, auto& builder) {
+  auto NoteInitializingParam(SemIR::InstId param_id, auto& builder) -> void {
     if (auto param = context().insts().TryGetAs<SemIR::SymbolicBindingPattern>(
             param_id)) {
       CARBON_DIAGNOSTIC(InitializingGenericParam, Note,
diff --git a/toolchain/check/import.cpp b/toolchain/check/import.cpp
index f299f1d96a25..314a4c49d6f8 100644
--- a/toolchain/check/import.cpp
+++ b/toolchain/check/import.cpp
@@ -97,11 +97,10 @@ struct NamespaceResult {
 // conflict. diagnose_duplicate_namespace is used when handling a cross-package
 // import, where an existing namespace is in the current package and the new
 // namespace is a different package.
-static auto AddNamespace(Context& context, SemIR::TypeId namespace_type_id,
-                         SemIR::NameId name_id,
-                         SemIR::NameScopeId parent_scope_id,
-                         bool diagnose_duplicate_namespace,
-                         llvm::function_ref<SemIR::InstId()> make_import_id)
+static auto AddNamespace(
+    Context& context, SemIR::TypeId namespace_type_id, SemIR::NameId name_id,
+    SemIR::NameScopeId parent_scope_id, bool diagnose_duplicate_namespace,
+    llvm::function_ref<auto()->SemIR::InstId> make_import_id)
     -> NamespaceResult {
   auto* parent_scope = &context.name_scopes().Get(parent_scope_id);
   auto [inserted, entry_id] = parent_scope->LookupOrAdd(
diff --git a/toolchain/diagnostics/diagnostic.h b/toolchain/diagnostics/diagnostic.h
index 5bce1d963d3d..4a80530a1842 100644
--- a/toolchain/diagnostics/diagnostic.h
+++ b/toolchain/diagnostics/diagnostic.h
@@ -104,7 +104,7 @@ struct DiagnosticMessage {
   llvm::SmallVector<llvm::Any> format_args;
 
   // Returns the formatted string. By default, this uses llvm::formatv.
-  std::function<std::string(const DiagnosticMessage&)> format_fn;
+  std::function<auto(const DiagnosticMessage&)->std::string> format_fn;
 };
 
 // An instance of a single error or warning.  Information about the diagnostic
diff --git a/toolchain/diagnostics/diagnostic_emitter.h b/toolchain/diagnostics/diagnostic_emitter.h
index 38080fa1dc5b..995590379372 100644
--- a/toolchain/diagnostics/diagnostic_emitter.h
+++ b/toolchain/diagnostics/diagnostic_emitter.h
@@ -164,7 +164,7 @@ class DiagnosticEmitter {
   // Note that the first parameter type is DiagnosticLoc rather than
   // LocT, because ConvertLoc must not recurse.
   using ContextFnT =
-      llvm::function_ref<void(DiagnosticLoc, const DiagnosticBase<>&)>;
+      llvm::function_ref<auto(DiagnosticLoc, const DiagnosticBase<>&)->void>;
 
   // Converts a LocT to a DiagnosticLoc and its `last_byte_offset` (see
   // `DiagnosticMessage`). ConvertLoc may invoke context_fn to provide context
diff --git a/toolchain/diagnostics/mocks.cpp b/toolchain/diagnostics/mocks.cpp
index 65af0af99056..31d4c13d0fa6 100644
--- a/toolchain/diagnostics/mocks.cpp
+++ b/toolchain/diagnostics/mocks.cpp
@@ -6,7 +6,7 @@
 
 namespace Carbon {
 
-void PrintTo(const Diagnostic& diagnostic, std::ostream* os) {
+auto PrintTo(const Diagnostic& diagnostic, std::ostream* os) -> void {
   *os << "Diagnostic{";
   PrintTo(diagnostic.level, os);
   for (const auto& message : diagnostic.messages) {
@@ -17,7 +17,7 @@ void PrintTo(const Diagnostic& diagnostic, std::ostream* os) {
   *os << "\"}";
 }
 
