[clang-tools-extra] Add bugprone-loop-variable-copied-then-modified clang-tidy check. (PR #157213)
Julia Hansbrough via cfe-commits
cfe-commits at lists.llvm.org
Tue Sep 30 14:56:28 PDT 2025
https://github.com/flowerhack updated https://github.com/llvm/llvm-project/pull/157213
>From fa8a0100ff5f53930624dac6d5e79432a220e99a Mon Sep 17 00:00:00 2001
From: Julia Hansbrough <flowerhack at google.com>
Date: Fri, 5 Sep 2025 21:27:14 +0000
Subject: [PATCH] Add bugprone-loop-variable-copied-then-modified clang-tidy
check.
Adds a clang-tidy check that alerts when a loop variable is copied and
subsequently modified.
This is a bugprone pattern because the programmer in this case often
assumes they are modifying the original value instead of a copy.
This warning can be suppressed by either converting the loop variable to
a const ref, or by performing the copy explicitly inside the body of the
loop.
Fix GH-155922
---
.../bugprone/BugproneTidyModule.cpp | 3 +
.../clang-tidy/bugprone/CMakeLists.txt | 1 +
.../LoopVariableCopiedThenModifiedCheck.cpp | 95 +++++++++++
.../LoopVariableCopiedThenModifiedCheck.h | 38 +++++
clang-tools-extra/docs/ReleaseNotes.rst | 11 ++
.../loop-variable-copied-then-modified.rst | 55 +++++++
.../docs/clang-tidy/checks/list.rst | 1 +
...variable-copied-then-modified-generics.cpp | 74 +++++++++
...opied-then-modified-ignore-inexpensive.cpp | 54 +++++++
...ed-then-modified-ignore-non-auto-types.cpp | 50 ++++++
.../loop-variable-copied-then-modified.cpp | 152 ++++++++++++++++++
11 files changed, 534 insertions(+)
create mode 100644 clang-tools-extra/clang-tidy/bugprone/LoopVariableCopiedThenModifiedCheck.cpp
create mode 100644 clang-tools-extra/clang-tidy/bugprone/LoopVariableCopiedThenModifiedCheck.h
create mode 100644 clang-tools-extra/docs/clang-tidy/checks/bugprone/loop-variable-copied-then-modified.rst
create mode 100644 clang-tools-extra/test/clang-tidy/checkers/bugprone/loop-variable-copied-then-modified-generics.cpp
create mode 100644 clang-tools-extra/test/clang-tidy/checkers/bugprone/loop-variable-copied-then-modified-ignore-inexpensive.cpp
create mode 100644 clang-tools-extra/test/clang-tidy/checkers/bugprone/loop-variable-copied-then-modified-ignore-non-auto-types.cpp
create mode 100644 clang-tools-extra/test/clang-tidy/checkers/bugprone/loop-variable-copied-then-modified.cpp
diff --git a/clang-tools-extra/clang-tidy/bugprone/BugproneTidyModule.cpp b/clang-tools-extra/clang-tidy/bugprone/BugproneTidyModule.cpp
index fe261e729539c..e5b55ce1c5283 100644
--- a/clang-tools-extra/clang-tidy/bugprone/BugproneTidyModule.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/BugproneTidyModule.cpp
@@ -40,6 +40,7 @@
#include "IntegerDivisionCheck.h"
#include "InvalidEnumDefaultInitializationCheck.h"
#include "LambdaFunctionNameCheck.h"
+#include "LoopVariableCopiedThenModifiedCheck.h"
#include "MacroParenthesesCheck.h"
#include "MacroRepeatedSideEffectsCheck.h"
#include "MisleadingSetterOfReferenceCheck.h"
@@ -154,6 +155,8 @@ class BugproneModule : public ClangTidyModule {
"bugprone-incorrect-enable-if");
CheckFactories.registerCheck<IncorrectEnableSharedFromThisCheck>(
"bugprone-incorrect-enable-shared-from-this");
+ CheckFactories.registerCheck<LoopVariableCopiedThenModifiedCheck>(
+ "bugprone-loop-variable-copied-then-modified");
