[clang] [clang] Output an error when [[lifetimebound]] attribute is applied on a function parameter while the function returns void (PR #113460)

[Boaz Brickner via cfe-commits]

https://github.com/bricknerb updated 
https://github.com/llvm/llvm-project/pull/113460

>From 4405d652029081cd63094e9a81dfa31e8611aad4 Mon Sep 17 00:00:00 2001
From: Boaz Brickner <brick...@google.com>
Date: Wed, 23 Oct 2024 15:50:43 +0200
Subject: [PATCH 1/2] [clang] Output a warning when [[lifetimebound]] attribute
 is applied on a function parameter while the function returns void.

---
 clang/include/clang/Basic/DiagnosticSemaKinds.td |  3 +++
 clang/lib/Sema/SemaDecl.cpp                      | 14 +++++++++++++-
 clang/test/SemaCXX/attr-lifetimebound.cpp        |  3 +--
 3 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td 
b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 8e4718008ece72..a1c20b22e736ed 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/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; did you mean 'lifetime_capture_by(X)'">;
 
 // CHECK: returning address/reference of stack memory
 def warn_ret_stack_addr_ref : Warning<
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 229c9080d558ec..e611bf60718bc5 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/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);
+        }
+      }
+    }
   }
 }
 
diff --git a/clang/test/SemaCXX/attr-lifetimebound.cpp 
b/clang/test/SemaCXX/attr-lifetimebound.cpp
index 1c5c79777c71c8..f790559b6b4769 100644
--- a/clang/test/SemaCXX/attr-lifetimebound.cpp
+++ b/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 voidreturn(int &param [[clang::lifetimebound]]); // expected-error 
{{'lifetimebound' attribute cannot be applied to a parameter of a function that 
returns void; did you mean 'lifetime_capture_by(X)'}}
 
   int *not_class_member() [[clang::lifetimebound]]; // expected-error 
{{non-member function has no implicit object parameter}}
   struct A {

>From 0a9a8629f5d188b1c1d648dc83438c33c14cc8cc Mon Sep 17 00:00:00 2001
From: Boaz Brickner <brick...@google.com>
Date: Thu, 24 Oct 2024 09:55:01 +0200
Subject: [PATCH 2/2] [clang] Add a test for lifetimebound on a parameter when
 the return type is dependent on template argument but actually void.

---
 clang/test/SemaCXX/attr-lifetimebound.cpp | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/clang/test/SemaCXX/attr-lifetimebound.cpp 
b/clang/test/SemaCXX/attr-lifetimebound.cpp
index f790559b6b4769..33cce02936dda1 100644
--- a/clang/test/SemaCXX/attr-lifetimebound.cpp
+++ b/clang/test/SemaCXX/attr-lifetimebound.cpp
@@ -30,6 +30,11 @@ 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);
+  }
+
   struct A {
     A();
     A(int);

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