-void PrintTo(DiagnosticLevel level, std::ostream* os) {
+auto PrintTo(DiagnosticLevel level, std::ostream* os) -> void {
   switch (level) {
     case DiagnosticLevel::LocationInfo:
       *os << "LocationInfo";
diff --git a/toolchain/diagnostics/mocks.h b/toolchain/diagnostics/mocks.h
index 0f42951583ef..91107a25b5f3 100644
--- a/toolchain/diagnostics/mocks.h
+++ b/toolchain/diagnostics/mocks.h
@@ -66,8 +66,8 @@ inline auto IsSingleDiagnostic(testing::Matcher<DiagnosticKind> kind,
 namespace Carbon {
 
 // Printing helpers for tests.
-void PrintTo(const Diagnostic& diagnostic, std::ostream* os);
-void PrintTo(DiagnosticLevel level, std::ostream* os);
+auto PrintTo(const Diagnostic& diagnostic, std::ostream* os) -> void;
+auto PrintTo(DiagnosticLevel level, std::ostream* os) -> void;
 
 }  // namespace Carbon
 
diff --git a/toolchain/diagnostics/sorting_diagnostic_consumer.h b/toolchain/diagnostics/sorting_diagnostic_consumer.h
index 3406690deced..0c55a0112a2c 100644
--- a/toolchain/diagnostics/sorting_diagnostic_consumer.h
+++ b/toolchain/diagnostics/sorting_diagnostic_consumer.h
@@ -39,7 +39,7 @@ class SortingDiagnosticConsumer : public DiagnosticConsumer {
   }
 
   // Sorts and flushes buffered diagnostics.
-  void Flush() override {
+  auto Flush() -> void override {
     llvm::stable_sort(diagnostics_,
                       [](const Diagnostic& lhs, const Diagnostic& rhs) {
                         return lhs.last_byte_offset < rhs.last_byte_offset;
diff --git a/toolchain/driver/codegen_options.cpp b/toolchain/driver/codegen_options.cpp
index 22c8567713c9..fdae3a6c4537 100644
--- a/toolchain/driver/codegen_options.cpp
+++ b/toolchain/driver/codegen_options.cpp
@@ -6,7 +6,7 @@
 
 namespace Carbon {
 
-void CodegenOptions::Build(CommandLine::CommandBuilder& b) {
+auto CodegenOptions::Build(CommandLine::CommandBuilder& b) -> void {
   b.AddStringOption(
       {
           .name = "target",
diff --git a/toolchain/driver/compile_subcommand.cpp b/toolchain/driver/compile_subcommand.cpp
index 044ae7dbcc25..6743f38491f1 100644
--- a/toolchain/driver/compile_subcommand.cpp
+++ b/toolchain/driver/compile_subcommand.cpp
@@ -372,8 +372,8 @@ class CompilationUnit {
   // with the actual function name, but marks timings with the appropriate
   // phase.
   auto LogCall(llvm::StringLiteral logging_label,
-               llvm::StringLiteral timing_label, llvm::function_ref<void()> fn)
-      -> void;
+               llvm::StringLiteral timing_label,
+               llvm::function_ref<auto()->void> fn) -> void;
 
   // Returns true if the current input file can be dumped.
   auto IncludeInDumps() const -> bool;
@@ -409,7 +409,7 @@ class CompilationUnit {
   std::optional<Lex::TokenizedBuffer> tokens_;
   std::optional<Parse::Tree> parse_tree_;
   std::optional<Parse::TreeAndSubtrees> parse_tree_and_subtrees_;
-  std::optional<std::function<const Parse::TreeAndSubtrees&()>>
+  std::optional<std::function<auto()->const Parse::TreeAndSubtrees&>>
       tree_and_subtrees_getter_;
   std::optional<SemIR::File> sem_ir_;
   std::unique_ptr<llvm::LLVMContext> llvm_context_;
@@ -710,7 +710,7 @@ auto CompilationUnit::GetParseTreeAndSubtrees()
 