CheckFactories.registerCheck<UnintendedCharOstreamOutputCheck>(
"bugprone-unintended-char-ostream-output");
CheckFactories.registerCheck<ReturnConstRefFromParameterCheck>(
diff --git a/clang-tools-extra/clang-tidy/bugprone/CMakeLists.txt b/clang-tools-extra/clang-tidy/bugprone/CMakeLists.txt
index 46bc8efd44bc5..52e81fed9f7cc 100644
--- a/clang-tools-extra/clang-tidy/bugprone/CMakeLists.txt
+++ b/clang-tools-extra/clang-tidy/bugprone/CMakeLists.txt
@@ -31,6 +31,7 @@ add_clang_library(clangTidyBugproneModule STATIC
IncorrectEnableIfCheck.cpp
IncorrectEnableSharedFromThisCheck.cpp
InvalidEnumDefaultInitializationCheck.cpp
+ LoopVariableCopiedThenModifiedCheck.cpp
UnintendedCharOstreamOutputCheck.cpp
ReturnConstRefFromParameterCheck.cpp
SuspiciousStringviewDataUsageCheck.cpp
diff --git a/clang-tools-extra/clang-tidy/bugprone/LoopVariableCopiedThenModifiedCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/LoopVariableCopiedThenModifiedCheck.cpp
new file mode 100644
index 0000000000000..7cc7ae35670da
--- /dev/null
+++ b/clang-tools-extra/clang-tidy/bugprone/LoopVariableCopiedThenModifiedCheck.cpp
@@ -0,0 +1,95 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 "LoopVariableCopiedThenModifiedCheck.h"
+#include "../utils/Matchers.h"
+#include "../utils/TypeTraits.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/Analysis/Analyses/ExprMutationAnalyzer.h"
+#include "clang/Basic/Diagnostic.h"
+#include "clang/Lex/Lexer.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::bugprone {
+
+namespace {
+AST_MATCHER(VarDecl, isInMacro) { return Node.getBeginLoc().isMacroID(); }
+} // namespace
+
+LoopVariableCopiedThenModifiedCheck::LoopVariableCopiedThenModifiedCheck(
+ StringRef Name, ClangTidyContext *Context)
+ : ClangTidyCheck(Name, Context), IgnoreInexpensiveVariables(Options.get(
+ "IgnoreInexpensiveVariables", false)),
+ WarnOnlyOnAutoCopies(Options.get("WarnOnlyOnAutoCopies", false)) {}
+
+void LoopVariableCopiedThenModifiedCheck::storeOptions(
+ ClangTidyOptions::OptionMap &Opts) {
+ Options.store(Opts, "IgnoreInexpensiveVariables", IgnoreInexpensiveVariables);
+ Options.store(Opts, "WarnOnlyOnAutoCopies", WarnOnlyOnAutoCopies);
+}
+
+void LoopVariableCopiedThenModifiedCheck::registerMatchers(
+ MatchFinder *Finder) {
+ const auto HasReferenceOrPointerType = hasType(qualType(
+ unless(hasCanonicalType(anyOf(referenceType(), pointerType())))));
+ const auto IteratorReturnsValueType = cxxOperatorCallExpr(
+ hasOverloadedOperatorName("*"),
+ callee(
+ cxxMethodDecl(returns(unless(hasCanonicalType(referenceType()))))));
+ const auto NotConstructedByCopy = cxxConstructExpr(
+ hasDeclaration(cxxConstructorDecl(unless(isCopyConstructor()))));
+ const auto ConstructedByConversion =
+ cxxMemberCallExpr(callee(cxxConversionDecl()));
+ const auto LoopVar =
+ varDecl(unless(isInMacro()), HasReferenceOrPointerType,
+ unless(hasInitializer(expr(hasDescendant(expr(
+ anyOf(materializeTemporaryExpr(), IteratorReturnsValueType,
+ NotConstructedByCopy, ConstructedByConversion)))))));
+ Finder->addMatcher(cxxForRangeStmt(hasLoopVariable(LoopVar.bind("loopVar")))
+ .bind("forRange"),
+ this);
+}
+
+void LoopVariableCopiedThenModifiedCheck::check(
+ const MatchFinder::MatchResult &Result) {
+ const auto *LoopVar = Result.Nodes.getNodeAs<VarDecl>("loopVar");
+ std::optional<bool> Expensive = utils::type_traits::isExpensiveToCopy(
