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

Fixes: #107556

Author: bricknerb

clang/docs/ReleaseNotes.rst

@@ -134,8 +134,8 @@ C++ Specific Potentially Breaking Changes
     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)
+  applied on a parameter or an implicit object parameter of a function that
+  returns void. This was previously ignored and had no effect. (#GH107556)
 
   .. code-block:: c++
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -10097,9 +10097,12 @@ 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<
+def err_lifetimebound_parameter_void_return_type : Error<
   "'lifetimebound' attribute cannot be applied to a parameter of a function "
   "that returns void">;
+def err_lifetimebound_implicit_object_parameter_void_return_type : Error<
+  "'lifetimebound' attribute cannot be applied to an implicit object "
+  "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

@@ -6967,6 +6967,11 @@ static void checkAttributesAfterMerging(Sema &S, NamedDecl &ND) {
         } else if (isa<CXXConstructorDecl>(MD) || isa<CXXDestructorDecl>(MD)) {
           S.Diag(A->getLocation(), diag::err_lifetimebound_ctor_dtor)
               << isa<CXXDestructorDecl>(MD) << A->getRange();
+        } else if (FD->getReturnType()->isVoidType()) {
+          S.Diag(
+              FD->getLocation(),
+              diag::
+                  err_lifetimebound_implicit_object_parameter_void_return_type);
         }
       }
     }
@@ -6978,7 +6983,8 @@ static void checkAttributesAfterMerging(Sema &S, NamedDecl &ND) {
       // 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);
+          S.Diag(A->getLocation(),
+                 diag::err_lifetimebound_parameter_void_return_type);
         }
       }
     }

clang/test/SemaCXX/attr-lifetimebound.cpp

@@ -11,8 +11,7 @@ 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]];
+    void void_return_member() [[clang::lifetimebound]]; // expected-error {{'lifetimebound' attribute cannot be applied to an implicit object parameter of a function that returns void}}
   };
   int *attr_with_param(int &param [[clang::lifetimebound(42)]]); // expected-error {{takes no arguments}}
 }