Index: clang-tidy/modernize/CMakeLists.txt
===================================================================
--- clang-tidy/modernize/CMakeLists.txt
+++ clang-tidy/modernize/CMakeLists.txt
@@ -30,6 +30,7 @@
   UseNoexceptCheck.cpp
   UseNullptrCheck.cpp
   UseOverrideCheck.cpp
+  UseTrailingReturnTypeCheck.cpp
   UseTransparentFunctorsCheck.cpp
   UseUncaughtExceptionsCheck.cpp
   UseUsingCheck.cpp
Index: clang-tidy/modernize/ModernizeTidyModule.cpp
===================================================================
--- clang-tidy/modernize/ModernizeTidyModule.cpp
+++ clang-tidy/modernize/ModernizeTidyModule.cpp
@@ -35,6 +35,7 @@
 #include "UseNoexceptCheck.h"
 #include "UseNullptrCheck.h"
 #include "UseOverrideCheck.h"
+#include "UseTrailingReturnTypeCheck.h"
 #include "UseTransparentFunctorsCheck.h"
 #include "UseUncaughtExceptionsCheck.h"
 #include "UseUsingCheck.h"
@@ -87,6 +88,8 @@
     CheckFactories.registerCheck<UseNoexceptCheck>("modernize-use-noexcept");
     CheckFactories.registerCheck<UseNullptrCheck>("modernize-use-nullptr");
     CheckFactories.registerCheck<UseOverrideCheck>("modernize-use-override");
+    CheckFactories.registerCheck<UseTrailingReturnTypeCheck>(
+        "modernize-use-trailing-return-type");
     CheckFactories.registerCheck<UseTransparentFunctorsCheck>(
         "modernize-use-transparent-functors");
     CheckFactories.registerCheck<UseUncaughtExceptionsCheck>(
Index: clang-tidy/modernize/UseTrailingReturnTypeCheck.h
===================================================================
--- /dev/null
+++ clang-tidy/modernize/UseTrailingReturnTypeCheck.h
@@ -0,0 +1,48 @@
+//===--- UseTrailingReturnTypeCheck.h - clang-tidy---------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MODERNIZE_USETRAILINGRETURNTYPECHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MODERNIZE_USETRAILINGRETURNTYPECHECK_H
+
+#include "../ClangTidy.h"
+
+namespace clang {
+namespace tidy {
+namespace modernize {
+
+/// Rewrites function signatures to use a trailing return type.
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/modernize-use-trailing-type-return.html
+class UseTrailingReturnTypeCheck : public ClangTidyCheck {
+public:
+  UseTrailingReturnTypeCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+
+private:
+  SourceLocation findTrailingReturnTypeSourceLocation(
+      const FunctionDecl &F, const FunctionTypeLoc &FTL, const ASTContext &Ctx,
+      const SourceManager &SM, const LangOptions &LangOpts);
+  llvm::Optional<SmallVector<Token, 8>>
+  nonMacroTokensBeforeFunctionName(const FunctionDecl &F, const ASTContext &Ctx,
+                                   const SourceManager &SM,
+                                   const LangOptions &LangOpts);
+  SourceRange findReturnTypeAndCVSourceRange(const FunctionDecl &F,
+                                             const ASTContext &Ctx,
+                                             const SourceManager &SM,
+                                             const LangOptions &LangOpts);
+};
+
+} // namespace modernize
+} // namespace tidy
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MODERNIZE_USETRAILINGRETURNTYPECHECK_H
Index: clang-tidy/modernize/UseTrailingReturnTypeCheck.cpp
===================================================================
--- /dev/null
+++ clang-tidy/modernize/UseTrailingReturnTypeCheck.cpp
@@ -0,0 +1,350 @@
+//===--- UseTrailingReturnTypeCheck.cpp - clang-tidy-----------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "UseTrailingReturnTypeCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/RecursiveASTVisitor.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/Tooling/FixIt.h"
+
+#include <cctype>
+
+using namespace clang::ast_matchers;
+
+namespace clang {
+namespace tidy {
+namespace modernize {
+namespace {
+struct UnqualNameVisitor : public RecursiveASTVisitor<UnqualNameVisitor> {
+public:
+  UnqualNameVisitor(const FunctionDecl &F, const SourceManager &SM)
+      : F(F), SM(SM) {}
+
+  bool Collision = false;
+
+  bool shouldWalkTypesOfTypeLocs() const { return false; }
+
+  bool VisitUnqualName(StringRef UnqualName) {
+    // Check for collisions with function arguments
+    for (ParmVarDecl *Param : F.parameters())
+      if (const IdentifierInfo *Ident = Param->getIdentifier())
+        if (Ident->getName() == UnqualName) {
+          Collision = true;
+          return true;
+        }
+    return false;
+  }
+
+  bool TraverseTypeLoc(TypeLoc TL, bool Elaborated = false) {
+    if (TL.isNull())
+      return true;
+
+    if (!Elaborated) {
+      switch (TL.getTypeLocClass()) {
+      case TypeLoc::Record:
+        if (VisitUnqualName(
+                TL.getAs<RecordTypeLoc>().getTypePtr()->getDecl()->getName()))
+          return false;
+        break;
+      case TypeLoc::Enum:
+        if (VisitUnqualName(
+                TL.getAs<EnumTypeLoc>().getTypePtr()->getDecl()->getName()))
+          return false;
+        break;
+      case TypeLoc::TemplateSpecialization:
+        if (VisitUnqualName(TL.getAs<TemplateSpecializationTypeLoc>()
+                                .getTypePtr()
+                                ->getTemplateName()
+                                .getAsTemplateDecl()
+                                ->getName()))
+          return false;
+        break;
+      default:
+        break;
+      }
+    }
+
+    return RecursiveASTVisitor<UnqualNameVisitor>::TraverseTypeLoc(TL);
+  }
+
+  // Replace the base method in order to call ower own
+  // TraverseTypeLoc().
