[clang-tidy] Add support for lambda-expression in use-trailing-return-type check (#135383)

Add support for lambda-expression in `use-trailing-return-type` check.

Added two new options:
1. `TransformFunctions` will trigger function declarations to use
trailing return type.
2. `TransformLambdas` will trigger lambda expression to use trailing
return type if it was not stated explicitly.

Fixed false positives when lambda was matched as a function in C++11
mode.

Closes #95711

Author: vbvictor

clang-tools-extra/clang-tidy/modernize/UseTrailingReturnTypeCheck.cpp

@@ -17,6 +17,30 @@
 #include <cctype>
 #include <optional>
 
+namespace clang::tidy {
+
+template <>
+struct OptionEnumMapping<
+    modernize::UseTrailingReturnTypeCheck::TransformLambda> {
+  static llvm::ArrayRef<std::pair<
+      modernize::UseTrailingReturnTypeCheck::TransformLambda, StringRef>>
+  getEnumMapping() {
+    static constexpr std::pair<
+        modernize::UseTrailingReturnTypeCheck::TransformLambda, StringRef>
+        Mapping[] = {
+            {modernize::UseTrailingReturnTypeCheck::TransformLambda::All,
+             "all"},
+            {modernize::UseTrailingReturnTypeCheck::TransformLambda::
+                 AllExceptAuto,
+             "all_except_auto"},
+            {modernize::UseTrailingReturnTypeCheck::TransformLambda::None,
+             "none"}};
+    return Mapping;
+  }
+};
+
+} // namespace clang::tidy
+
 using namespace clang::ast_matchers;
 
 namespace clang::tidy::modernize {
@@ -111,10 +135,17 @@ struct UnqualNameVisitor : public RecursiveASTVisitor<UnqualNameVisitor> {
 private:
   const FunctionDecl &F;
 };
+
+AST_MATCHER(LambdaExpr, hasExplicitResultType) {
+  return Node.hasExplicitResultType();
+}
+
 } // namespace
 
 constexpr llvm::StringLiteral ErrorMessageOnFunction =
     "use a trailing return type for this function";
+constexpr llvm::StringLiteral ErrorMessageOnLambda =
+    "use a trailing return type for this lambda";
 
 static SourceLocation expandIfMacroId(SourceLocation Loc,
                                       const SourceManager &SM) {
@@ -329,6 +360,48 @@ findReturnTypeAndCVSourceRange(const FunctionDecl &F, const TypeLoc &ReturnLoc,
   return ReturnTypeRange;
 }
 
+static SourceLocation findLambdaTrailingReturnInsertLoc(
+    const CXXMethodDecl *Method, const SourceManager &SM,
+    const LangOptions &LangOpts, const ASTContext &Ctx) {
+  // 'requires' keyword is present in lambda declaration
+  if (Method->getTrailingRequiresClause()) {
+    SourceLocation ParamEndLoc;
+    if (Method->param_empty())
+      ParamEndLoc = Method->getBeginLoc();
+    else
+      ParamEndLoc = Method->getParametersSourceRange().getEnd();
+
+    std::pair<FileID, unsigned> ParamEndLocInfo =
+        SM.getDecomposedLoc(ParamEndLoc);
+    StringRef Buffer = SM.getBufferData(ParamEndLocInfo.first);
+
+    Lexer Lexer(SM.getLocForStartOfFile(ParamEndLocInfo.first), LangOpts,
+                Buffer.begin(), Buffer.data() + ParamEndLocInfo.second,
+                Buffer.end());
+
+    Token Token;
+    while (!Lexer.LexFromRawLexer(Token)) {
+      if (Token.is(tok::raw_identifier)) {
+        IdentifierInfo &Info = Ctx.Idents.get(StringRef(
+            SM.getCharacterData(Token.getLocation()), Token.getLength()));
+        Token.setIdentifierInfo(&Info);
+        Token.setKind(Info.getTokenID());
+      }
+
+      if (Token.is(tok::kw_requires))
+        return Token.getLocation().getLocWithOffset(-1);
+    }
+
+    return {};
+  }
+
+  // If no requires clause, insert before the body
+  if (const Stmt *Body = Method->getBody())
+    return Body->getBeginLoc().getLocWithOffset(-1);
+
+  return {};
+}
+
 static void keepSpecifiers(std::string &ReturnType, std::string &Auto,
                            SourceRange ReturnTypeCVRange, const FunctionDecl &F,
                            const FriendDecl *Fr, const ASTContext &Ctx,
@@ -382,14 +455,43 @@ static void keepSpecifiers(std::string &ReturnType, std::string &Auto,
   }
 }
 
