[clang] Output an error when [[lifetimebound]] attribute is applied on a function parameter while the function returns void

clang/docs/ReleaseNotes.rst

@@ -133,6 +133,15 @@ C++ Specific Potentially Breaking Changes
     // Fixed version:
     unsigned operator""_udl_name(unsigned long long);
 
+- Clang will now produce an error diagnostic when [[clang::lifetimebound]] is
+  applied on a parameter of a function that returns void. This was previously 
+  ignored and had no effect. (#GH107556)
+
+  .. code-block:: c++
+
+    // Now diagnoses with an error.
+    void f(int& i [[clang::lifetimebound]]);
+
 ABI Changes in This Version
 ---------------------------
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -10097,6 +10097,9 @@ def err_lifetimebound_no_object_param : Error<
 def err_lifetimebound_ctor_dtor : Error<
   "'lifetimebound' attribute cannot be applied to a "
   "%select{constructor|destructor}0">;
+def err_lifetimebound_void_return_type : Error<
+  "'lifetimebound' attribute cannot be applied to a parameter of a function "
+  "that returns void">;
 
 // CHECK: returning address/reference of stack memory
 def warn_ret_stack_addr_ref : Warning<

clang/lib/Sema/SemaDecl.cpp

@@ -6940,7 +6940,7 @@ static void checkAttributesAfterMerging(Sema &S, NamedDecl &ND) {
     }
   }
 
-  // Check the attributes on the function type, if any.
+  // Check the attributes on the function type and function params, if any.
   if (const auto *FD = dyn_cast<FunctionDecl>(&ND)) {
     // Don't declare this variable in the second operand of the for-statement;
     // GCC miscompiles that by ending its lifetime before evaluating the
@@ -6970,6 +6970,18 @@ static void checkAttributesAfterMerging(Sema &S, NamedDecl &ND) {
         }
       }
     }
+
+    for (unsigned int I = 0; I < FD->getNumParams(); ++I) {
+      const ParmVarDecl *P = FD->getParamDecl(I);
+
+      // The [[lifetimebound]] attribute can be applied to a function parameter
+      // only if the function returns a value.
+      if (auto *A = P->getAttr<LifetimeBoundAttr>()) {
+        if (!isa<CXXConstructorDecl>(FD) && FD->getReturnType()->isVoidType()) {
+          S.Diag(A->getLocation(), diag::err_lifetimebound_void_return_type);
+        }
+      }
+    }
   }
 }
 

clang/test/SemaCXX/attr-lifetimebound.cpp

@@ -1,8 +1,7 @@
 // RUN: %clang_cc1 -std=c++23 -verify %s
 
 namespace usage_invalid {
-  // FIXME: Should we diagnose a void return type?
-  void voidreturn(int &param [[clang::lifetimebound]]);
+  void void_return(int &param [[clang::lifetimebound]]); // expected-error {{'lifetimebound' attribute cannot be applied to a parameter of a function that returns void}}
 
   int *not_class_member() [[clang::lifetimebound]]; // expected-error {{non-member function has no implicit object parameter}}
   struct A {
@@ -12,6 +11,8 @@ namespace usage_invalid {
     int *explicit_object(this A&) [[clang::lifetimebound]]; // expected-error {{explicit object member function has no implicit object parameter}}
     int not_function [[clang::lifetimebound]]; // expected-error {{only applies to parameters and implicit object parameters}}
     int [[clang::lifetimebound]] also_not_function; // expected-error {{cannot be applied to types}}
+    // FIXME: Should diagnose a void return type.
+    void void_return_member() [[clang::lifetimebound]];
   };
   int *attr_with_param(int &param [[clang::lifetimebound(42)]]); // expected-error {{takes no arguments}}
 }
@@ -31,6 +32,13 @@ namespace usage_ok {
     return *(int*)param;
   }
 
+  template <class T, class R = void> R dependent_void(const T& t [[clang::lifetimebound]]);
+  void dependent_void_instantiation() {
+    dependent_void<int>(1); // OK: Returns void.
+    int x = dependent_void<int, int>(1); // expected-warning {{temporary whose address is used as value of local variable 'x' will be destroyed at the end of the full-expression}}
+    dependent_void<int, int>(1); // OK: Returns an unused value.
+  }
+
   struct A {
     A();
     A(int);