+  bool TraverseQualifiedTypeLoc(QualifiedTypeLoc TL) {
+    return TraverseTypeLoc(TL.getUnqualifiedLoc());
+  }
+
+  // Replace the base version to inform TraverseTypeLoc that the type is
+  // elaborated.
+  bool TraverseElaboratedTypeLoc(ElaboratedTypeLoc TL) {
+    if (TL.getQualifierLoc())
+      if (!TraverseNestedNameSpecifierLoc(TL.getQualifierLoc()))
+        return false;
+    if (!TraverseTypeLoc(TL.getNamedTypeLoc(), true))
+      return false;
+    return true;
+  }
+
+  bool VisitDeclRefExpr(DeclRefExpr *S) {
+    const DeclarationName Name = S->getNameInfo().getName();
+    if (!S->getQualifierLoc() && Name.isIdentifier() &&
+        VisitUnqualName(Name.getAsIdentifierInfo()->getName()))
+      return false;
+    return true;
+  }
+
+private:
+  const FunctionDecl &F;
+  const SourceManager &SM;
+};
+} // namespace
+
+constexpr llvm::StringLiteral Message =
+    "use a trailing return type for this function";
+
+static SourceLocation expandIfMacroId(SourceLocation Loc,
+                                      const SourceManager &SM) {
+  if (Loc.isMacroID())
+    Loc = SM.getImmediateExpansionRange(Loc).getBegin();
+  return Loc;
+}
+
+SourceLocation UseTrailingReturnTypeCheck::findTrailingReturnTypeSourceLocation(
+    const FunctionDecl &F, const FunctionTypeLoc &FTL, const ASTContext &Ctx,
+    const SourceManager &SM, const LangOptions &LangOpts) {
+  // We start with the location of the closing parenthesis.
+  SourceRange ExceptionSpecRange = F.getExceptionSpecSourceRange();
+  if (ExceptionSpecRange.isValid())
+    return Lexer::getLocForEndOfToken(ExceptionSpecRange.getEnd(), 0, SM,
+                                      LangOpts);
+
+  // If the function argument list ends inside of a macro, it is dangerous to
+  // start lexing from here - bail out.
+  SourceLocation ClosingParen = FTL.getRParenLoc();
+  if (ClosingParen.isMacroID())
+    return {};
+
+  SourceLocation Result =
+      Lexer::getLocForEndOfToken(ClosingParen, 0, SM, LangOpts);
+
+  // Skip subsequent CV and ref qualifiers.
+  std::pair<FileID, unsigned> Loc = SM.getDecomposedLoc(Result);
+  StringRef File = SM.getBufferData(Loc.first);
+  const char *TokenBegin = File.data() + Loc.second;
+  Lexer Lexer(SM.getLocForStartOfFile(Loc.first), LangOpts, File.begin(),
+              TokenBegin, File.end());
+  Token T;
+  while (!Lexer.LexFromRawLexer(T)) {
+    if (T.is(tok::raw_identifier)) {
+      IdentifierInfo &Info = Ctx.Idents.get(
+          StringRef(SM.getCharacterData(T.getLocation()), T.getLength()));
+      T.setIdentifierInfo(&Info);
+      T.setKind(Info.getTokenID());
+    }
+
+    if (T.isOneOf(tok::amp, tok::ampamp, tok::kw_const, tok::kw_volatile)) {
+      Result = T.getEndLoc();
+      continue;
+    }
+
+    break;
+  }
+
+  return Result;
+}
+
+llvm::Optional<SmallVector<Token, 8>>
+UseTrailingReturnTypeCheck::nonMacroTokensBeforeFunctionName(
+    const FunctionDecl &F, const ASTContext &Ctx, const SourceManager &SM,
+    const LangOptions &LangOpts) {
+  SourceLocation BeginF = expandIfMacroId(F.getBeginLoc(), SM);
+  SourceLocation BeginNameF = expandIfMacroId(F.getLocation(), SM);
+
+  // Create tokens for everything before the name of the function.