+ LoopVar->getType(), *Result.Context);
+ if ((!Expensive || !*Expensive) && IgnoreInexpensiveVariables)
+ return;
+ if (WarnOnlyOnAutoCopies) {
+ if (!isa<AutoType>(LoopVar->getType())) {
+ return;
+ }
+ }
+ const auto *ForRange = Result.Nodes.getNodeAs<CXXForRangeStmt>("forRange");
+
+ if (!ExprMutationAnalyzer(*ForRange->getBody(), *Result.Context)
+ .isMutated(LoopVar)) {
+ return;
+ }
+
+ clang::SourceRange LoopVarSourceRange =
+ LoopVar->getTypeSourceInfo()->getTypeLoc().getSourceRange();
+ clang::SourceLocation EndLoc = clang::Lexer::getLocForEndOfToken(
+ LoopVarSourceRange.getEnd(), 0, Result.Context->getSourceManager(),
+ Result.Context->getLangOpts());
+ diag(LoopVar->getLocation(),
+ "loop variable '%0' is copied and then (possibly) modified; use an "
+ "explicit copy inside the body of the loop or make the variable a "
+ "reference")
+ << LoopVar->getName();
+ diag(LoopVar->getLocation(), "consider making '%0' a reference",
+ DiagnosticIDs::Note)
+ << LoopVar->getName()
+ << FixItHint::CreateInsertion(LoopVarSourceRange.getBegin(), "const ")
+ << FixItHint::CreateInsertion(EndLoc, "&");
+}
+
+} // namespace clang::tidy::bugprone
diff --git a/clang-tools-extra/clang-tidy/bugprone/LoopVariableCopiedThenModifiedCheck.h b/clang-tools-extra/clang-tidy/bugprone/LoopVariableCopiedThenModifiedCheck.h
new file mode 100644
index 0000000000000..dfc18f98d2f80
--- /dev/null
+++ b/clang-tools-extra/clang-tidy/bugprone/LoopVariableCopiedThenModifiedCheck.h
@@ -0,0 +1,38 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_BUGPRONE_LOOPVARIABLECOPIEDTHENMODIFIEDCHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BUGPRONE_LOOPVARIABLECOPIEDTHENMODIFIEDCHECK_H
+
+#include "../ClangTidyCheck.h"
+
+namespace clang::tidy::bugprone {
+
+/// Finds loop variables that are copied and subsequently modified.
+///
+/// For the user-facing documentation see:
+/// https://clang.llvm.org/extra/clang-tidy/checks/bugprone/loop-variable-copied-then-modified.html
+class LoopVariableCopiedThenModifiedCheck : public ClangTidyCheck {
+public:
+ LoopVariableCopiedThenModifiedCheck(StringRef Name,
+ ClangTidyContext *Context);
+ void storeOptions(ClangTidyOptions::OptionMap &Opts) override;
+ 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;
+ }
+
+private:
+ const bool IgnoreInexpensiveVariables;
+ const bool WarnOnlyOnAutoCopies;
+};
+
+} // namespace clang::tidy::bugprone
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BUGPRONE_LOOPVARIABLECOPIEDTHENMODIFIEDCHECK_H
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 23d757b5e6f2e..d220351be3551 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -142,12 +142,23 @@ Improvements to clang-tidy
New checks
^^^^^^^^^^
+- New :doc:`bugprone-derived-method-shadowing-base-method
+ <clang-tidy/checks/bugprone/derived-method-shadowing-base-method>` check.
+
+ Finds derived class methods that shadow a (non-virtual) base class method.
+
- New :doc:`bugprone-invalid-enum-default-initialization
<clang-tidy/checks/bugprone/invalid-enum-default-initialization>` check.
Detects default initialization (to 0) of variables with ``enum`` type where
the enum has no enumerator with value of 0.
+- New :doc:`bugprone-loop-variable-copied-then-modified
+ <clang-tidy/checks/bugprone/loop-variable-copied-then-modified>` check.
