[clang] [LifetimeSafety] Model pointer-to-data-member access in the fact generator (PR #204612)

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llvmorg-github-actions[bot] wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-analysis

Author: Gábor Horváth (Xazax-hun)

<details>
<summary>Changes</summary>

VisitBinaryOperator had no case for `obj.*pm` (BO_PtrMemD) / `objptr->*pm` (BO_PtrMemI), so a borrow of the accessed member (`&(obj.*pm)`) dropped the object's loan to an empty origin and a use-after-scope was missed. Flow the object operand's origin into the result, mirroring a member access: for `.*` the object is the LHS, for `->*` it is the LHS pointer's pointee.

Assisted-by: Claude Opus 4.8

---
Full diff: https://github.com/llvm/llvm-project/pull/204612.diff


2 Files Affected:

- (modified) clang/lib/Analysis/LifetimeSafety/FactsGenerator.cpp (+14) 
- (modified) clang/test/Sema/LifetimeSafety/safety.cpp (+46) 


``````````diff
diff --git a/clang/lib/Analysis/LifetimeSafety/FactsGenerator.cpp b/clang/lib/Analysis/LifetimeSafety/FactsGenerator.cpp
index d56703a4b29c4..e2f934e0b6ad2 100644
--- a/clang/lib/Analysis/LifetimeSafety/FactsGenerator.cpp
+++ b/clang/lib/Analysis/LifetimeSafety/FactsGenerator.cpp
@@ -468,6 +468,20 @@ void FactsGenerator::handlePointerArithmetic(const BinaryOperator *BO) {
 }
 
 void FactsGenerator::VisitBinaryOperator(const BinaryOperator *BO) {
+  if (BO->getOpcode() == BO_PtrMemD || BO->getOpcode() == BO_PtrMemI) {
+    // `obj.*pm` / `objptr->*pm` names a member of the object, so a borrow of it
+    // borrows the object; flow the object's origin into the result. For `.*` the
+    // object is the LHS; for `->*` it is the LHS pointer's pointee.
+    OriginList *Dst = getOriginsList(*BO);
+    OriginList *ObjSrc =
+        BO->getOpcode() == BO_PtrMemD
+            ? getOriginsList(*BO->getLHS())
+            : getRValueOrigins(BO->getLHS(), getOriginsList(*BO->getLHS()));
+    if (Dst && ObjSrc && Dst->getLength() == ObjSrc->getLength())
+      flow(Dst, ObjSrc, /*Kill=*/true);
+    handleUse(BO->getLHS());
+    return;
+  }
   if (BO->getOpcode() == BO_Comma) {
     killAndFlowOrigin(*BO, *BO->getRHS());
     return;
diff --git a/clang/test/Sema/LifetimeSafety/safety.cpp b/clang/test/Sema/LifetimeSafety/safety.cpp
index 6fc275b51a9d0..69c1605ce9708 100644
--- a/clang/test/Sema/LifetimeSafety/safety.cpp
+++ b/clang/test/Sema/LifetimeSafety/safety.cpp
@@ -1962,6 +1962,52 @@ std::string_view refViewMemberReturnRefView1(RefMember a) { return a.view_ref; }
 std::string_view& refViewMemberReturnRefView2(RefMember a) { return a.view_ref; }
 } // namespace field_access
 
+namespace pointer_to_member {
+struct S { int x; void f() const; };
+
+// `&(obj.*pm)` borrows the object, like `&obj.field`.
+void via_dot_star() {
+  const int *p;
+  {
+    S s{5};
+    int S::*pm = &S::x;
+    p = &(s.*pm); // expected-warning {{local variable 's' does not live long enough}}
+  }               // expected-note {{destroyed here}}
+  (void)*p;       // expected-note {{later used here}}
+}
+
+void via_arrow_star() {
+  const int *p;
+  {
+    S s{5};
+    int S::*pm = &S::x;
+    S *sp = &s;     // expected-warning {{local variable 's' does not live long enough}}
+    p = &(sp->*pm); // expected-note {{local variable 'sp' aliases the storage of local variable 's'}}
+  }                 // expected-note {{destroyed here}}
+  (void)*p;         // expected-note {{later used here}}
+}
+
+// Negative: a long-lived object borrowed through `.*` stays silent.
+void via_dot_star_ok() {
+  static S s{5};
+  int S::*pm = &S::x;
+  const int *p = &(s.*pm);
+  (void)*p; // no-warning
+}
+
+// A pointer-to-member-function result is only callable (not storable), so it
+// carries no borrow -- but calling one on a dangling object is still caught.
+void via_member_function_ptr() {
+  void (S::*pmf)() const = &S::f;
+  S *sp;
+  {
+    S s;
+    sp = &s; // expected-warning {{local variable 's' does not live long enough}}
+  }          // expected-note {{destroyed here}}
+  (sp->*pmf)(); // expected-note {{later used here}}
+}
+} // namespace pointer_to_member
+
 namespace attr_on_template_params {
 struct MyObj {
   ~MyObj();

``````````

</details>


https://github.com/llvm/llvm-project/pull/204612