[clang-tidy] Make copy-and-swap idiom more general for bugprone-unhandled-self-assignment (#147066)

This change enhances the `bugprone-unhandled-self-assignment` checker by
adding an additional matcher that generalizes the copy-and-swap idiom
pattern detection.

# What Changed

Added a new matcher that checks for:
- An instance of the current class being created in operator=
(regardless of constructor arguments)
- That instance being passed to a `swap` function call

# Problem Solved
This fix reduces false positives in PMR-like scenarios where "extended"
constructors are used (typically taking an additional allocator
argument). The checker now properly recognizes copy-and-swap
implementations that use extended copy/move constructors instead of
flagging them as unhandled self-assignment cases.

Fixes #146324

---------

Co-authored-by: Baranov Victor <[email protected]>

Author: negativ

clang-tools-extra/clang-tidy/bugprone/UnhandledSelfAssignmentCheck.cpp

@@ -69,6 +69,28 @@ void UnhandledSelfAssignmentCheck::registerMatchers(MatchFinder *Finder) {
       cxxMethodDecl(unless(hasDescendant(cxxMemberCallExpr(callee(cxxMethodDecl(
           hasName("operator="), ofClass(equalsBoundNode("class"))))))));
 
+  // Checking that some kind of constructor is called and followed by a `swap`:
+  // T& operator=(const T& other) {
+  //    T tmp{this->internal_data(), some, other, args};
+  //    swap(tmp);
+  //    return *this;
+  // }
+  const auto HasCopyAndSwap = cxxMethodDecl(
+      ofClass(cxxRecordDecl()),
+      hasBody(compoundStmt(
+          hasDescendant(
+              varDecl(hasType(cxxRecordDecl(equalsBoundNode("class"))))
+                  .bind("tmp_var")),
+          hasDescendant(stmt(anyOf(
+              cxxMemberCallExpr(hasArgument(
+                  0, declRefExpr(to(varDecl(equalsBoundNode("tmp_var")))))),
+              callExpr(
+                  callee(functionDecl(hasName("swap"))), argumentCountIs(2),
+                  hasAnyArgument(
+                      declRefExpr(to(varDecl(equalsBoundNode("tmp_var"))))),
+                  hasAnyArgument(unaryOperator(has(cxxThisExpr()),
+                                               hasOperatorName("*"))))))))));
+
   DeclarationMatcher AdditionalMatcher = cxxMethodDecl();
   if (WarnOnlyIfThisHasSuspiciousField) {
     // Matcher for standard smart pointers.
@@ -89,14 +111,14 @@ void UnhandledSelfAssignmentCheck::registerMatchers(MatchFinder *Finder) {
                             hasType(arrayType())))))));
   }
 
-  Finder->addMatcher(cxxMethodDecl(ofClass(cxxRecordDecl().bind("class")),
-                                   isCopyAssignmentOperator(), IsUserDefined,
-                                   HasReferenceParam, HasNoSelfCheck,
-                                   unless(HasNonTemplateSelfCopy),
-                                   unless(HasTemplateSelfCopy),
-                                   HasNoNestedSelfAssign, AdditionalMatcher)
-                         .bind("copyAssignmentOperator"),
-                     this);
+  Finder->addMatcher(
+      cxxMethodDecl(
+          ofClass(cxxRecordDecl().bind("class")), isCopyAssignmentOperator(),
+          IsUserDefined, HasReferenceParam, HasNoSelfCheck,
+          unless(HasNonTemplateSelfCopy), unless(HasTemplateSelfCopy),
+          unless(HasCopyAndSwap), HasNoNestedSelfAssign, AdditionalMatcher)
+          .bind("copyAssignmentOperator"),
+      this);
 }
 
 void UnhandledSelfAssignmentCheck::check(

clang-tools-extra/docs/ReleaseNotes.rst

@@ -114,6 +114,11 @@ Changes in existing checks
   <clang-tidy/checks/bugprone/infinite-loop>` check by adding detection for
   variables introduced by structured bindings.
 