+  std::pair<FileID, unsigned> Loc = SM.getDecomposedLoc(BeginF);
+  StringRef File = SM.getBufferData(Loc.first);
+  const char *TokenBegin = File.data() + Loc.second;
+  Lexer Lexer(SM.getLocForStartOfFile(Loc.first), LangOpts, File.begin(),
+              TokenBegin, File.end());
+  Token T;
+  SmallVector<Token, 8> Tokens;
+  while (!Lexer.LexFromRawLexer(T) &&
+         SM.isBeforeInTranslationUnit(T.getLocation(), BeginNameF)) {
+    if (T.is(tok::raw_identifier)) {
+      IdentifierInfo &Info = Ctx.Idents.get(
+          StringRef(SM.getCharacterData(T.getLocation()), T.getLength()));
+      if (Info.hasMacroDefinition()) {
+        // The CV qualifiers of the return type are inside macros
+        diag(F.getLocation(), Message);
+        return {};
+      }
+      T.setIdentifierInfo(&Info);
+      T.setKind(Info.getTokenID());
+    }
+    Tokens.push_back(T);
+  }
+  return Tokens;
+}
+
+static bool hasAnyNestedLocalQualifiers(QualType Type) {
+  bool Result = Type.hasLocalQualifiers();
+  if (Type->isPointerType())
+    Result = Result || hasAnyNestedLocalQualifiers(
+                           Type->castAs<PointerType>()->getPointeeType());
+  if (Type->isReferenceType())
+    Result = Result || hasAnyNestedLocalQualifiers(
+                           Type->castAs<ReferenceType>()->getPointeeType());
+  return Result;
+}
+
+SourceRange UseTrailingReturnTypeCheck::findReturnTypeAndCVSourceRange(
+    const FunctionDecl &F, const ASTContext &Ctx, const SourceManager &SM,
+    const LangOptions &LangOpts) {
+
+  // We start with the range of the return type and expand to neighboring
+  // 'const' and 'volatile'.
+  SourceRange ReturnTypeRange = F.getReturnTypeSourceRange();
+  if (ReturnTypeRange.isInvalid()) {
+    // Happens if e.g. clang cannot resolve all includes and the return type is
+    // unknown.
+    diag(F.getLocation(), Message);
+    return {};
+  }
+
+  // If the return type has no local qualifiers, it's source range is accurate.
+  if (!hasAnyNestedLocalQualifiers(F.getReturnType()))
+    return ReturnTypeRange;
+
+  // Include const and volatile to the left and right of the return type.
+  llvm::Optional<SmallVector<Token, 8>> MaybeTokens =
+      nonMacroTokensBeforeFunctionName(F, Ctx, SM, LangOpts);
+  if (!MaybeTokens)
+    return {};
+  const SmallVector<Token, 8> &Tokens = *MaybeTokens;
+
+  auto IsCV = [](Token T) {
+    return T.isOneOf(tok::kw_const, tok::kw_volatile);
+  };
+
+  bool ExtendedLeft = false;
+  for (size_t I = 0; I < Tokens.size(); I++) {
+    // If we found the beginning of the return type, include const and volatile
+    // to the left.
+    if (!SM.isBeforeInTranslationUnit(Tokens[I].getLocation(),
+                                      ReturnTypeRange.getBegin()) &&
+        !ExtendedLeft) {
+      for (int J = static_cast<int>(I) - 1; J >= 0 && IsCV(Tokens[J]); J--)
+        ReturnTypeRange.setBegin(Tokens[J].getLocation());
+      ExtendedLeft = true;
+    }
+    // If we found the end of the return type, include const and volatile to the
+    // right.
+    if (SM.isBeforeInTranslationUnit(ReturnTypeRange.getEnd(),
+                                     Tokens[I].getLocation())) {
+      for (size_t J = I; J < Tokens.size() && IsCV(Tokens[J]); J++)
+        ReturnTypeRange.setEnd(Tokens[J].getLocation());
+      break;
+    }
+  }
+
+  return ReturnTypeRange;
+}
+
+void UseTrailingReturnTypeCheck::registerMatchers(MatchFinder *Finder) {
+  if (!getLangOpts().CPlusPlus11)
+    return;
+
+  Finder->addMatcher(
+      functionDecl(unless(anyOf(hasTrailingReturn(), returns(voidType()),
+                                returns(autoType()), cxxConversionDecl(),
+                                cxxMethodDecl(isImplicit()))))
+          .bind("f"),
+      this);
+}
+
+void UseTrailingReturnTypeCheck::check(const MatchFinder::MatchResult &Result) {
+  const auto *F = Result.Nodes.getNodeAs<FunctionDecl>("f");
+  assert(F && "Matcher is expected to find only FunctionDecls");
+
+  if (F->getLocation().isInvalid())
+    return;
+
+  // TODO: implement those
+  if (F->getDeclaredReturnType()->isFunctionPointerType() ||
+      F->getDeclaredReturnType()->isMemberFunctionPointerType() ||
+      F->getDeclaredReturnType()->isMemberPointerType() ||
+      F->getDeclaredReturnType()->getAs<DecltypeType>() != nullptr) {
+    diag(F->getLocation(), Message);
+    return;
+  }
+
+  const ASTContext &Ctx = *Result.Context;
+  const SourceManager &SM = *Result.SourceManager;
+  const LangOptions &LangOpts = getLangOpts();
+
+  const TypeSourceInfo *TSI = F->getTypeSourceInfo();
+  if (!TSI) {
+    diag(F->getLocation(), Message);
+    return;
+  }
+  FunctionTypeLoc FTL =
+      TSI->getTypeLoc().IgnoreParens().getAs<FunctionTypeLoc>();
+  if (!FTL) {
+    diag(F->getLocation(), Message);
+    return;
+  }
+
+  SourceLocation InsertionLoc =
+      findTrailingReturnTypeSourceLocation(*F, FTL, Ctx, SM, LangOpts);
+  if (InsertionLoc.isInvalid()) {
+    diag(F->getLocation(), Message);
+    return;
+  }
+
+  // Using the declared return type via F->getDeclaredReturnType().getAsString()
+  // discards user formatting and order of const, volatile, type, whitespace,
+  // space before & ... .