 auto CompilationUnit::LogCall(llvm::StringLiteral logging_label,
                               llvm::StringLiteral timing_label,
-                              llvm::function_ref<void()> fn) -> void {
+                              llvm::function_ref<auto()->void> fn) -> void {
   CARBON_VLOG("*** {0}: {1} ***\n", logging_label, input_filename_);
   Timings::ScopedTiming timing(timings_ ? &*timings_ : nullptr, timing_label);
   fn();
diff --git a/toolchain/language_server/handle.h b/toolchain/language_server/handle.h
index 08ba21d2d208..82aa3a1b6616 100644
--- a/toolchain/language_server/handle.h
+++ b/toolchain/language_server/handle.h
@@ -29,21 +29,22 @@ auto HandleDidOpenTextDocument(
 auto HandleDocumentSymbol(
     Context& context, const clang::clangd::DocumentSymbolParams& params,
     llvm::function_ref<
-        void(llvm::Expected<std::vector<clang::clangd::DocumentSymbol>>)>
+        auto(llvm::Expected<std::vector<clang::clangd::DocumentSymbol>>)->void>
         on_done) -> void;
 
 // Tells the client what features are supported.
 auto HandleInitialize(
     Context& /*context*/,
     const clang::clangd::NoParams& /*client_capabilities*/,
-    llvm::function_ref<void(llvm::Expected<llvm::json::Object>)> on_done)
+    llvm::function_ref<auto(llvm::Expected<llvm::json::Object>)->void> on_done)
     -> void;
 
 // Prepares LSP for shutdown.
 auto HandleShutdown(
     Context& /*context*/,
     const clang::clangd::NoParams& /*client_capabilities*/,
-    llvm::function_ref<void(llvm::Expected<std::nullptr_t>)> on_done) -> void;
+    llvm::function_ref<auto(llvm::Expected<std::nullptr_t>)->void> on_done)
+    -> void;
 
 }  // namespace Carbon::LanguageServer
 
diff --git a/toolchain/language_server/handle_document_symbol.cpp b/toolchain/language_server/handle_document_symbol.cpp
index 84e7b168d85b..c78d004e2774 100644
--- a/toolchain/language_server/handle_document_symbol.cpp
+++ b/toolchain/language_server/handle_document_symbol.cpp
@@ -74,7 +74,7 @@ static auto GetSymbolRange(const Parse::TreeAndSubtrees& tree_and_subtrees,
 auto HandleDocumentSymbol(
     Context& context, const clang::clangd::DocumentSymbolParams& params,
     llvm::function_ref<
-        void(llvm::Expected<std::vector<clang::clangd::DocumentSymbol>>)>
+        auto(llvm::Expected<std::vector<clang::clangd::DocumentSymbol>>)->void>
         on_done) -> void {
   auto* file = context.LookupFile(params.textDocument.uri.file());
   if (!file) {
diff --git a/toolchain/language_server/handle_initialize.cpp b/toolchain/language_server/handle_initialize.cpp
index 77884820853b..b1adf0188e05 100644
--- a/toolchain/language_server/handle_initialize.cpp
+++ b/toolchain/language_server/handle_initialize.cpp
@@ -9,7 +9,7 @@ namespace Carbon::LanguageServer {
 auto HandleInitialize(
     Context& /*context*/,
     const clang::clangd::NoParams& /*client_capabilities*/,
-    llvm::function_ref<void(llvm::Expected<llvm::json::Object>)> on_done)
+    llvm::function_ref<auto(llvm::Expected<llvm::json::Object>)->void> on_done)
     -> void {
   llvm::json::Object capabilities{{"documentSymbolProvider", true},
                                   {"textDocumentSync", /*Full=*/1}};
diff --git a/toolchain/language_server/handle_shutdown.cpp b/toolchain/language_server/handle_shutdown.cpp
index 269bd76c2e4d..64d78dbd8b56 100644
--- a/toolchain/language_server/handle_shutdown.cpp
+++ b/toolchain/language_server/handle_shutdown.cpp
@@ -9,7 +9,8 @@ namespace Carbon::LanguageServer {
 auto HandleShutdown(
     Context& /*context*/,
     const clang::clangd::NoParams& /*client_capabilities*/,
-    llvm::function_ref<void(llvm::Expected<std::nullptr_t>)> on_done) -> void {
+    llvm::function_ref<auto(llvm::Expected<std::nullptr_t>)->void> on_done)
+    -> void {
   // TODO: Track that `shutdown` was called, and:
   // - Warn on duplicate calls.
   // - Make `exit` return `1` if `shutdown` wasn't called.
diff --git a/toolchain/language_server/incoming_messages.cpp b/toolchain/language_server/incoming_messages.cpp
index 42a733a5bf08..219da3b9e1b1 100644
--- a/toolchain/language_server/incoming_messages.cpp
+++ b/toolchain/language_server/incoming_messages.cpp
@@ -29,14 +29,14 @@ inline auto Parse(llvm::StringRef name, const llvm::json::Value& raw_params)
 template <typename ParamsT, typename ResultT>
 auto IncomingMessages::AddCallHandler(
     llvm::StringRef name,
-    void (*handler)(Context&, const ParamsT&,
-                    llvm::function_ref<void(llvm::Expected<ResultT>)>))
-    -> void {
+    auto (*handler)(Context&, const ParamsT&,
+                    llvm::function_ref<auto(llvm::Expected<ResultT>)->void>)
+        ->void) -> void {
   CallHandler parsing_handler =
       [name, handler](
           Context& context, llvm::json::Value raw_params,
-          llvm::function_ref<void(llvm::Expected<llvm::json::Value>)> on_done)
-      -> void {
+          llvm::function_ref<auto(llvm::Expected<llvm::json::Value>)->void>
+              on_done) -> void {
     auto params = Parse<ParamsT>(name, raw_params);
     if (!params) {
       on_done(params.takeError());
@@ -49,9 +49,8 @@ auto IncomingMessages::AddCallHandler(
 }
 
