[clang] [LifetimeSafety] Fix off-by-one crash in `lifetime_capture_by` argument indexing (PR #227231)

[Utkarsh Saxena via cfe-commits]

https://github.com/usx95 updated https://github.com/llvm/llvm-project/pull/227231

>From e3d9d651b17e77bce3a8ea2c499c2f139507a657 Mon Sep 17 00:00:00 2001
From: Utkarsh Saxena <usx at google.com>
Date: Tue, 29 Sep 2026 09:09:19 +0000
Subject: [PATCH] capture-by-crash

---
 .../LifetimeSafety/FactsGenerator.cpp         | 11 +++++----
 clang/test/Sema/LifetimeSafety/safety.cpp     | 23 +++++++++++++++++++
 2 files changed, 29 insertions(+), 5 deletions(-)

diff --git a/clang/lib/Analysis/LifetimeSafety/FactsGenerator.cpp b/clang/lib/Analysis/LifetimeSafety/FactsGenerator.cpp
index 292e3279233bb..2457b8270cb80 100644
--- a/clang/lib/Analysis/LifetimeSafety/FactsGenerator.cpp
+++ b/clang/lib/Analysis/LifetimeSafety/FactsGenerator.cpp
@@ -1044,11 +1044,12 @@ void FactsGenerator::handleLifetimeCaptureBy(const FunctionDecl *FD,
           CapturingArgIdx == LifetimeCaptureByAttr::Unknown ||
           CapturingArgIdx == LifetimeCaptureByAttr::Invalid)
         continue;
-      ArrayRef<const Expr *> CallArgs = IsInstance ? Args.drop_front() : Args;
-      const Expr *CapturedByArg =
-          (CapturingArgIdx == LifetimeCaptureByAttr::This)
-              ? Args[0]
-              : CallArgs[CapturingArgIdx];
+      // FIXME: Diagnose bad CapturingArgIdx.
+      if (CapturingArgIdx != LifetimeCaptureByAttr::This &&
+          (CapturingArgIdx < 0 ||
+           static_cast<size_t>(CapturingArgIdx) >= Args.size()))
+        continue;
+      const Expr *CapturedByArg = Args[CapturingArgIdx];
       assert(CapturedByArg && "Capturer expression must be valid");
 
       OriginList *Dest = readValue(CapturedByArg);
diff --git a/clang/test/Sema/LifetimeSafety/safety.cpp b/clang/test/Sema/LifetimeSafety/safety.cpp
index bf49e86577678..fb91fb30713db 100644
--- a/clang/test/Sema/LifetimeSafety/safety.cpp
+++ b/clang/test/Sema/LifetimeSafety/safety.cpp
@@ -4076,6 +4076,29 @@ void capturing_multiple_locals() {
     use(v);                         // expected-note 2 {{later used here}}
 }
 
+namespace off_by_one_crash {
+struct Item {
+  const int* ptr;
+};
+
+struct Container {
+  const Item* saved;
+};
+
+struct Helper {
+  void AddItem(const Item& item [[clang::lifetime_capture_by(c)]],
+               Container& c) const;
+  void Populate() const {
+    Container c{};
+    {
+      Item item;
+      AddItem(item, c); // expected-warning {{local variable 'item' does not live long enough}}
+    }                   // expected-note {{local variable 'item' is destroyed here}}
+    use(c);             // expected-note {{later used here}}
+  }
+};
+} // namespace off_by_one_crash
+
 struct [[gsl::Pointer()]] PtrWithInt { int x; };
 PtrWithInt f() {
   return PtrWithInt{10};