+UseTrailingReturnTypeCheck::UseTrailingReturnTypeCheck(
+    StringRef Name, ClangTidyContext *Context)
+    : ClangTidyCheck(Name, Context),
+      TransformFunctions(Options.get("TransformFunctions", true)),
+      TransformLambdas(Options.get("TransformLambdas", TransformLambda::All)) {
+
+  if (TransformFunctions == false && TransformLambdas == TransformLambda::None)
+    this->configurationDiag(
+        "The check 'modernize-use-trailing-return-type' will not perform any "
+        "analysis because 'TransformFunctions' and 'TransformLambdas' are "
+        "disabled.");
+}
+
+void UseTrailingReturnTypeCheck::storeOptions(
+    ClangTidyOptions::OptionMap &Opts) {
+  Options.store(Opts, "TransformFunctions", TransformFunctions);
+  Options.store(Opts, "TransformLambdas", TransformLambdas);
+}
+
 void UseTrailingReturnTypeCheck::registerMatchers(MatchFinder *Finder) {
-  auto F = functionDecl(
-               unless(anyOf(hasTrailingReturn(), returns(voidType()),
-                            cxxConversionDecl(), cxxMethodDecl(isImplicit()))))
-               .bind("Func");
+  auto F =
+      functionDecl(
+          unless(anyOf(
+              hasTrailingReturn(), returns(voidType()), cxxConversionDecl(),
+              cxxMethodDecl(
+                  anyOf(isImplicit(),
+                        hasParent(cxxRecordDecl(hasParent(lambdaExpr()))))))))
+          .bind("Func");
+
+  if (TransformFunctions) {
+    Finder->addMatcher(F, this);
+    Finder->addMatcher(friendDecl(hasDescendant(F)).bind("Friend"), this);
+  }
 
-  Finder->addMatcher(F, this);
-  Finder->addMatcher(friendDecl(hasDescendant(F)).bind("Friend"), this);
+  if (TransformLambdas != TransformLambda::None)
+    Finder->addMatcher(
+        lambdaExpr(unless(hasExplicitResultType())).bind("Lambda"), this);
 }
 
 void UseTrailingReturnTypeCheck::registerPPCallbacks(
@@ -401,8 +503,13 @@ void UseTrailingReturnTypeCheck::check(const MatchFinder::MatchResult &Result) {
   assert(PP && "Expected registerPPCallbacks() to have been called before so "
                "preprocessor is available");
 
-  const auto *F = Result.Nodes.getNodeAs<FunctionDecl>("Func");
+  if (const auto *Lambda = Result.Nodes.getNodeAs<LambdaExpr>("Lambda")) {
+    diagOnLambda(Lambda, Result);
+    return;
+  }
+
   const auto *Fr = Result.Nodes.getNodeAs<FriendDecl>("Friend");
+  const auto *F = Result.Nodes.getNodeAs<FunctionDecl>("Func");
   assert(F && "Matcher is expected to find only FunctionDecls");
 
   // Three-way comparison operator<=> is syntactic sugar and generates implicit
@@ -495,4 +602,41 @@ void UseTrailingReturnTypeCheck::check(const MatchFinder::MatchResult &Result) {
       << FixItHint::CreateInsertion(InsertionLoc, " -> " + ReturnType);
 }
 
+void UseTrailingReturnTypeCheck::diagOnLambda(
+    const LambdaExpr *Lambda,
+    const ast_matchers::MatchFinder::MatchResult &Result) {
+
+  const CXXMethodDecl *Method = Lambda->getCallOperator();
+  if (!Method || Lambda->hasExplicitResultType())
+    return;
+
+  const ASTContext *Ctx = Result.Context;
+  const QualType ReturnType = Method->getReturnType();
+
+  // We can't write 'auto' in C++11 mode, try to write generic msg and bail out.
+  if (ReturnType->isDependentType() &&
+      Ctx->getLangOpts().LangStd == LangStandard::lang_cxx11) {
+    if (TransformLambdas == TransformLambda::All)
+      diag(Lambda->getBeginLoc(), ErrorMessageOnLambda);
+    return;
+  }
+
+  if (ReturnType->isUndeducedAutoType() &&
+      TransformLambdas == TransformLambda::AllExceptAuto)
+    return;
+
+  const SourceLocation TrailingReturnInsertLoc =
+      findLambdaTrailingReturnInsertLoc(Method, *Result.SourceManager,
+                                        getLangOpts(), *Result.Context);
+
+  if (TrailingReturnInsertLoc.isValid())
+    diag(Lambda->getBeginLoc(), "use a trailing return type for this lambda")
+        << FixItHint::CreateInsertion(
+               TrailingReturnInsertLoc,
+               " -> " +
+                   ReturnType.getAsString(Result.Context->getPrintingPolicy()));
+  else
+    diag(Lambda->getBeginLoc(), ErrorMessageOnLambda);
+}
+
 } // namespace clang::tidy::modernize

