[clang-tools-extra] Add check 'modernize-use-enum-class' (PR #138282)
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Fri May 2 07:18:15 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang-tidy
Author: Philipp Jung (JungPhilipp)
<details>
<summary>Changes</summary>
Warn on non-class enum definitions as suggested by the Core Guidelines: https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Renum-class
---
Full diff: https://github.com/llvm/llvm-project/pull/138282.diff
8 Files Affected:
- (modified) clang-tools-extra/clang-tidy/modernize/CMakeLists.txt (+1)
- (modified) clang-tools-extra/clang-tidy/modernize/ModernizeTidyModule.cpp (+2)
- (added) clang-tools-extra/clang-tidy/modernize/UseEnumClassCheck.cpp (+34)
- (added) clang-tools-extra/clang-tidy/modernize/UseEnumClassCheck.h (+34)
- (modified) clang-tools-extra/docs/ReleaseNotes.rst (+5)
- (modified) clang-tools-extra/docs/clang-tidy/checks/list.rst (+1)
- (added) clang-tools-extra/docs/clang-tidy/checks/modernize/use-enum-class.rst (+26)
- (added) clang-tools-extra/test/clang-tidy/checkers/modernize/use-enum-class.cpp (+58)
``````````diff
diff --git a/clang-tools-extra/clang-tidy/modernize/CMakeLists.txt b/clang-tools-extra/clang-tidy/modernize/CMakeLists.txt
index 4f68c487cac9d..92e6866c5f17b 100644
--- a/clang-tools-extra/clang-tidy/modernize/CMakeLists.txt
+++ b/clang-tools-extra/clang-tidy/modernize/CMakeLists.txt
@@ -34,6 +34,7 @@ add_clang_library(clangTidyModernizeModule
UseDefaultMemberInitCheck.cpp
UseDesignatedInitializersCheck.cpp
UseEmplaceCheck.cpp
+ UseEnumClassCheck.cpp
UseEqualsDefaultCheck.cpp
UseEqualsDeleteCheck.cpp
UseNodiscardCheck.cpp
diff --git a/clang-tools-extra/clang-tidy/modernize/ModernizeTidyModule.cpp b/clang-tools-extra/clang-tidy/modernize/ModernizeTidyModule.cpp
index 1860759332063..30e6938dbd7db 100644
--- a/clang-tools-extra/clang-tidy/modernize/ModernizeTidyModule.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/ModernizeTidyModule.cpp
@@ -35,6 +35,7 @@
#include "UseDefaultMemberInitCheck.h"
#include "UseDesignatedInitializersCheck.h"
#include "UseEmplaceCheck.h"
+#include "UseEnumClassCheck.h"
#include "UseEqualsDefaultCheck.h"
#include "UseEqualsDeleteCheck.h"
#include "UseNodiscardCheck.h"
@@ -107,6 +108,7 @@ class ModernizeModule : public ClangTidyModule {
CheckFactories.registerCheck<UseDefaultMemberInitCheck>(
"modernize-use-default-member-init");
CheckFactories.registerCheck<UseEmplaceCheck>("modernize-use-emplace");
+ CheckFactories.registerCheck<UseEnumClassCheck>("modernize-use-enum-class");
CheckFactories.registerCheck<UseEqualsDefaultCheck>("modernize-use-equals-default");
CheckFactories.registerCheck<UseEqualsDeleteCheck>(
"modernize-use-equals-delete");
diff --git a/clang-tools-extra/clang-tidy/modernize/UseEnumClassCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseEnumClassCheck.cpp
new file mode 100644
index 0000000000000..9fc3614aaf498
--- /dev/null
+++ b/clang-tools-extra/clang-tidy/modernize/UseEnumClassCheck.cpp
@@ -0,0 +1,34 @@
+//===--- UseEnumClassCheck.cpp - clang-tidy -------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "UseEnumClassCheck.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::modernize {
+
+void UseEnumClassCheck::registerMatchers(MatchFinder *Finder) {
+ Finder->addMatcher(
+ traverse(TK_AsIs,
+ enumDecl(unless(isScoped()), unless(hasParent(recordDecl()))))
+ .bind("unscoped_enum"),
+ this);
+}
+
+void UseEnumClassCheck::check(const MatchFinder::MatchResult &Result) {
+ const auto *UnscopedEnum = Result.Nodes.getNodeAs<EnumDecl>("unscoped_enum");
+
+ diag(UnscopedEnum->getLocation(),
+ "enum %0 is unscoped, use enum class instead")
+ << UnscopedEnum;
+ diag(UnscopedEnum->getLocation(), "insert 'class'", DiagnosticIDs::Note)
+ << FixItHint::CreateInsertion(UnscopedEnum->getLocation(), "class ");
+}
+
+} // namespace clang::tidy::modernize
diff --git a/clang-tools-extra/clang-tidy/modernize/UseEnumClassCheck.h b/clang-tools-extra/clang-tidy/modernize/UseEnumClassCheck.h
new file mode 100644
index 0000000000000..9cfb2024b9cfd
--- /dev/null
+++ b/clang-tools-extra/clang-tidy/modernize/UseEnumClassCheck.h
@@ -0,0 +1,34 @@
+//===--- UseEnumClassCheck.h - clang-tidy -----------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MODERNIZE_USEENUMCLASSCHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MODERNIZE_USEENUMCLASSCHECK_H
+
+#include "../ClangTidyCheck.h"
+
+namespace clang::tidy::modernize {
+
+/// Check for unscoped enums that are not contained in classes/structs.