+
+ Detects when a loop variable is copied and then subsequently (possibly) modified
+ and suggests replacing with a reference or an explicit copy.
+
- New :doc:`cppcoreguidelines-pro-bounds-avoid-unchecked-container-access
<clang-tidy/checks/cppcoreguidelines/pro-bounds-avoid-unchecked-container-access>`
check.
diff --git a/clang-tools-extra/docs/clang-tidy/checks/bugprone/loop-variable-copied-then-modified.rst b/clang-tools-extra/docs/clang-tidy/checks/bugprone/loop-variable-copied-then-modified.rst
new file mode 100644
index 0000000000000..d2a476d918e94
--- /dev/null
+++ b/clang-tools-extra/docs/clang-tidy/checks/bugprone/loop-variable-copied-then-modified.rst
@@ -0,0 +1,55 @@
+.. title:: clang-tidy - bugprone-loop-variable-copied-then-modified
+
+bugprone-loop-variable-copied-then-modified
+===========================================
+
+Detects when a loop variable is copied and then subsequently (possibly) modified
+and suggests replacing with a reference or an explicit copy.
+
+This pattern is considered bugprone because, frequently, programmers do not
+realize that they are modifying a *copy* rather than an underlying value,
+resulting in subtly erroneous code.
+
+For instance, the following code attempts to null out a value in a map, but only
+succeeds in nulling out a value in a *copy* of the map:
+
+.. code-block:: c++
+
+ for (auto target : target_map) {
+ target.value = nullptr;
+ }
+
+The programmer is likely to have intended this code instead:
+
+.. code-block:: c++
+
+ for (auto& target : target_map) {
+ target.value = nullptr;
+ }
+
+This code can be fixed in one of two ways:
+ - In cases where the programmer did not intend to create a copy, they can
+ convert the loop variable to a reference or a ``const`` reference. A
+ fix-note message will provide a naive suggestion of how to achieve this,
+ which works in most cases.
+ - In cases where the intent is in fact to modify a copy, they may perform the
+ copy explicitly, inside the body of the loop, and perform whatever
+ operations they like on that copy.
+
+This is a conservative check: in cases where it cannot be determined at compile
+time whether or not a particular function modifies the variable, it assumes a
+modification has ocurred and warns accordingly. However, in such cases, the
+warning can still be suppressed by doing one of the actions described above.
+
+Options
+-------
+
+.. option:: IgnoreInexpensiveVariables
+
+ When `true`, this check will only alert on types that are expensive to copy.
+ This will lead to fewer false positives, but will also overlook some
+ instances where there may be an actual bug. Default is `false`.
+
+.. option:: WarnOnlyOnAutoCopies
+
+ When `true`, this check will only alert on `auto` types. Default is `false`.
\ No newline at end of file
diff --git a/clang-tools-extra/docs/clang-tidy/checks/list.rst b/clang-tools-extra/docs/clang-tidy/checks/list.rst
index c490d2ece2e0a..3ee2cae65accb 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/list.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/list.rst
@@ -108,6 +108,7 @@ Clang-Tidy Checks
:doc:`bugprone-integer-division <bugprone/integer-division>`,
:doc:`bugprone-invalid-enum-default-initialization <bugprone/invalid-enum-default-initialization>`,
:doc:`bugprone-lambda-function-name <bugprone/lambda-function-name>`,
+ :doc:`bugprone-loop-variable-copied-then-modified <bugprone/loop-variable-copied-then-modified>`, "Yes"
:doc:`bugprone-macro-parentheses <bugprone/macro-parentheses>`, "Yes"
:doc:`bugprone-macro-repeated-side-effects <bugprone/macro-repeated-side-effects>`,
:doc:`bugprone-misleading-setter-of-reference <bugprone/misleading-setter-of-reference>`,