clang-tools-extra/clang-tidy/modernize/UseTrailingReturnTypeCheck.h

@@ -26,18 +26,25 @@ struct ClassifiedToken {
 /// http://clang.llvm.org/extra/clang-tidy/checks/modernize/use-trailing-return-type.html
 class UseTrailingReturnTypeCheck : public ClangTidyCheck {
 public:
-  UseTrailingReturnTypeCheck(StringRef Name, ClangTidyContext *Context)
-      : ClangTidyCheck(Name, Context) {}
+  UseTrailingReturnTypeCheck(StringRef Name, ClangTidyContext *Context);
   bool isLanguageVersionSupported(const LangOptions &LangOpts) const override {
     return LangOpts.CPlusPlus11;
   }
+  void storeOptions(ClangTidyOptions::OptionMap &Opts) override;
   void registerMatchers(ast_matchers::MatchFinder *Finder) override;
   void registerPPCallbacks(const SourceManager &SM, Preprocessor *PP,
                            Preprocessor *ModuleExpanderPP) override;
   void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
 
+  enum TransformLambda { All, AllExceptAuto, None };
+
 private:
   Preprocessor *PP = nullptr;
+  const bool TransformFunctions;
+  const TransformLambda TransformLambdas;
+
+  void diagOnLambda(const LambdaExpr *Lambda,
+                    const ast_matchers::MatchFinder::MatchResult &Result);
 };
 
 } // namespace clang::tidy::modernize

clang-tools-extra/docs/ReleaseNotes.rst

@@ -236,6 +236,13 @@ Changes in existing checks
   <clang-tidy/checks/modernize/use-std-numbers>` check to support math
   functions of different precisions.
 
+- Improved :doc:`modernize-use-trailing-return-type
+  <clang-tidy/checks/modernize/use-trailing-return-type>` check by adding
+  support to modernize lambda signatures to use trailing return type and adding
+  two new options: `TransformFunctions` and `TransformLambdas` to control
+  whether function declarations and lambdas should be transformed by the check.
+  Fixed false positives when lambda was matched as a function in C++11 mode.
+
 - Improved :doc:`performance-move-const-arg
   <clang-tidy/checks/performance/move-const-arg>` check by fixing false
   negatives on ternary operators calling ``std::move``.

clang-tools-extra/docs/clang-tidy/checks/modernize/use-trailing-return-type.rst

@@ -3,7 +3,7 @@
 modernize-use-trailing-return-type
 ==================================
 
-Rewrites function signatures to use a trailing return type
+Rewrites function and lambda 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).
@@ -16,6 +16,7 @@ Example
   int f1();
   inline int f2(int arg) noexcept;
   virtual float f3() const && = delete;
+  auto lambda = []() {};
 
 transforms to:
 
@@ -24,6 +25,7 @@ transforms to:
   auto f1() -> int;
   inline auto f2(int arg) -> int noexcept;
   virtual auto f3() const && -> float = delete;
+  auto lambda = []() -> void {};
 
 Known Limitations
 -----------------
@@ -66,3 +68,25 @@ a careless rewrite would produce the following output:
 This code fails to compile because the S in the context of f refers to the equally named function parameter.
 Similarly, the S in the context of m refers to the equally named class member.
 The check can currently only detect and avoid a clash with a function parameter name.
+
+Options
+-------
+
+.. option:: TransformFunctions
+
+  When set to `true`, function declarations will be transformed to use trailing
+  return. Default is `true`.
+
+.. option:: TransformLambdas
+
+  Controls how lambda expressions are transformed to use trailing
+  return type. Possible values are:
+
+  * `all` - Transform all lambda expressions without an explicit return type
+    to use trailing return type. If type can not be deduced, ``auto`` will be
+    used since C++14 and generic message will be emitted otherwise.
+  * `all_except_auto` - Transform all lambda expressions except those whose return
+    type can not be deduced.
+  * `none` - Do not transform any lambda expressions.
+
+  Default is `all`.