 template <typename ParamsT>
-auto IncomingMessages::AddNotificationHandler(llvm::StringRef name,
-                                              void (*handler)(Context&,
-                                                              const ParamsT&))
+auto IncomingMessages::AddNotificationHandler(
+    llvm::StringRef name, auto (*handler)(Context&, const ParamsT&)->void)
     -> void {
   NotificationHandler parsing_handler =
       [name, handler](Context& context, llvm::json::Value raw_params) -> void {
diff --git a/toolchain/language_server/incoming_messages.h b/toolchain/language_server/incoming_messages.h
index 407dc98a7f94..fa1534dd7b8e 100644
--- a/toolchain/language_server/incoming_messages.h
+++ b/toolchain/language_server/incoming_messages.h
@@ -40,21 +40,23 @@ class IncomingMessages : public clang::clangd::Transport::MessageHandler {
 
  private:
   // These are the signatures expected for handlers.
-  using CallHandler = std::function<void(
-      Context& context, llvm::json::Value raw_params,
-      llvm::function_ref<void(llvm::Expected<llvm::json::Value>)> on_done)>;
+  using CallHandler = std::function<
+      auto(Context& context, llvm::json::Value raw_params,
+           llvm::function_ref<auto(llvm::Expected<llvm::json::Value>)->void>
+               on_done)
+          ->void>;
   using NotificationHandler =
-      std::function<void(Context& context, llvm::json::Value raw_params)>;
+      std::function<auto(Context& context, llvm::json::Value raw_params)->void>;
 
   template <typename ParamsT, typename ResultT>
   auto AddCallHandler(
       llvm::StringRef name,
-      void (*handler)(Context&, const ParamsT&,
-                      llvm::function_ref<void(llvm::Expected<ResultT>)>))
-      -> void;
+      auto (*handler)(Context&, const ParamsT&,
+                      llvm::function_ref<auto(llvm::Expected<ResultT>)->void>)
+          ->void) -> void;
   template <typename ParamsT>
   auto AddNotificationHandler(llvm::StringRef name,
-                              void (*handler)(Context&, const ParamsT&))
+                              auto (*handler)(Context&, const ParamsT&)->void)
       -> void;
 
   // The connection to the client.
diff --git a/toolchain/lex/tokenized_buffer_benchmark.cpp b/toolchain/lex/tokenized_buffer_benchmark.cpp
index 3ed9b1f91f1a..5ec07304bd16 100644
--- a/toolchain/lex/tokenized_buffer_benchmark.cpp
+++ b/toolchain/lex/tokenized_buffer_benchmark.cpp
@@ -75,7 +75,7 @@ struct RandomSourceOptions {
   int comment_line_percent = 0;
   int blank_line_percent = 0;
 