+  SourceRange ReturnTypeCVRange =
+      findReturnTypeAndCVSourceRange(*F, Ctx, SM, LangOpts);
+  if (ReturnTypeCVRange.isInvalid())
+    return;
+
+  // Check if unqualified names in the return type conflict with other entities
+  // after the rewrite.
+  // FIXME: this could be done better, by performing a lookup of all
+  // unqualified names in the return type in the scope of the function. If the
+  // lookup finds a different entity than the original entity identified by the
+  // name, then we can either not perform a rewrite or explicitely qualify the
+  // entity. Such entities could be function parameter names, (inherited) class
+  // members, template parameters, etc.
+  UnqualNameVisitor UNV{*F, SM};
+  UNV.TraverseTypeLoc(FTL.getReturnLoc());
+  if (UNV.Collision) {
+    diag(F->getLocation(), Message);
+    return;
+  }
+
+  SourceLocation ReturnTypeEnd =
+      Lexer::getLocForEndOfToken(ReturnTypeCVRange.getEnd(), 0, SM, LangOpts);
+  StringRef CharAfterReturnType = Lexer::getSourceText(
+      CharSourceRange::getCharRange(ReturnTypeEnd,
+                                    ReturnTypeEnd.getLocWithOffset(1)),
+      SM, LangOpts);
+  bool NeedSpaceAfterAuto =
+      CharAfterReturnType.empty() || !std::isspace(CharAfterReturnType[0]);
+
+  StringRef ReturnType = tooling::fixit::getText(ReturnTypeCVRange, Ctx);
+  StringRef Auto = NeedSpaceAfterAuto ? "auto " : "auto";
+  diag(F->getLocation(), Message)
+      << FixItHint::CreateReplacement(ReturnTypeCVRange, Auto)
+      << FixItHint::CreateInsertion(InsertionLoc, (" -> " + ReturnType).str());
+}
+
+} // namespace modernize
+} // namespace tidy
+} // namespace clang
Index: docs/ReleaseNotes.rst
===================================================================
--- docs/ReleaseNotes.rst
+++ docs/ReleaseNotes.rst
@@ -102,6 +102,11 @@
   :doc:`objc-property-declaration <clang-tidy/checks/objc-property-declaration>`
   check have been removed.
 
+- New :doc:`modernize-use-trailing-type-return
+  <clang-tidy/checks/modernize-use-trailing-type-return>` check.
+
+  Rewrites function signatures to use a trailing return type.
+
 Improvements to include-fixer
 -----------------------------
 
Index: docs/clang-tidy/checks/list.rst
===================================================================
--- docs/clang-tidy/checks/list.rst
+++ docs/clang-tidy/checks/list.rst
@@ -215,6 +215,7 @@
    modernize-use-noexcept
    modernize-use-nullptr
    modernize-use-override
+   modernize-use-trailing-return-type
    modernize-use-transparent-functors
    modernize-use-uncaught-exceptions
    modernize-use-using
Index: docs/clang-tidy/checks/modernize-use-trailing-return-type.rst
===================================================================
--- /dev/null
+++ docs/clang-tidy/checks/modernize-use-trailing-return-type.rst
@@ -0,0 +1,68 @@
+.. title:: clang-tidy - modernize-use-trailing-return-type
+
+modernize-use-trailing-return-type
+==================================
+
+Rewrites function signatures to use a trailing return type
+(introduced in C++11). This transformation is purely stylistic.
+The return type before the function name is replaced by ``auto``
+and inserted after the function parameter list (and qualifiers).
+
+Example
+-------
+
+.. code-block:: c++
+
+  int f1();
+  inline int f2(int arg) noexcept;
+  virtual float f3() const && = delete;
+
+transforms to:
+
+.. code-block:: c++
+
+  auto f1() -> int;
+  inline auto f2(int arg) -> int noexcept;
+  virtual auto f3() const && -> float = delete;
+
+Known Limitations
+-----------------
+
+The following categories of return types cannot be rewritten currently:
+* function pointers
+* member function pointers
+* member pointers
+* decltype, when it is the top level expression
+
+Unqualified names in the return type might erroneously refer to different entities after the rewrite.
