[clang-tidy] `modernize-use-trailing-return-type`: add an option to apply to `void`-returning functions as well #129406

khuldraeseth · 2025-03-01T23:54:55Z

Adds an option modernize-use-trailing-return-type.WarnOnNontrailingVoid that when true enforces this check for declarations like void foo();. Default is false to preserve existing behavior.

github-actions · 2025-03-01T23:55:13Z

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llvmbot · 2025-03-01T23:55:45Z

@llvm/pr-subscribers-clang-tools-extra

@llvm/pr-subscribers-clang-tidy

Author: None (khuldraeseth)

Changes

Adds an option modernize-use-trailing-return-type.EvenWhenVoid that when true enforces this check for declarations like void foo();. Default is false to preserve existing behavior.

Full diff: https://github.com/llvm/llvm-project/pull/129406.diff

5 Files Affected:

(modified) clang-tools-extra/clang-tidy/modernize/UseTrailingReturnTypeCheck.cpp (+11-5)
(modified) clang-tools-extra/clang-tidy/modernize/UseTrailingReturnTypeCheck.h (+7-1)
(modified) clang-tools-extra/docs/ReleaseNotes.rst (+5)
(modified) clang-tools-extra/docs/clang-tidy/checks/modernize/use-trailing-return-type.rst (+8)
(modified) clang-tools-extra/test/clang-tidy/checkers/modernize/use-trailing-return-type.cpp (+27-5)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseTrailingReturnTypeCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseTrailingReturnTypeCheck.cpp
index 9774e988d71e2..12448303fdcd8 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseTrailingReturnTypeCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseTrailingReturnTypeCheck.cpp
@@ -8,8 +8,10 @@
 
 #include "UseTrailingReturnTypeCheck.h"
 #include "clang/AST/ASTContext.h"
+#include "clang/AST/DeclCXX.h"
 #include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
 #include "clang/Lex/Preprocessor.h"
 #include "clang/Tooling/FixIt.h"
 #include "llvm/ADT/StringExtras.h"
@@ -287,7 +289,6 @@ SourceRange UseTrailingReturnTypeCheck::findReturnTypeAndCVSourceRange(
     return {};
   }
 
-
   // If the return type has no local qualifiers, it's source range is accurate.
   if (!hasAnyNestedLocalQualifiers(F.getReturnType()))
     return ReturnTypeRange;
@@ -384,10 +385,15 @@ void UseTrailingReturnTypeCheck::keepSpecifiers(
 }
 
 void UseTrailingReturnTypeCheck::registerMatchers(MatchFinder *Finder) {
-  auto F = functionDecl(
-               unless(anyOf(hasTrailingReturn(), returns(voidType()),
-                            cxxConversionDecl(), cxxMethodDecl(isImplicit()))))
-               .bind("Func");
+  const auto hasNoWrittenReturnType =
+      anyOf(cxxConversionDecl(), cxxConstructorDecl(), cxxDestructorDecl(),
+            cxxMethodDecl(isImplicit()));
+
+  const auto where = functionDecl(
+      unless(anyOf(hasTrailingReturn(), hasNoWrittenReturnType,
+                   EvenWhenVoid ? unless(anything()) : returns(voidType()))));
+
+  auto F = where.bind("Func");
 
   Finder->addMatcher(F, this);
   Finder->addMatcher(friendDecl(hasDescendant(F)).bind("Friend"), this);
diff --git a/clang-tools-extra/clang-tidy/modernize/UseTrailingReturnTypeCheck.h b/clang-tools-extra/clang-tidy/modernize/UseTrailingReturnTypeCheck.h
index 5fb6ae945f466..fdd8be285c03b 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseTrailingReturnTypeCheck.h
+++ b/clang-tools-extra/clang-tidy/modernize/UseTrailingReturnTypeCheck.h
@@ -28,7 +28,8 @@ struct ClassifiedToken {
 class UseTrailingReturnTypeCheck : public ClangTidyCheck {
 public:
   UseTrailingReturnTypeCheck(StringRef Name, ClangTidyContext *Context)
-      : ClangTidyCheck(Name, Context) {}
+      : ClangTidyCheck(Name, Context),
+        EvenWhenVoid(Options.get("EvenWhenVoid", false)) {}
   bool isLanguageVersionSupported(const LangOptions &LangOpts) const override {
     return LangOpts.CPlusPlus11;
   }
@@ -39,6 +40,7 @@ class UseTrailingReturnTypeCheck : public ClangTidyCheck {
 
 private:
   Preprocessor *PP = nullptr;
+  bool const EvenWhenVoid;
 