+- Improved :doc:`bugprone-unhandled-self-assignment
+  <clang-tidy/checks/bugprone/unhandled-self-assignment>` check by adding
+  an additional matcher that generalizes the copy-and-swap idiom pattern
+  detection.
+  
 Removed checks
 ^^^^^^^^^^^^^^
 

clang-tools-extra/test/clang-tidy/checkers/bugprone/unhandled-self-assignment.cpp

@@ -28,6 +28,13 @@ template <class T>
 class auto_ptr {
 };
 
+namespace pmr {
+    template <typename TYPE = void>
+    class allocator {};
+}
+
+struct allocator_arg_t {} allocator_arg;
+
 } // namespace std
 
 void assert(int expression){};
@@ -229,6 +236,122 @@ bool operator!=(Foo2 &, Foo2 &) {
   };
 }
 
+// Missing call to `swap` function
+class AllocatorAwareClassNoSwap {
+  // pointer member to trigger bugprone-unhandled-self-assignment
+  void *foo = nullptr;
+
+  public:
+    using allocator_type = std::pmr::allocator<>;
+
+    AllocatorAwareClassNoSwap(const AllocatorAwareClassNoSwap& other) {
+    }
+
+    AllocatorAwareClassNoSwap(const AllocatorAwareClassNoSwap& other, const allocator_type& alloc) {
+    }
+
+    AllocatorAwareClassNoSwap& operator=(const AllocatorAwareClassNoSwap& other) {
+        // CHECK-MESSAGES: [[@LINE-1]]:32: warning: operator=() does not handle self-assignment properly [bugprone-unhandled-self-assignment]
+        AllocatorAwareClassNoSwap tmp(other, get_allocator());
+        return *this;
+    }
+
+    allocator_type get_allocator() const {
+        return allocator_type();
+    }
+};
+
+// "Wrong" type is passed to member `swap` function
+class AllocatorAwareClassSwapWrongArgType {
+  // pointer member to trigger bugprone-unhandled-self-assignment
+  void *foo = nullptr;
+
+  public:
+    using allocator_type = std::pmr::allocator<>;
+
+    AllocatorAwareClassSwapWrongArgType(const AllocatorAwareClassSwapWrongArgType& other) {
+    }
+
+    AllocatorAwareClassSwapWrongArgType(const AllocatorAwareClassSwapWrongArgType& other, const allocator_type& alloc) {
+    }
+
+    AllocatorAwareClassSwapWrongArgType& operator=(const AllocatorAwareClassSwapWrongArgType& other) {
+        // CHECK-MESSAGES: [[@LINE-1]]:42: warning: operator=() does not handle self-assignment properly [bugprone-unhandled-self-assignment]
+        int tmp = 0;
+        swap(tmp);
+        
+        return *this;
+    }
+
+    void swap(int&) noexcept {
+    }
+
+    allocator_type get_allocator() const {
+        return allocator_type();
+    }
+};
+
+
+// Making ADL `swap` call but with wrong argument type
+class AllocatorAwareClassAdlSwapWrongArgType {
+  // pointer member to trigger bugprone-unhandled-self-assignment
+  void *foo = nullptr;
+
+  public:
+    using allocator_type = std::pmr::allocator<>;
+
+    AllocatorAwareClassAdlSwapWrongArgType(const AllocatorAwareClassAdlSwapWrongArgType& other) {
+    }
+
+    AllocatorAwareClassAdlSwapWrongArgType(const AllocatorAwareClassAdlSwapWrongArgType& other, const allocator_type& alloc) {
+    }
+
+    AllocatorAwareClassAdlSwapWrongArgType& operator=(const AllocatorAwareClassAdlSwapWrongArgType& other) {
+        // CHECK-MESSAGES: [[@LINE-1]]:45: warning: operator=() does not handle self-assignment properly [bugprone-unhandled-self-assignment]
+        int tmp = 0;
+        swap(*this, tmp);
+        
+        return *this;
+    }
+
+    allocator_type get_allocator() const {
+        return allocator_type();
+    }
+    
+    friend void swap(AllocatorAwareClassAdlSwapWrongArgType&, int&) {
+    }
+};
+
+// `this` isn't passed to `swap`
+class AllocatorAwareClassAdlSwapWrongArgs {
+  // pointer member to trigger bugprone-unhandled-self-assignment
+  void *foo = nullptr;
+
+  public:
+    using allocator_type = std::pmr::allocator<>;
+
+    AllocatorAwareClassAdlSwapWrongArgs(const AllocatorAwareClassAdlSwapWrongArgs& other) {
+    }
+
+    AllocatorAwareClassAdlSwapWrongArgs(const AllocatorAwareClassAdlSwapWrongArgs& other, const allocator_type& alloc) {
+    }
+
+    AllocatorAwareClassAdlSwapWrongArgs& operator=(const AllocatorAwareClassAdlSwapWrongArgs& other) {
+        // CHECK-MESSAGES: [[@LINE-1]]:42: warning: operator=() does not handle self-assignment properly [bugprone-unhandled-self-assignment]
+        AllocatorAwareClassAdlSwapWrongArgs tmp1(other, get_allocator());
+        AllocatorAwareClassAdlSwapWrongArgs tmp2(*this, get_allocator());
+        swap(tmp1, tmp2);
+        return *this;
+    }
+
+    allocator_type get_allocator() const {
+        return allocator_type();
+    }
+    
+    friend void swap(AllocatorAwareClassAdlSwapWrongArgs&, AllocatorAwareClassAdlSwapWrongArgs&) {
+    }
+};
+
 ///////////////////////////////////////////////////////////////////
 /// Test cases correctly ignored by the check.
 