+Preventing such errors requires a full lookup of all unqualified names present in the return type in the scope of the trailing return type location.
+This location includes e.g. function parameter names and members of the enclosing class (including all inherited classes).
+Such a lookup is currently not implemented.
+
+Given the following piece of code
+
+.. code-block:: c++
+
+  struct Object { long long value; };
+  Object f(unsigned Object) { return {Object * 2}; }
+  class CC {
+    int Object;
+    struct Object m();
+  };
+  Object CC::m() { return {0}; }
+
+a careless rewrite would produce the following output:
+
+.. code-block:: c++
+
+  struct Object { long long value; };
+  auto f(unsigned Object) -> Object { return {Object * 2}; } // error
+  class CC {
+    int Object;
+    auto m() -> struct Object;
+  };
+  auto CC::m() -> Object { return {0}; } // error
+
+This code fails to compile because the Object in the context of f refers to the equally named function parameter.
+Similarly, the Object in the context of m refers to the equally named class member.
+The check can currently only detect a clash with a function parameter name.
Index: test/clang-tidy/modernize-use-trailing-return-type.cpp
===================================================================
--- /dev/null
+++ test/clang-tidy/modernize-use-trailing-return-type.cpp
@@ -0,0 +1,437 @@
+// RUN: %check_clang_tidy %s modernize-use-trailing-return-type %t -- -- --std=c++14 -fdeclspec
+
+namespace std {
+    template <typename T>
+    class vector;
+
+    template <typename T, int N>
+    class array;
+
+    class string;
+
+    template <typename T>
+    auto declval() -> T;
+}
+
+//
+// Functions
+//
+
+int f();
+// CHECK-MESSAGES: :[[@LINE-1]]:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto f() -> int;{{$}}
+int ((f))();
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto ((f))() -> int;{{$}}
+int f(int);
+// CHECK-MESSAGES: :[[@LINE-1]]:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto f(int) -> int;{{$}}
+int f(int arg);
+// CHECK-MESSAGES: :[[@LINE-1]]:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto f(int arg) -> int;{{$}}
+int f(int arg1, int arg2, int arg3);
+// CHECK-MESSAGES: :[[@LINE-1]]:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto f(int arg1, int arg2, int arg3) -> int;{{$}}
+int f(int arg1, int arg2, int arg3, ...);
+// CHECK-MESSAGES: :[[@LINE-1]]:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto f(int arg1, int arg2, int arg3, ...) -> int;{{$}}
+template <typename T> int f(T t);
+// CHECK-MESSAGES: :[[@LINE-1]]:27: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}template <typename T> auto f(T t) -> int;{{$}}
+
+//
+// Functions with formatting
+//
+
+int a1() { return 42; }
+// CHECK-MESSAGES: :[[@LINE-1]]:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto a1() -> int { return 42; }{{$}}
+int a2() {
+    return 42;
+}
+// CHECK-MESSAGES: :[[@LINE-3]]:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto a2() -> int {{{$}}
+int a3()
+{
+    return 42;
+}
+// CHECK-MESSAGES: :[[@LINE-4]]:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto a3() -> int{{$}}
+int a4(int   arg   )   ;
+// CHECK-MESSAGES: :[[@LINE-1]]:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto a4(int   arg   ) -> int   ;{{$}}
+int a5
+// CHECK-MESSAGES: :[[@LINE-1]]:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto a5{{$}}
+(int arg);
+// CHECK-FIXES: {{^}}(int arg) -> int;{{$}}
+const
+int
+*
+a7
+// CHECK-MESSAGES: :[[@LINE-1]]:1: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+()
+// CHECK-FIXES: {{^}}() -> const{{$}}
+// CHECK-FIXES: {{^}}int{{$}}
+// CHECK-FIXES: {{^}}*{{$}}
+;
+
+int*a7(int arg);
+// CHECK-MESSAGES: :[[@LINE-1]]:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto a7(int arg) -> int*;{{$}}
+template<template <typename> class C>
+C<int>a8(int arg);
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto a8(int arg) -> C<int>;{{$}}
+
+
+//
+// Functions with qualifiers and specifiers
+//
+
+inline int d1(int arg);
+// CHECK-MESSAGES: :[[@LINE-1]]:12: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}inline auto d1(int arg) -> int;{{$}}
+extern "C" int d2(int arg);
+// CHECK-MESSAGES: :[[@LINE-1]]:16: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}extern "C" auto d2(int arg) -> int;{{$}}
+inline int d3(int arg) noexcept(true);
+// CHECK-MESSAGES: :[[@LINE-1]]:12: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}inline auto d3(int arg) noexcept(true) -> int;{{$}}
+inline int d4(int arg) try { } catch(...) { }
+// CHECK-MESSAGES: :[[@LINE-1]]:12: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}inline auto d4(int arg) -> int try { } catch(...) { }{{$}}
+int d5(int arg) throw();