diff --git a/clang-tools-extra/test/clang-tidy/checkers/bugprone/loop-variable-copied-then-modified-generics.cpp b/clang-tools-extra/test/clang-tidy/checkers/bugprone/loop-variable-copied-then-modified-generics.cpp
new file mode 100644
index 0000000000000..a7bf7017c8aec
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/bugprone/loop-variable-copied-then-modified-generics.cpp
@@ -0,0 +1,74 @@
+// RUN: %check_clang_tidy -std=c++17-or-later %s bugprone-loop-variable-copied-then-modified %t --fix-notes
+
+template <typename T>
+struct Iterator {
+ void operator++() {}
+ const T& operator*() {
+ static T* TT = new T();
+ return *TT;
+ }
+ bool operator!=(const Iterator &) { return false; }
+};
+template <typename T>
+struct View {
+ T begin() { return T(); }
+ T begin() const { return T(); }
+ T end() { return T(); }
+ T end() const { return T(); }
+};
+
+struct ConstructorConvertible {
+};
+
+struct S {
+ int value;
+
+ S() : value(0) {};
+ S(const S &);
+ S(const ConstructorConvertible&) {}
+ ~S();
+ S &operator=(const S &);
+ void modify() {
+ value++;
+ }
+};
+
+struct Convertible {
+ operator S() const {
+ return S();
+ }
+};
+
+struct PairLike {
+ int id;
+ S data;
+};
+
+template <typename V>
+struct Generic {
+ V value;
+
+ Generic() : value{} {};
+ Generic(const Generic &);
+ ~Generic();
+ Generic &operator=(const Generic &);
+ void modify() {
+ value++;
+ }
+};
+
+void PositiveLoopVariableCopiedAndThenModfiedGeneric() {
+ for (Generic G : View<Iterator<Generic<double>>>()) {
+ // CHECK-MESSAGES: [[@LINE-1]]:16: warning: loop variable 'G' is copied and then (possibly) modified; use an explicit copy inside the body of the loop or make the variable a reference
+ // CHECK-MESSAGES: [[@LINE-2]]:16: note: consider making 'G' a reference
+ // CHECK-FIXES: for (const Generic& G : View<Iterator<Generic<double>>>()) {
+ G.modify();
+ }
+}
+
+void NegativeLoopVariableIsReferenceAndModifiedGeneric() {
+ for (const Generic<double>& G : View<Iterator<Generic<double>>>()) {
+ // It's fine to copy-by-value G into some other G.
+ Generic<double> G2 = G;
+ }
+}
diff --git a/clang-tools-extra/test/clang-tidy/checkers/bugprone/loop-variable-copied-then-modified-ignore-inexpensive.cpp b/clang-tools-extra/test/clang-tidy/checkers/bugprone/loop-variable-copied-then-modified-ignore-inexpensive.cpp
new file mode 100644
index 0000000000000..13b47dcdd2b63
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/bugprone/loop-variable-copied-then-modified-ignore-inexpensive.cpp
@@ -0,0 +1,54 @@
+// RUN: %check_clang_tidy %s bugprone-loop-variable-copied-then-modified %t --fix-notes \
+// RUN: -config="{CheckOptions: \
+// RUN: {bugprone-loop-variable-copied-then-modified.IgnoreInexpensiveVariables: true}}" \
+// RUN: -- -I%S
+#include "Inputs/system-header-simulator/sim_initializer_list"
+#include "Inputs/system-header-simulator/sim_vector"
+
+template <typename T>
+struct Iterator {
+ void operator++() {}
+ const T& operator*() {
+ static T* TT = new T();
+ return *TT;
+ }
+ bool operator!=(const Iterator &) { return false; }
+};
+template <typename T>
+struct View {
+ T begin() { return T(); }
+ T begin() const { return T(); }
+ T end() { return T(); }
+ T end() const { return T(); }
+};
+
+struct S {
+ int value;
+
+ S() : value(0) {};
+ S(const S &);
+ ~S();
+ S &operator=(const S &);
+ void modify() {
+ value++;
+ }
+};
+
+void NegativeOnlyCopyingInts() {
+ std::vector<int> foo;
+ foo.push_back(1);
+ foo.push_back(2);
+ foo.push_back(3);
+ for (int v : foo) {
+ v += 1;
+ }
+}
+
+void PositiveLoopVariableCopiedAndThenModfied() {
+ for (S S1 : View<Iterator<S>>()) {
