[clang-tools-extra] [clang-tidy] Fix false positive in bugprone-use-after-move with std::forward on derived classes (PR #199905)

[via cfe-commits]

https://github.com/voyager-jhk updated https://github.com/llvm/llvm-project/pull/199905

>From 61bd57c542fee8fcd7c12ebb0d0e59342ca37291 Mon Sep 17 00:00:00 2001
From: voyager-jhk <voyager.lpq at gmail.com>
Date: Wed, 27 May 2026 16:54:08 +0800
Subject: [PATCH] [clang-tidy] Fix false positive in bugprone-use-after-move
 with std::forward on derived classes

The `bugprone-use-after-move` check correctly identified partial moves when using `std::move` by matching the `ImplicitCastExpr` (DerivedToBase) as the parent of the call. However, when using `std::forward<Base>`, the cast occurs inside the argument, causing the matcher to miss the cast and falsely report a use-after-move.

This patch manually checks the first argument of the moving call for a `DerivedToBase` cast if the parent matcher fails, ensuring both `move` and `forward` are correctly identified as partial moves.

Fixes #63202
---
 .../clang-tidy/bugprone/UseAfterMoveCheck.cpp |  8 +++++
 clang-tools-extra/docs/ReleaseNotes.rst       |  3 ++
 .../checkers/bugprone/use-after-move.cpp      | 35 +++++++++++++++++++
 3 files changed, 46 insertions(+)

diff --git a/clang-tools-extra/clang-tidy/bugprone/UseAfterMoveCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/UseAfterMoveCheck.cpp
index 399442f52bd33..1570c3a39ab07 100644
--- a/clang-tools-extra/clang-tidy/bugprone/UseAfterMoveCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/UseAfterMoveCheck.cpp
@@ -641,6 +641,14 @@ void UseAfterMoveCheck::check(const MatchFinder::MatchResult &Result) {
   const CXXRecordDecl *MovedAs =
       ParentCast ? ParentCast->getType()->getAsCXXRecordDecl() : nullptr;
 
+  if (!MovedAs && CallMove && CallMove->getNumArgs() > 0) {
+    if (const auto *ArgCast =
+            dyn_cast<ImplicitCastExpr>(CallMove->getArg(0)->IgnoreParens())) {
+      if (ArgCast->getCastKind() == CK_DerivedToBase)
+        MovedAs = ArgCast->getType()->getAsCXXRecordDecl();
+    }
+  }
+
   for (Stmt *CodeBlock : CodeBlocks) {
     UseAfterMoveFinder Finder(Result.Context, InvalidationFunctions,
                               ReinitializationFunctions, MovedAs);
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index f1b49e2cb6056..dfbd7f7d7bdbc 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -441,6 +441,9 @@ Changes in existing checks
   - Avoid false positives when moving object is reinitialized via the base
     class's ``operator=``.
 
+  - Avoid false positives when forwarding derived objects to base class
+    initializers.
+
 - Improved :doc:`cppcoreguidelines-avoid-capturing-lambda-coroutines
   <clang-tidy/checks/cppcoreguidelines/avoid-capturing-lambda-coroutines>`
   check by adding the `AllowExplicitObjectParameters` option. When enabled,
diff --git a/clang-tools-extra/test/clang-tidy/checkers/bugprone/use-after-move.cpp b/clang-tools-extra/test/clang-tidy/checkers/bugprone/use-after-move.cpp
index d4e78d359b654..6b9ac8c4a3861 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/bugprone/use-after-move.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/bugprone/use-after-move.cpp
@@ -1814,3 +1814,38 @@ namespace GH62206 {
     (d) = b; // Should not warn
   }
 } // namespace GH62206
+
+namespace GH63202 {
+
+struct Person {
+  Person() = default;
+  Person(Person&&) = default;
+  Person& operator=(Person&&) = default;
+};
+
+struct SpecialPerson : Person {
+  int surname;
+
+  // Valid: partial move
+  SpecialPerson(SpecialPerson&& sp)
+    : Person(std::move(sp)), surname(std::move(sp.surname)) {}
+
+  // Valid: partial forward — case fixed by this patch
+  SpecialPerson(SpecialPerson&& sp, int)
+    : Person(std::forward<Person>(sp)), surname(std::move(sp.surname)) {}
+};
+
+void invalidFullMove(SpecialPerson&& sp) {
+  SpecialPerson other(std::move(sp));
+  sp.surname = 1;
+  // CHECK-NOTES: [[@LINE-1]]:3: warning: 'sp' used after it was moved [bugprone-use-after-move]
+  // CHECK-NOTES: [[@LINE-3]]:17: note: move occurred here
+}
+
+void invalidFullForward(SpecialPerson&& sp) {
+  SpecialPerson other(std::forward<SpecialPerson>(sp));
+  sp.surname = 1;
+  // CHECK-NOTES: [[@LINE-1]]:3: warning: 'sp' used after it was forwarded [bugprone-use-after-move]
+  // CHECK-NOTES: [[@LINE-3]]:17: note: forward occurred here
+}
+} // namespace GH63202