+// CHECK-MESSAGES: :[[@LINE-1]]:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto d5(int arg) throw() -> int;{{$}}
+
+//
+// Functions in namespaces
+//
+
+namespace N {
+    int e1();
+}
+// CHECK-MESSAGES: :[[@LINE-2]]:9: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}    auto e1() -> int;{{$}}
+int N::e1() {}
+// CHECK-MESSAGES: :[[@LINE-1]]:8: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto N::e1() -> int {}{{$}}
+
+//
+// Functions with unsupported return types
+//
+int (*e3())(double);
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}int (*e3())(double);{{$}}
+struct A;
+int A::* e5();
+// CHECK-MESSAGES: :[[@LINE-1]]:10: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}int A::* e5();{{$}}
+int std::vector<std::string>::* e6();
+// CHECK-MESSAGES: :[[@LINE-1]]:33: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}int std::vector<std::string>::* e6();{{$}}
+int (std::vector<std::string>::*e7())(double);
+// CHECK-MESSAGES: :[[@LINE-1]]:33: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}int (std::vector<std::string>::*e7())(double);{{$}}
+
+//
+// Functions with complex return types
+//
+
+inline volatile const std::vector<std::string> e2();
+// CHECK-MESSAGES: :[[@LINE-1]]:48: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}inline auto e2() -> volatile const std::vector<std::string>;{{$}}
+inline const std::vector<std::string> volatile e2();
+// CHECK-MESSAGES: :[[@LINE-1]]:48: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}inline auto e2() -> const std::vector<std::string> volatile;{{$}}
+inline std::vector<std::string> const volatile e2();
+// CHECK-MESSAGES: :[[@LINE-1]]:48: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}inline auto e2() -> std::vector<std::string> const volatile;{{$}}
+int* e8();
+// CHECK-MESSAGES: :[[@LINE-1]]:6: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto e8() -> int*;{{$}}
+static const char* e9(void* user_data);
+// CHECK-MESSAGES: :[[@LINE-1]]:20: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}static auto e9(void* user_data) -> const char*;{{$}}
+static const char* const e10(void* user_data);
+// CHECK-MESSAGES: :[[@LINE-1]]:26: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}static auto e10(void* user_data) -> const char* const;{{$}}
+static const char** volatile * const & e11(void* user_data);
+// CHECK-MESSAGES: :[[@LINE-1]]:40: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}static auto e11(void* user_data) -> const char** volatile * const &;{{$}}
+static const char* const * const * const e12(void* user_data);
+// CHECK-MESSAGES: :[[@LINE-1]]:42: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}static auto e12(void* user_data) -> const char* const * const * const;{{$}}
+struct A e13();
+// CHECK-MESSAGES: :[[@LINE-1]]:10: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto e13() -> struct A;{{$}}
+
+//
+// decltype (unsupported if top level expression)
+//
+
+decltype(1 + 2) dec1() { return 1 + 2; }
+// CHECK-MESSAGES: :[[@LINE-1]]:17: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// TODO: source range of DecltypeTypeLoc not yet implemented
+// _HECK-FIXES: {{^}}auto dec1() -> decltype(1 + 2) { return 1 + 2; }{{$}}
+template <typename F, typename T>
+decltype(std::declval<F>(std::declval<T>)) dec2(F f, T t) { return f(t); }
+// CHECK-MESSAGES: :[[@LINE-1]]:44: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// TODO: source range of DecltypeTypeLoc not yet implemented
+// _HECK-FIXES: {{^}}auto dec2(F f, T t) -> decltype(std::declval<F>(std::declval<T>)) { return f(t); }{{$}}
+template <typename T>
+typename decltype(std::declval<T>())::value_type dec3();
+// CHECK-MESSAGES: :[[@LINE-1]]:50: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto dec3() -> typename decltype(std::declval<T>())::value_type;{{$}}
+template <typename T>
+decltype(std::declval<T>())* dec4();
+// CHECK-MESSAGES: :[[@LINE-1]]:30: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto dec4() -> decltype(std::declval<T>())*;{{$}}
+
+//
+// Methods
+//
+
+struct B {
+    B& operator=(const B&);
+// CHECK-MESSAGES: :[[@LINE-1]]:8: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}    auto operator=(const B&) -> B&;{{$}}
+    
+    double base1(int, bool b);
+// CHECK-MESSAGES: :[[@LINE-1]]:12: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}    auto base1(int, bool b) -> double;{{$}}
+
+    virtual double base2(int, bool b) {}
+// CHECK-MESSAGES: :[[@LINE-1]]:20: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}    virtual auto base2(int, bool b) -> double {}{{$}}
+
+    virtual float base3() const = 0;
+// CHECK-MESSAGES: :[[@LINE-1]]:19: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}    virtual auto base3() const -> float = 0;{{$}}