   SourceLocation findTrailingReturnTypeSourceLocation(
       const FunctionDecl &F, const FunctionTypeLoc &FTL, const ASTContext &Ctx,
@@ -56,6 +58,10 @@ class UseTrailingReturnTypeCheck : public ClangTidyCheck {
                       SourceRange ReturnTypeCVRange, const FunctionDecl &F,
                       const FriendDecl *Fr, const ASTContext &Ctx,
                       const SourceManager &SM, const LangOptions &LangOpts);
+
+  auto storeOptions(ClangTidyOptions::OptionMap &Opts) -> void override {
+    Options.store(Opts, "EvenWhenVoid", EvenWhenVoid);
+  }
 };
 
 } // namespace clang::tidy::modernize
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 2dcefa2ddec83..93b2202ce8635 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -120,6 +120,11 @@ Changes in existing checks
   tolerating fix-it breaking compilation when functions is used as pointers 
   to avoid matching usage of functions within the current compilation unit.
 
+- Improved :doc:`modernize/use-trailing-return-type
+  <clang-tidy/checks/modernize/use-trailing-return-type>` check by adding the
+  option ``EvenWhenVoid`` that applies the check to ``void``-returning functions
+  that by default are excluded from this check.
+
 Removed checks
 ^^^^^^^^^^^^^^
 
diff --git a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-trailing-return-type.rst b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-trailing-return-type.rst
index 0593a35326aaa..1187c931fca43 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-trailing-return-type.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-trailing-return-type.rst
@@ -25,6 +25,14 @@ transforms to:
   inline auto f2(int arg) -> int noexcept;
   virtual auto f3() const && -> float = delete;
 
+Options
+-------
+
+.. option:: EvenWhenVoid
+
+  If the option is set to `true`, the check will apply even to function signatures
+  with return type `void`. Default is `false`.
+
 Known Limitations
 -----------------
 
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-trailing-return-type.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-trailing-return-type.cpp
index e1f36c52a7c01..d8bef1b64f217 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-trailing-return-type.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-trailing-return-type.cpp
@@ -1,4 +1,12 @@
-// RUN: %check_clang_tidy -std=c++14-or-later %s modernize-use-trailing-return-type %t -- -- -fdeclspec -fexceptions -DCOMMAND_LINE_INT=int
+// RUN: %check_clang_tidy                                                   \
+// RUN:   -std=c++14-or-later %s modernize-use-trailing-return-type %t --   \
+// RUN:   -- -fdeclspec -fexceptions -DCOMMAND_LINE_INT=int
+// RUN: %check_clang_tidy -check-suffixes=,EVEN-WHEN-VOID                   \
+// RUN:   -std=c++14-or-later %s modernize-use-trailing-return-type %t --   \
+// RUN:   -config="{CheckOptions: {                                         \
+// RUN:              modernize-use-trailing-return-type.EvenWhenVoid: true, \
+// RUN:            }}"                                                      \
+// RUN:   -- -fdeclspec -fexceptions -DCOMMAND_LINE_INT=int
 
 namespace std {
     template <typename T>
@@ -566,6 +574,24 @@ ostream& operator<<(ostream& ostream, int i);
 // CHECK-MESSAGES: :[[@LINE-1]]:10: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
 // CHECK-FIXES: {{^}}ostream& operator<<(ostream& ostream, int i);{{$}}
 
+//
+// EvenWhenVoid option
+//
+
+void leadingVoid();
+// CHECK-MESSAGES-EVEN-WHEN-VOID: :[[@LINE-1]]:6: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES-EVEN-WHEN-VOID: {{^}}auto leadingVoid() -> void;{{$}}
+void leadingVoid(int arg);
+// CHECK-MESSAGES-EVEN-WHEN-VOID: :[[@LINE-1]]:6: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES-EVEN-WHEN-VOID: {{^}}auto leadingVoid(int arg) -> void;{{$}}
+void leadingVoid(int arg) { return; }
+// CHECK-MESSAGES-EVEN-WHEN-VOID: :[[@LINE-1]]:6: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
+// CHECK-FIXES-EVEN-WHEN-VOID: {{^}}auto leadingVoid(int arg) -> void { return; }{{$}}
+
+auto trailingVoid() -> void;
+auto trailingVoid(int arg) -> void;
+auto trailingVoid(int arg) -> void { return; }
+
 //
 // Samples which do not trigger the check
 //
@@ -579,10 +605,6 @@ template <typename T> auto f(T t) -> int;
 
 auto ff();
 
-void c();
-void c(int arg);
-void c(int arg) { return; }
-
 struct D2 : B {
     D2();
     virtual ~D2();

clang-tools-extra/docs/ReleaseNotes.rst

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

clang-tools-extra/docs/ReleaseNotes.rst

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

vbvictor · 2025-03-02T18:00:33Z

Consider renaming your option EvenOnVoid to a name that has WarnOn prefix, that is more or less
established approach for naming (see other option names) . In your case I think it's better to name WarnOnVoidFunctions.

khuldraeseth · 2025-03-02T18:07:06Z

Consider renaming your option EvenOnVoid to a name that has WarnOn prefix, that is more or less established approach for naming (see other option names) . In your case I think it's better to name WarnOnVoidFunctions.