+ // CHECK-MESSAGES: [[@LINE-1]]:10: warning: loop variable 'S1' is copied and then (possibly) modified; use an explicit copy inside the body of the loop or make the variable a reference
+ // CHECK-MESSAGES: [[@LINE-2]]:10: note: consider making 'S1' a reference
+ // CHECK-FIXES: for (const S& S1 : View<Iterator<S>>()) {
+ S1.modify();
+ }
+}
diff --git a/clang-tools-extra/test/clang-tidy/checkers/bugprone/loop-variable-copied-then-modified-ignore-non-auto-types.cpp b/clang-tools-extra/test/clang-tidy/checkers/bugprone/loop-variable-copied-then-modified-ignore-non-auto-types.cpp
new file mode 100644
index 0000000000000..c82a3453e9a8b
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/bugprone/loop-variable-copied-then-modified-ignore-non-auto-types.cpp
@@ -0,0 +1,50 @@
+// RUN: %check_clang_tidy %s bugprone-loop-variable-copied-then-modified %t --fix-notes \
+// RUN: -config="{CheckOptions: \
+// RUN: {bugprone-loop-variable-copied-then-modified.WarnOnlyOnAutoCopies: true}}" \
+// RUN: -- -I%S
+#include "Inputs/system-header-simulator/sim_initializer_list"
+#include "Inputs/system-header-simulator/sim_vector"
+
+template <typename T>
+struct Iterator {
+ void operator++() {}
+ const T& operator*() {
+ static T* TT = new T();
+ return *TT;
+ }
+ bool operator!=(const Iterator &) { return false; }
+};
+template <typename T>
+struct View {
+ T begin() { return T(); }
+ T begin() const { return T(); }
+ T end() { return T(); }
+ T end() const { return T(); }
+};
+
+struct S {
+ int value;
+
+ S() : value(0) {};
+ S(const S &);
+ ~S();
+ S &operator=(const S &);
+ void modify() {
+ value++;
+ }
+};
+
+void NegativeLoopVariableIsNotAuto() {
+ for (S S1 : View<Iterator<S>>()) {
+ S1.modify();
+ }
+}
+
+void PositiveLoopVariableIsAuto() {
+ for (auto S1 : View<Iterator<S>>()) {
+ // CHECK-MESSAGES: [[@LINE-1]]:13: warning: loop variable 'S1' is copied and then (possibly) modified; use an explicit copy inside the body of the loop or make the variable a reference
+ // CHECK-MESSAGES: [[@LINE-2]]:13: note: consider making 'S1' a reference
+ // CHECK-FIXES: for (const auto& S1 : View<Iterator<S>>()) {
+ S1.modify();
+ }
+}
diff --git a/clang-tools-extra/test/clang-tidy/checkers/bugprone/loop-variable-copied-then-modified.cpp b/clang-tools-extra/test/clang-tidy/checkers/bugprone/loop-variable-copied-then-modified.cpp
new file mode 100644
index 0000000000000..1fb6d6116c8b3
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/bugprone/loop-variable-copied-then-modified.cpp
@@ -0,0 +1,152 @@
+// RUN: %check_clang_tidy %s bugprone-loop-variable-copied-then-modified %t --fix-notes
+
+template <typename T>
+struct Iterator {
+ void operator++() {}
+ const T& operator*() {
+ static T* TT = new T();
+ return *TT;
+ }
+ bool operator!=(const Iterator &) { return false; }
+};
+template <typename T>
+struct View {
+ T begin() { return T(); }
+ T begin() const { return T(); }
+ T end() { return T(); }
+ T end() const { return T(); }
+};
+
+struct ConstructorConvertible {
+};
+
+struct S {
+ int value;
+
+ S() : value(0) {};
+ S(const S &);
+ S(const ConstructorConvertible&) {}
+ ~S();
+ S &operator=(const S &);
+ void modify() {
+ value++;
+ }
+};
+
+struct Convertible {
+ operator S() const {
+ return S();
+ }
+};
+
+struct PairLike {
+ int id;
+ S data;
+};
+
+template <typename V>
+struct Generic {
+ V value;
+
+ Generic() : value{} {};
+ Generic(const Generic &);
+ ~Generic();
+ Generic &operator=(const Generic &);
+ void modify() {
+ value++;
+ }
+};
+
+#define LOOP_AND_MODIFY(VAR_NAME, TYPE) \
+ for (TYPE VAR_NAME : View<Iterator<TYPE>>()) { \
+ VAR_NAME.modify(); \
+ }
+
+void NegativeLoopVariableNotCopied() {
+ for (const S& S1 : View<Iterator<S>>()) {
+ // It's fine to copy-by-value S1 into some other S.