+
+    virtual float base4() volatile = 0;
+// CHECK-MESSAGES: :[[@LINE-1]]:19: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}    virtual auto base4() volatile -> float = 0;{{$}}
+
+    double base5(int, bool b) &&;
+// CHECK-MESSAGES: :[[@LINE-1]]:12: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}    auto base5(int, bool b) && -> double;{{$}}
+
+    double base6(int, bool b) const &&;
+// CHECK-MESSAGES: :[[@LINE-1]]:12: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}    auto base6(int, bool b) const && -> double;{{$}}
+
+    double base7(int, bool b) const & = delete;
+// CHECK-MESSAGES: :[[@LINE-1]]:12: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}    auto base7(int, bool b) const & -> double = delete;{{$}}
+
+    double base8(int, bool b) const volatile & = delete;
+// CHECK-MESSAGES: :[[@LINE-1]]:12: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}    auto base8(int, bool b) const volatile & -> double = delete;{{$}}
+
+    virtual const char * base9() const noexcept { return ""; }
+// CHECK-MESSAGES: :[[@LINE-1]]:26: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}    virtual auto base9() const noexcept -> const char * { return ""; }{{$}}
+};
+
+double B::base1(int, bool b) {}
+// CHECK-MESSAGES: :[[@LINE-1]]:11: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto B::base1(int, bool b) -> double {}{{$}}
+
+struct D : B {
+    virtual double f1(int, bool b) final;
+// CHECK-MESSAGES: :[[@LINE-1]]:20: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}    virtual auto f1(int, bool b) -> double final;{{$}}
+
+    virtual double base2(int, bool b) override;
+// CHECK-MESSAGES: :[[@LINE-1]]:20: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}    virtual auto base2(int, bool b) -> double override;{{$}}
+
+    virtual float base3() const override final { }
+// CHECK-MESSAGES: :[[@LINE-1]]:19: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}    virtual auto base3() const -> float override final { }{{$}}
+
+    const char * base9() const noexcept override { return ""; }
+// CHECK-MESSAGES: :[[@LINE-1]]:18: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}    auto base9() const noexcept -> const char * override { return ""; }{{$}}
+};
+
+//
+// Functions with attributes
+//
+
+int g1() [[asdf]];
+// CHECK-MESSAGES: :[[@LINE-1]]:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto g1() -> int {{[[][[]}}asdf{{[]][]]}};{{$}}
+[[noreturn]] int g2();
+// CHECK-MESSAGES: :[[@LINE-1]]:18: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}{{[[][[]}}noreturn{{[]][]]}} auto g2() -> int;{{$}}
+int g2 [[noreturn]] ();
+// CHECK-MESSAGES: :[[@LINE-1]]:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto g2 {{[[][[]}}noreturn{{[]][]]}} () -> int;{{$}}
+
+//
+// Templates
+//
+template <typename Container>
+[[maybe_unused]] typename Container::value_type const volatile&& t1(Container& C) noexcept;
+// CHECK-MESSAGES: :[[@LINE-1]]:66: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}{{[[][[]}}maybe_unused{{[]][]]}} auto t1(Container& C) noexcept -> typename Container::value_type const volatile&&;{{$}}
+
+//
+// Macros
+//
+
+#define DWORD unsigned int
+DWORD h1();
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto h1() -> DWORD;{{$}}
+#define INT int
+#define UNSIGNED unsigned
+UNSIGNED INT h2();
+// CHECK-MESSAGES: :[[@LINE-1]]:14: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto h2() -> UNSIGNED INT;{{$}}
+#define CONST const
+CONST int h3();
+// CHECK-MESSAGES: :[[@LINE-1]]:11: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+#define ALWAYS_INLINE inline
+#define DLL_EXPORT __declspec(dllexport)
+ALWAYS_INLINE DLL_EXPORT int h4();
+// CHECK-MESSAGES: :[[@LINE-1]]:30: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}ALWAYS_INLINE DLL_EXPORT auto h4() -> int;{{$}}
+#define ANOTHER_ATTRIBUTE __attribute__((deprecated))
+int h5() ANOTHER_ATTRIBUTE;
+// CHECK-MESSAGES: :[[@LINE-1]]:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto h5() -> int ANOTHER_ATTRIBUTE;{{$}}
+#define FUNCTION_NAME(a, b) a##b
+int FUNCTION_NAME(foo, bar)();
+// CHECK-MESSAGES: :[[@LINE-1]]:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto FUNCTION_NAME(foo, bar)() -> int;{{$}}
+#define DEFINE_FUNCTION_1(a, b) int a##b()
+DEFINE_FUNCTION_1(foo, bar);
+// CHECK-MESSAGES: :[[@LINE-1]]:1: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+#define DEFINE_FUNCTION_2 int foo(int arg);
+DEFINE_FUNCTION_2
+// CHECK-MESSAGES: :[[@LINE-1]]:1: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+#define DLL_EXPORT_CONST __declspec(dllexport) const
+DLL_EXPORT_CONST int h6();
+// CHECK-MESSAGES: :[[@LINE-1]]:22: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+
+template <typename T>