+/// Suggest to use scoped enums (enum class) instead.
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/modernize/use-enum-class.html
+class UseEnumClassCheck : public ClangTidyCheck {
+public:
+ UseEnumClassCheck(StringRef Name, ClangTidyContext *Context)
+ : ClangTidyCheck(Name, Context) {}
+ void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+ void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+ bool isLanguageVersionSupported(const LangOptions &LangOpts) const override {
+ return LangOpts.CPlusPlus;
+ }
+};
+
+} // namespace clang::tidy::modernize
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MODERNIZE_USE_ENUM_CLASS_H
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index b4792d749a86c..f39dcecae63f2 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -115,6 +115,11 @@ New checks
Gives warnings for tagged unions, where the number of tags is
different from the number of data members inside the union.
+- New :doc:`modernize-use-enum-class
+ <clang-tidy/checks/modernize/use-enum-class>` check.
+
+ Finds plain non-class enum definitions that could use ``enum class``.
+
New check aliases
^^^^^^^^^^^^^^^^^
diff --git a/clang-tools-extra/docs/clang-tidy/checks/list.rst b/clang-tools-extra/docs/clang-tidy/checks/list.rst
index e3dfabba8fad1..6533828a96fdf 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/list.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/list.rst
@@ -297,6 +297,7 @@ Clang-Tidy Checks
:doc:`modernize-use-default-member-init <modernize/use-default-member-init>`, "Yes"
:doc:`modernize-use-designated-initializers <modernize/use-designated-initializers>`, "Yes"
:doc:`modernize-use-emplace <modernize/use-emplace>`, "Yes"
+ :doc:`modernize-use-enum-class <modernize/use-enum-class>`, "Yes"
:doc:`modernize-use-equals-default <modernize/use-equals-default>`, "Yes"
:doc:`modernize-use-equals-delete <modernize/use-equals-delete>`, "Yes"
:doc:`modernize-use-nodiscard <modernize/use-nodiscard>`, "Yes"
diff --git a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-enum-class.rst b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-enum-class.rst
new file mode 100644
index 0000000000000..3adb6e204ad92
--- /dev/null
+++ b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-enum-class.rst
@@ -0,0 +1,26 @@
+.. title:: clang-tidy - modernize-use-enum-class
+
+modernize-use-enum-class
+=============================
+
+Scoped enums (enum class) should be preferred over unscoped enums:
+https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Renum-class
+
+Unscoped enums in classes are not reported since it is a well
+established pattern to limit the scope of plain enums.
+
+Example:
+
+.. code-block:: c++
+
+ enum E {}; // use "enum class E {};" instead
+ enum class E {}; // OK
+
+ struct S {
+ enum E {}; // OK, scope already limited
+ };
+
+ namespace N {
+ enum E {}; // use "enum class E {};" instead
+ // report since it is hard to detect how large the surrounding namespace is
+ }
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-enum-class.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-enum-class.cpp
new file mode 100644
index 0000000000000..9712fbc0aac4a
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-enum-class.cpp
@@ -0,0 +1,58 @@
+// RUN: %check_clang_tidy -std=c++17-or-later %s modernize-use-enum-class %t
+
+enum E {};
+// CHECK-MESSAGES: :[[@LINE-1]]:6: warning: enum 'E' is unscoped, use enum class instead [modernize-use-enum-class]
+
+enum class EC {};
+
+struct S {
+ enum E {};
+ // CHECK-MESSAGES-NOT: :[[@LINE-1]]:12: warning: enum 'E' is unscoped, use enum class instead [modernize-use-enum-class]
+ // Ignore unscoped enums in recordDecl
+ enum class EC {};
+};
+
+class C {
+ enum E {};
+ // CHECK-MESSAGES-NOT: :[[@LINE-1]]:12: warning: enum 'E' is unscoped, use enum class instead [modernize-use-enum-class]
+ // Ignore unscoped enums in recordDecl
+ enum class EC {};
+};
+
+template<class T>
+class TC {
+ enum E {};
+ // CHECK-MESSAGES-NOT: :[[@LINE-1]]:12: warning: enum 'E' is unscoped, use enum class instead [modernize-use-enum-class]
+ // Ignore unscoped enums in recordDecl
+ enum class EC {};
+};
+
+union U {
+ enum E {};
+ // CHECK-MESSAGES-NOT: :[[@LINE-1]]:12: warning: enum 'E' is unscoped, use enum class instead [modernize-use-enum-class]
+ // Ignore unscoped enums in recordDecl
+ enum class EC {};
+};
+
+namespace {
+enum E {};
+// CHECK-MESSAGES: :[[@LINE-1]]:6: warning: enum 'E' is unscoped, use enum class instead [modernize-use-enum-class]
+enum class EC {};
+} // namespace
+
+namespace N {
+enum E {};
+// CHECK-MESSAGES: :[[@LINE-1]]:6: warning: enum 'E' is unscoped, use enum class instead [modernize-use-enum-class]
+enum class EC {};
+} // namespace N
+
+template<enum ::EC>
+static void foo();
+
+using enum S::E;
+using enum S::EC;
+
+enum ForwardE : int;
+// CHECK-MESSAGES: :[[@LINE-1]]:6: warning: enum 'ForwardE' is unscoped, use enum class instead [modernize-use-enum-class]
+
+enum class ForwardEC : int;
``````````
</details>
https://github.com/llvm/llvm-project/pull/138282
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