Fix lifetimebound for field access (#100197)

Fixes: #81589

There is no way to switch this off without  `-Wno-dangling`.

Author: usx95

clang/docs/ReleaseNotes.rst

@@ -124,6 +124,8 @@ Improvements to Clang's diagnostics
       template <typename> int i; // error: non-static data member 'i' cannot be declared as a template
      };
 
+- Clang now diagnoses dangling references to fields of temporary objects. Fixes #GH81589.
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/lib/Sema/CheckExprLifetime.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "CheckExprLifetime.h"
+#include "clang/AST/Decl.h"
 #include "clang/AST/Expr.h"
 #include "clang/Basic/DiagnosticSema.h"
 #include "clang/Sema/Initialization.h"
@@ -548,6 +549,14 @@ static void visitLocalsRetainedByReferenceBinding(IndirectLocalPath &Path,
                                        EnableLifetimeWarnings);
   }
 
+  if (auto *M = dyn_cast<MemberExpr>(Init)) {
+    // Lifetime of a non-reference type field is same as base object.
+    if (auto *F = dyn_cast<FieldDecl>(M->getMemberDecl());
+        F && !F->getType()->isReferenceType())
+      visitLocalsRetainedByInitializer(Path, M->getBase(), Visit, true,
+                                       EnableLifetimeWarnings);
+  }
+
   if (isa<CallExpr>(Init)) {
     if (EnableLifetimeWarnings)
       handleGslAnnotatedTypes(Path, Init, Visit);

clang/test/SemaCXX/attr-lifetimebound.cpp

@@ -47,6 +47,31 @@ namespace usage_ok {
     q = A(); // expected-warning {{object backing the pointer q will be destroyed at the end of the full-expression}}
     r = A(1); // expected-warning {{object backing the pointer r will be destroyed at the end of the full-expression}}
   }
+
+  struct FieldCheck {
+    struct Set {
+      int a;
+    };
+    struct Pair {
+      const int& a;
+      int b;
+      Set c;
+      int * d;
+    };
+    Pair p;  
+    FieldCheck(const int& a): p(a){}
+    Pair& getR() [[clang::lifetimebound]] { return p; }
+    Pair* getP() [[clang::lifetimebound]] { return &p; }
+    Pair* getNoLB() { return &p; }
+  };
+  void test_field_access() {
+    int x = 0;
+    const int& a = FieldCheck{x}.getR().a;
+    const int& b = FieldCheck{x}.getP()->b;   // expected-warning {{temporary bound to local reference 'b' will be destroyed at the end of the full-expression}}
+    const int& c = FieldCheck{x}.getP()->c.a; // expected-warning {{temporary bound to local reference 'c' will be destroyed at the end of the full-expression}}
+    const int& d = FieldCheck{x}.getNoLB()->c.a;
+    const int* e = FieldCheck{x}.getR().d;
+  }
 }
 
 # 1 "<std>" 1 3
@@ -239,3 +264,4 @@ namespace move_forward_et_al_examples {
   S X;
   S *AddressOfOk = std::addressof(X);
 } // namespace move_forward_et_al_examples
+