+using Real = T;
+#define PRECISION float
+Real<PRECISION> h7() { return 0.; }
+// CHECK-MESSAGES: :[[@LINE-1]]:17: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto h7() -> Real<PRECISION> { return 0.; }{{$}}
+
+#define MAYBE_UNUSED_MACRO [[maybe_unused]]
+template <typename Container>
+MAYBE_UNUSED_MACRO typename Container::value_type const volatile** const h8(Container& C) noexcept;
+// CHECK-MESSAGES: :[[@LINE-1]]:74: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}MAYBE_UNUSED_MACRO auto h8(Container& C) noexcept -> typename Container::value_type const volatile** const;{{$}}
+
+#define NOEXCEPT noexcept
+int h9(int arg) NOEXCEPT;
+// CHECK-MESSAGES: :[[@LINE-1]]:5: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto h9(int arg) NOEXCEPT -> int;{{$}}
+
+//
+// Name collisions
+//
+struct Object { long long value; };
+
+Object j1(unsigned Object) { return {Object * 2}; }
+// CHECK-MESSAGES: :[[@LINE-1]]:8: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}Object j1(unsigned Object) { return {Object * 2}; }{{$}}
+::Object j1(unsigned Object);
+// CHECK-MESSAGES: :[[@LINE-1]]:10: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto j1(unsigned Object) -> ::Object;{{$}}
+const Object& j2(unsigned a, int b, char Object, long l);
+// CHECK-MESSAGES: :[[@LINE-1]]:15: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}const Object& j2(unsigned a, int b, char Object, long l);{{$}}
+const struct Object& j2(unsigned a, int b, char Object, long l);
+// CHECK-MESSAGES: :[[@LINE-1]]:22: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto j2(unsigned a, int b, char Object, long l) -> const struct Object&;{{$}}
+std::vector<Object> j3(unsigned Object);
+// CHECK-MESSAGES: :[[@LINE-1]]:21: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}std::vector<Object> j3(unsigned Object);{{$}}
+std::vector<const Object> j7(unsigned Object);
+// CHECK-MESSAGES: :[[@LINE-1]]:27: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}std::vector<const Object> j7(unsigned Object);{{$}}
+std::vector<Object> j4(unsigned vector);
+// CHECK-MESSAGES: :[[@LINE-1]]:21: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto j4(unsigned vector) -> std::vector<Object>;{{$}}
+std::vector<::Object> j4(unsigned vector);
+// CHECK-MESSAGES: :[[@LINE-1]]:23: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto j4(unsigned vector) -> std::vector<::Object>;{{$}}
+std::vector<struct Object> j4(unsigned vector);
+// CHECK-MESSAGES: :[[@LINE-1]]:28: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto j4(unsigned vector) -> std::vector<struct Object>;{{$}}
+std::vector<Object> j4(unsigned Vector);
+// CHECK-MESSAGES: :[[@LINE-1]]:21: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto j4(unsigned Vector) -> std::vector<Object>;{{$}}
+using std::vector;
+vector<Object> j5(unsigned vector);
+// CHECK-MESSAGES: :[[@LINE-1]]:16: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}vector<Object> j5(unsigned vector);{{$}}
+constexpr auto Size = 5;
+std::array<int, Size> j6(unsigned Size);
+// CHECK-MESSAGES: :[[@LINE-1]]:23: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}std::array<int, Size> j6(unsigned Size);{{$}}
+std::array<decltype(Size), (Size * 2) + 1> j8(unsigned Size);
+// CHECK-MESSAGES: :[[@LINE-1]]:44: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}std::array<decltype(Size), (Size * 2) + 1> j8(unsigned Size);{{$}}
+
+class CC {
+    int Object;
+    struct Object m();
+// CHECK-MESSAGES: :[[@LINE-1]]:19: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}    auto m() -> struct Object;{{$}}
+};
+Object CC::m() { return {0}; }
+// CHECK-MESSAGES: :[[@LINE-1]]:12: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto CC::m() -> Object { return {0}; }{{$}}
+class DD : public CC {
+    ::Object g();
+// CHECK-MESSAGES: :[[@LINE-1]]:14: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}    auto g() -> ::Object;{{$}}
+};
+Object DD::g() {
+// CHECK-MESSAGES: :[[@LINE-1]]:12: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES: {{^}}auto DD::g() -> Object {{{$}}
+    return {0};
+}
+
+
+//
+// Samples which do not trigger the check
+//
+
+auto f() -> int;
+auto f(int) -> int;
+auto f(int arg) -> int;
+auto f(int arg1, int arg2, int arg3) -> int;
+auto f(int arg1, int arg2, int arg3, ...) -> int;
+template <typename T> auto f(T t) -> int;
+
+auto ff();
+decltype(auto) fff();
+
+void c();
+void c(int arg);
+void c(int arg) { return; }
+
+struct D2 : B {
+    D2();
+    virtual ~D2();
+    
+    virtual auto f1(int, bool b) -> double final;
+    virtual auto base2(int, bool b) -> double override;
+    virtual auto base3() const -> float override final { }
+
+    operator double();
+};
+
+auto l1 = [](int arg) {};
+auto l2 = [](int arg) -> double {};