@@ -540,6 +663,89 @@ class NotACopyAssignmentOperator {
   Uy *getUy() const { return Ptr2; }
 };
 
+// Support "extended" copy/move constructors
+class AllocatorAwareClass {
+  // pointer member to trigger bugprone-unhandled-self-assignment
+  void *foo = nullptr;
+
+  public:
+    using allocator_type = std::pmr::allocator<>;
+
+    AllocatorAwareClass(const AllocatorAwareClass& other) {
+    }
+
+    AllocatorAwareClass(const AllocatorAwareClass& other, const allocator_type& alloc) {
+    }
+
+    AllocatorAwareClass& operator=(const AllocatorAwareClass& other) {
+        AllocatorAwareClass tmp(other, get_allocator());
+        swap(tmp);
+        return *this;
+    }
+
+    void swap(AllocatorAwareClass& other) noexcept {
+    }
+
+    allocator_type get_allocator() const {
+        return allocator_type();
+    }
+};
+
+// Support "extended" copy/move constructors + std::swap
+class AllocatorAwareClassStdSwap {
+  // pointer member to trigger bugprone-unhandled-self-assignment
+  void *foo = nullptr;
+
+  public:
+    using allocator_type = std::pmr::allocator<>;
+
+    AllocatorAwareClassStdSwap(const AllocatorAwareClassStdSwap& other) {
+    }
+
+    AllocatorAwareClassStdSwap(const AllocatorAwareClassStdSwap& other, const allocator_type& alloc) {
+    }
+
+    AllocatorAwareClassStdSwap& operator=(const AllocatorAwareClassStdSwap& other) {
+        using std::swap;
+        
+        AllocatorAwareClassStdSwap tmp(other, get_allocator());
+        swap(*this, tmp);
+        return *this;
+    }
+
+    allocator_type get_allocator() const {
+        return allocator_type();
+    }
+};
+
+// Support "extended" copy/move constructors + ADL swap
+class AllocatorAwareClassAdlSwap {
+  // pointer member to trigger bugprone-unhandled-self-assignment
+  void *foo = nullptr;
+
+  public:
+    using allocator_type = std::pmr::allocator<>;
+
+    AllocatorAwareClassAdlSwap(const AllocatorAwareClassAdlSwap& other) {
+    }
+
+    AllocatorAwareClassAdlSwap(const AllocatorAwareClassAdlSwap& other, const allocator_type& alloc) {
+    }
+
+    AllocatorAwareClassAdlSwap& operator=(const AllocatorAwareClassAdlSwap& other) {
+        AllocatorAwareClassAdlSwap tmp(other, get_allocator());
+        swap(*this, tmp);
+        return *this;
+    }
+
+    allocator_type get_allocator() const {
+        return allocator_type();
+    }
+    
+    friend void swap(AllocatorAwareClassAdlSwap&, AllocatorAwareClassAdlSwap&) {
+    }
+};
+
 ///////////////////////////////////////////////////////////////////
 /// Test cases which should be caught by the check.