+ S S2 = S1;
+ }
+}
+
+void NegativeLoopVariableCopiedButNotModified() {
+ for (S S1 : View<Iterator<S>>()) {
+ }
+}
+
+void PositiveLoopVariableCopiedAndThenModfied() {
+ for (S S1 : View<Iterator<S>>()) {
+ // CHECK-MESSAGES: [[@LINE-1]]:10: warning: loop variable 'S1' is copied and then (possibly) modified; use an explicit copy inside the body of the loop or make the variable a reference
+ // CHECK-MESSAGES: [[@LINE-2]]:10: note: consider making 'S1' a reference
+ // CHECK-FIXES: for (const S& S1 : View<Iterator<S>>()) {
+ S1.modify();
+ }
+}
+
+void PositiveLoopVariableCopiedAndThenModifiedAuto() {
+ for (auto S1 : View<Iterator<S>>()) {
+ // CHECK-MESSAGES: [[@LINE-1]]:13: warning: loop variable 'S1' is copied and then (possibly) modified; use an explicit copy inside the body of the loop or make the variable a reference
+ // CHECK-MESSAGES: [[@LINE-2]]:13: note: consider making 'S1' a reference
+ // CHECK-FIXES: for (const auto& S1 : View<Iterator<S>>()) {
+ S1.modify();
+ }
+}
+
+void PositiveLoopVariableIsTypedefType() {
+ typedef int TypedefInt;
+ for (TypedefInt ti : View<Iterator<TypedefInt>>()) {
+ // CHECK-MESSAGES: [[@LINE-1]]:19: warning: loop variable 'ti' is copied and then (possibly) modified; use an explicit copy inside the body of the loop or make the variable a reference
+ // CHECK-MESSAGES: [[@LINE-2]]:19: note: consider making 'ti' a reference
+ // CHECK-FIXES: for (const TypedefInt& ti : View<Iterator<TypedefInt>>()) {
+ ti += 1;
+ }
+}
+
+void NegativeLoopVariableIsInsideMacro() {
+ LOOP_AND_MODIFY(S1, S);
+}
+
+template <typename T>
+struct ValueReturningIterator {
+ void operator++() {}
+ T operator*() { return T(); }
+ bool operator!=(const ValueReturningIterator &) { return false; }
+ typedef const T &const_reference;
+};
+
+void NegativeValueIterator() {
+ // Check does not trigger for iterators that return elements by value.
+ for (S S1 : View<ValueReturningIterator<S>>()) {
+ S1.modify();
+ }
+}
+
+void NegativeConstructedByConversion() {
+ Convertible C[0];
+ for (S S1 : C) {
+ S1.modify();
+ }
+}
+
+void NegativeNotConstructedByCopy() {
+ // Designed to exercise the unless(NotConstructedByCopy) clause in the check's source code.
+ // Distinct from NegativeConstructedByConversion because it exercises a converting constructor rather than a conversion operator.
+ ConstructorConvertible C[0];
+ for (S S1 : C) {
+ S1.modify();
+ }
+}
+
+void PositiveLoopVariableIsStructuredBinding() {
+ for (auto [id, data] : View<Iterator<PairLike>>()) {
+ // CHECK-MESSAGES: [[@LINE-1]]:13: warning: loop variable '' is copied and then (possibly) modified; use an explicit copy inside the body of the loop or make the variable a reference
+ // CHECK-MESSAGES: [[@LINE-2]]:13: note: consider making '' a reference
+ // CHECK-FIXES: for (const auto& [id, data] : View<Iterator<PairLike>>()) {
+ data.modify();
+ }
+}
+
+void NegativeLoopVariableIsStructuredBinding() {
+ for (const auto& [id, data] : View<Iterator<PairLike>>()) {
+ }
+}
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