-  void Validate() {
+  auto Validate() -> void {
     auto is_percentage = [](int n) { return 0 <= n && n <= 100; };
     CARBON_CHECK(is_percentage(symbol_percent));
     CARBON_CHECK(is_percentage(keyword_percent));
@@ -244,7 +244,7 @@ class LexerBenchHelper {
   SourceBuffer source_;
 };
 
-void BM_ValidKeywords(benchmark::State& state) {
+auto BM_ValidKeywords(benchmark::State& state) -> void {
   absl::BitGen gen;
   std::array<llvm::StringRef, NumTokens> tokens;
   for (int i : llvm::seq(NumTokens)) {
@@ -266,7 +266,7 @@ void BM_ValidKeywords(benchmark::State& state) {
 }
 BENCHMARK(BM_ValidKeywords);
 
-void BM_ValidKeywordsAsRawIdentifiers(benchmark::State& state) {
+auto BM_ValidKeywordsAsRawIdentifiers(benchmark::State& state) -> void {
   absl::BitGen gen;
   std::array<llvm::StringRef, NumTokens> tokens;
   for (int i : llvm::seq(NumTokens)) {
@@ -291,7 +291,7 @@ BENCHMARK(BM_ValidKeywordsAsRawIdentifiers);
 
 // This benchmark does a 50-50 split of r-prefixed and r#-prefixed identifiers
 // to directly compare raw and non-raw performance.
-void BM_RawIdentifierFocus(benchmark::State& state) {
+auto BM_RawIdentifierFocus(benchmark::State& state) -> void {
   llvm::SmallVector<llvm::StringRef> ids =
       Testing::SourceGen::Global().GetIdentifiers(NumTokens / 2);
 
@@ -328,7 +328,7 @@ void BM_RawIdentifierFocus(benchmark::State& state) {
 BENCHMARK(BM_RawIdentifierFocus);
 
 template <int MinLength, int MaxLength, bool Uniform>
-void BM_ValidIdentifiers(benchmark::State& state) {
+auto BM_ValidIdentifiers(benchmark::State& state) -> void {
   std::string source = RandomIdentifierSeq(MinLength, MaxLength, Uniform);
 
   LexerBenchHelper helper(source);
@@ -361,7 +361,7 @@ BENCHMARK(BM_ValidIdentifiers<80, 80, /*Uniform=*/true>);
 // Benchmark to stress the lexing of horizontal whitespace. This sets up what is
 // nearly a worst-case scenario of short-but-expensive-to-lex tokens with runs
 // of horizontal whitespace between them.
-void BM_HorizontalWhitespace(benchmark::State& state) {
+auto BM_HorizontalWhitespace(benchmark::State& state) -> void {
   int num_spaces = state.range(0);
   std::string separator(num_spaces, ' ');
   std::string source = RandomIdentifierSeq(3, 5, /*uniform=*/true, separator);
@@ -381,7 +381,7 @@ void BM_HorizontalWhitespace(benchmark::State& state) {
 }
 BENCHMARK(BM_HorizontalWhitespace)->RangeMultiplier(4)->Range(1, 128);
 
-void BM_RandomSource(benchmark::State& state) {
+auto BM_RandomSource(benchmark::State& state) -> void {
   std::string source = RandomSource(DefaultSourceDist);
 
   LexerBenchHelper helper(source);
@@ -407,7 +407,7 @@ void BM_RandomSource(benchmark::State& state) {
 BENCHMARK(BM_RandomSource);
 
 // Benchmark to stress opening and closing grouped symbols.
-void BM_GroupingSymbols(benchmark::State& state) {
+auto BM_GroupingSymbols(benchmark::State& state) -> void {
   int curly_brace_depth = state.range(0);
   int paren_depth = state.range(1);
   int square_bracket_depth = state.range(2);
@@ -495,7 +495,7 @@ BENCHMARK(BM_GroupingSymbols)
 