How's WarnOnLeadingVoid or WarnOnNontrailingVoid? WarnOnVoidFunctions suggests different behavior to me.

vbvictor · 2025-03-02T18:35:30Z

Consider renaming your option EvenOnVoid to a name that has WarnOn prefix, that is more or less established approach for naming (see other option names) . In your case I think it's better to name WarnOnVoidFunctions.

How's WarnOnLeadingVoid or WarnOnNontrailingVoid? WarnOnVoidFunctions suggests different behavior to me.

WarnOnNontrailingVoid is also good in my opinion

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

HerrCai0907

LGTM

vbvictor

LGTM, few nits

vbvictor · 2025-03-09T12:40:22Z

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

+
+.. option:: WarnOnNontrailingVoid
+
+  If the option is set to `true`, the check will apply even to function signatures


Please, make lines no more than 80 characters long.

vbvictor · 2025-03-09T12:41:23Z

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

+// RUN: %check_clang_tidy                                                            \
+// RUN:   -std=c++14-or-later %s modernize-use-trailing-return-type %t --            \
+// RUN:   -- -fdeclspec -fexceptions -DCOMMAND_LINE_INT=int
+// RUN: %check_clang_tidy -check-suffixes=,EVEN-WHEN-VOID                            \


Please, use new suffix, not EVEN-WHEN-VOID

PiotrZSL

Except current comments, looks fine.

PiotrZSL · 2025-03-10T18:34:28Z

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

@@ -28,7 +28,8 @@ struct ClassifiedToken {
 class UseTrailingReturnTypeCheck : public ClangTidyCheck {
 public:
  UseTrailingReturnTypeCheck(StringRef Name, ClangTidyContext *Context)
-      : ClangTidyCheck(Name, Context) {}
+      : ClangTidyCheck(Name, Context),
+        WarnOnNontrailingVoid(Options.get("WarnOnNontrailingVoid", false)) {}


shoudn't be WarnOnNonTrailingVoid ?

PiotrZSL · 2025-03-10T18:35:03Z

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

@@ -494,4 +500,9 @@ void UseTrailingReturnTypeCheck::check(const MatchFinder::MatchResult &Result) {
      << FixItHint::CreateInsertion(InsertionLoc, " -> " + ReturnType);
 }

+auto UseTrailingReturnTypeCheck::storeOptions(ClangTidyOptions::OptionMap &Opts)
+    -> void {


follow exist style = no trailing return types.

PiotrZSL · 2025-03-10T18:38:39Z

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

-               .bind("Func");
+  const auto HasNoWrittenReturnType =
+      anyOf(cxxConversionDecl(), cxxConstructorDecl(), cxxDestructorDecl(),
+            cxxMethodDecl(isImplicit()));


Note: This check is missing TK_IgnoreUnlessSpelledInSource, in such case those isImplicit woudn't be needed, and check could be faster.

PiotrZSL · 2025-03-10T18:39:09Z

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

@@ -56,6 +58,8 @@ class UseTrailingReturnTypeCheck : public ClangTidyCheck {
                      SourceRange ReturnTypeCVRange, const FunctionDecl &F,
                      const FriendDecl *Fr, const ASTContext &Ctx,
                      const SourceManager &SM, const LangOptions &LangOpts);
+
+  auto storeOptions(ClangTidyOptions::OptionMap &Opts) -> void override;


follow exist style = no trailing return types.

vbvictor · 2025-06-04T10:38:34Z

@khuldraeseth, gentle ping, do you still wish to work on this PR? 21th LLVM release will be soon, If you wish to have your changes in the upcoming release please rebase on main and fix suggested issues

khuldraeseth added 5 commits March 1, 2025 15:25

add EvenWhenVoid option to modernize-use-trailing-return-type check

cd7e8c6

document EvenWhenVoid check option

305cbb6

update modernize-use-trailing-return-type tests for EvenWhenVoid

a0757c0

fix ctor/dtor false positive with EvenWhenVoid

ee659eb

update release notes

57cbd50

llvmbot added clang-tools-extra clang-tidy labels Mar 1, 2025

EugeneZelenko requested review from PiotrZSL, 5chmidti and HerrCai0907 March 2, 2025 15:34

EugeneZelenko reviewed Mar 2, 2025

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alphabetize release notes

b268de6

EugeneZelenko reviewed Mar 2, 2025

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clang-tools-extra/docs/ReleaseNotes.rst Outdated Show resolved Hide resolved

fix check name in release notes

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