clang-tools-extra/test/clang-tidy/checkers/modernize/use-trailing-return-type-transform-lambdas-cxx14.cpp

@@ -0,0 +1,24 @@
+// RUN: %check_clang_tidy -std=c++14-or-later %s modernize-use-trailing-return-type %t -- -- -fno-delayed-template-parsing
+
+namespace std {
+    template <typename T>
+    class vector {};
+
+    class string {};
+} // namespace std
+
+void test_lambda_positive() {
+  auto l1 = [](auto x) { return x; };
+  // CHECK-MESSAGES: :[[@LINE-1]]:13: warning: use a trailing return type for this lambda [modernize-use-trailing-return-type]
+  // CHECK-FIXES: {{^}}  auto l1 = [](auto x) -> auto { return x; };{{$}}
+}
+
+template <template <typename> class C>
+void test_lambda_positive_template() {
+  auto l1 = []() { return C<int>{}; };
+  // CHECK-MESSAGES: :[[@LINE-1]]:13: warning: use a trailing return type for this lambda [modernize-use-trailing-return-type]
+  // CHECK-FIXES: {{^}}  auto l1 = []() -> auto { return C<int>{}; };{{$}}
+  auto l2 = []() { return 0; };
+  // CHECK-MESSAGES: :[[@LINE-1]]:13: warning: use a trailing return type for this lambda [modernize-use-trailing-return-type]
+  // CHECK-FIXES: {{^}}  auto l2 = []() -> auto { return 0; };{{$}}
+}

clang-tools-extra/test/clang-tidy/checkers/modernize/use-trailing-return-type-transform-lambdas-cxx20.cpp

@@ -0,0 +1,35 @@
+// RUN: %check_clang_tidy -std=c++20-or-later %s modernize-use-trailing-return-type %t -- -- -fno-delayed-template-parsing
+
+namespace std {
+template <typename T, typename U>
+struct is_same { static constexpr auto value = false; };
+
+template <typename T>
+struct is_same<T, T> { static constexpr auto value = true; };
+
+template <typename T>
+concept floating_point = std::is_same<T, float>::value || std::is_same<T, double>::value || std::is_same<T, long double>::value;
+}
+
+void test_lambda_positive() {
+  auto l1 = []<typename T, typename U>(T x, U y) { return x + y; };
+  // CHECK-MESSAGES: :[[@LINE-1]]:13: warning: use a trailing return type for this lambda [modernize-use-trailing-return-type]
+  // CHECK-FIXES: {{^}}  auto l1 = []<typename T, typename U>(T x, U y) -> auto { return x + y; };{{$}}
+  auto l2 = [](auto x) requires requires { x + x; } { return x; };
+  // CHECK-MESSAGES: :[[@LINE-1]]:13: warning: use a trailing return type for this lambda [modernize-use-trailing-return-type]
+  // CHECK-FIXES: {{^}}  auto l2 = [](auto x) -> auto requires requires { x + x; } { return x; };{{$}}
+  auto l3 = [](auto x) requires std::floating_point<decltype(x)> { return x; };
+  // CHECK-MESSAGES: :[[@LINE-1]]:13: warning: use a trailing return type for this lambda [modernize-use-trailing-return-type]
+  // CHECK-FIXES: {{^}}  auto l3 = [](auto x) -> auto requires std::floating_point<decltype(x)> { return x; };{{$}}
+  auto l4 = [](int x) consteval { return x; };
+  // CHECK-MESSAGES: :[[@LINE-1]]:13: warning: use a trailing return type for this lambda [modernize-use-trailing-return-type]
+  // CHECK-FIXES: {{^}}  auto l4 = [](int x) consteval -> int { return x; };{{$}}
+  // Complete complex example
+  auto l5 = []<typename T, typename U>(T x, U y) constexpr noexcept
+    requires std::floating_point<T> && std::floating_point<U>
+  { return x * y; };
+  // CHECK-MESSAGES: :[[@LINE-3]]:13: warning: use a trailing return type for this lambda [modernize-use-trailing-return-type]
+  // CHECK-FIXES: {{^}}  auto l5 = []<typename T, typename U>(T x, U y) constexpr noexcept{{$}}
+  // CHECK-FIXES: {{^}}    -> auto requires std::floating_point<T> && std::floating_point<U>{{$}}
+  // CHECK-FIXES: {{^}}  { return x * y; };{{$}}
+}