 // Benchmark to stress the lexing of blank lines. This uses a simple, easy to
 // lex token, but separates each one by varying numbers of blank lines.
-void BM_BlankLines(benchmark::State& state) {
+auto BM_BlankLines(benchmark::State& state) -> void {
   int num_blank_lines = state.range(0);
   std::string separator(num_blank_lines, '\n');
   std::string source = RandomIdentifierSeq(3, 5, /*uniform=*/true, separator);
@@ -521,7 +521,7 @@ BENCHMARK(BM_BlankLines)->RangeMultiplier(4)->Range(1, 128);
 // Benchmark to stress the lexing of comment lines. This uses a simple, easy to
 // lex token, but separates each one by varying numbers of comment lines, with
 // varying comment line length and indentation.
-void BM_CommentLines(benchmark::State& state) {
+auto BM_CommentLines(benchmark::State& state) -> void {
   int num_comment_lines = state.range(0);
   int comment_length = state.range(1);
   int comment_indent = state.range(2);
diff --git a/toolchain/lower/function_context.h b/toolchain/lower/function_context.h
index fee0c328525c..5cdacda9a356 100644
--- a/toolchain/lower/function_context.h
+++ b/toolchain/lower/function_context.h
@@ -142,7 +142,7 @@ class FunctionContext {
         : inst_namer_(inst_namer) {}
 
     // Sets the instruction we are currently emitting.
-    void SetCurrentInstId(SemIR::InstId inst_id) { inst_id_ = inst_id; }
+    auto SetCurrentInstId(SemIR::InstId inst_id) -> void { inst_id_ = inst_id; }
 
    private:
     auto InsertHelper(llvm::Instruction* inst, const llvm::Twine& name,
diff --git a/toolchain/parse/handle_import_and_package.cpp b/toolchain/parse/handle_import_and_package.cpp
index b58271f21b81..5112a5189488 100644
--- a/toolchain/parse/handle_import_and_package.cpp
+++ b/toolchain/parse/handle_import_and_package.cpp
@@ -37,7 +37,7 @@ static auto HasModifier(Context& context, Context::StateStackEntry state,
 static auto HandleDeclContent(Context& context, Context::StateStackEntry state,
                               NodeKind declaration, bool is_export,
                               bool is_impl,
-                              llvm::function_ref<void()> on_parse_error)
+                              llvm::function_ref<auto()->void> on_parse_error)
     -> void {
   Tree::PackagingNames names{
       .node_id = ImportDeclId(NodeId(state.subtree_start)),
diff --git a/toolchain/parse/tree_and_subtrees.h b/toolchain/parse/tree_and_subtrees.h
index 10f1447e7f1c..5c2aa515fe3d 100644
--- a/toolchain/parse/tree_and_subtrees.h
+++ b/toolchain/parse/tree_and_subtrees.h
@@ -194,7 +194,7 @@ class TreeAndSubtrees {
 };
 
 // A standard signature for a callback to support lazy construction.
-using GetTreeAndSubtreesFn = llvm::function_ref<const TreeAndSubtrees&()>;
+using GetTreeAndSubtreesFn = llvm::function_ref<auto()->const TreeAndSubtrees&>;
 
 // A forward iterator across the siblings at a particular level in the parse
 // tree. It produces `Tree::NodeId` objects which are opaque handles and must
diff --git a/toolchain/sem_ir/inst_kind.h b/toolchain/sem_ir/inst_kind.h
index eb603e377c80..5f632e04b8c5 100644
--- a/toolchain/sem_ir/inst_kind.h
+++ b/toolchain/sem_ir/inst_kind.h
@@ -145,7 +145,7 @@ class InstKind : public CARBON_ENUM_BASE(InstKind) {
 
   // Compute a fingerprint for this instruction kind, allowing its use as part
   // of the key in a `FoldingSet`.
-  void Profile(llvm::FoldingSetNodeID& id) { id.AddInteger(AsInt()); }
+  auto Profile(llvm::FoldingSetNodeID& id) -> void { id.AddInteger(AsInt()); }
 
  private:
   // Returns the DefinitionInfo for the kind.