[Clang] disallow attributes on void parameters (#124920)

Fixes #108819 

--- 

This PR introduces diagnostics to disallow the use of attributes on void
parameters

```cpp
void f([[deprecated]] void) {}
```

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -117,6 +117,7 @@ Attribute Changes in Clang
 --------------------------
 
 - The ``no_sanitize`` attribute now accepts both ``gnu`` and ``clang`` names.
+- Clang now diagnoses use of declaration attributes on void parameters. (#GH108819)
 
 Improvements to Clang's diagnostics
 -----------------------------------

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3832,6 +3832,9 @@ def warn_type_attribute_wrong_type : Warning<
   "'%0' only applies to %select{function|pointer|"
   "Objective-C object or block pointer}1 types; type here is %2">,
   InGroup<IgnoredAttributes>;
+def warn_attribute_on_void_param: Warning<
+  "attribute %0 cannot be applied to a 'void' parameter">,
+  InGroup<IgnoredAttributes>;
 def err_type_attribute_wrong_type : Error<
   warn_type_attribute_wrong_type.Summary>;
 def warn_incomplete_encoded_type : Warning<

clang/lib/Sema/SemaAttr.cpp

@@ -1189,6 +1189,11 @@ void Sema::ActOnPragmaAttributePop(SourceLocation PragmaLoc,
 void Sema::AddPragmaAttributes(Scope *S, Decl *D) {
   if (PragmaAttributeStack.empty())
     return;
+
+  if (const auto *P = dyn_cast<ParmVarDecl>(D))
+    if (P->getType()->isVoidType())
+      return;
+
   for (auto &Group : PragmaAttributeStack) {
     for (auto &Entry : Group.Entries) {
       ParsedAttr *Attribute = Entry.Attribute;

clang/lib/Sema/SemaType.cpp

@@ -5183,6 +5183,11 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
               if (ParamTy.hasQualifiers())
                 S.Diag(DeclType.Loc, diag::err_void_param_qualified);
 
+              for (const auto *A : Param->attrs()) {
+                S.Diag(A->getLoc(), diag::warn_attribute_on_void_param)
+                    << A << A->getRange();
+              }
+
               // Reject, but continue to parse 'float(this void)' as
               // 'float(void)'.
               if (Param->isExplicitObjectParameter()) {

clang/test/SemaCXX/attr-cxx0x.cpp

@@ -53,3 +53,26 @@ alignas(4) auto PR19252 = 0;
 
 // Check the diagnostic message
 class alignas(void) AlignasVoid {}; // expected-error {{invalid application of 'alignas' to an incomplete type 'void'}}
+
+namespace GH108819 {
+void a([[maybe_unused]] void) {}                 // expected-warning {{attribute 'maybe_unused' cannot be applied to a 'void' parameter}}\
+                                                 // expected-warning {{use of the 'maybe_unused' attribute is a C++17 extension}}
+void b([[deprecated, maybe_unused]] void) {}     // expected-warning {{attribute 'deprecated' cannot be applied to a 'void' parameter}} \
+                                                 // expected-warning {{attribute 'maybe_unused' cannot be applied to a 'void' parameter}} \
+                                                 // expected-warning {{use of the 'deprecated' attribute is a C++14 extension}} \
+                                                 // expected-warning {{use of the 'maybe_unused' attribute is a C++17 extension}}
+void c([[clang::lifetimebound]] void) {}         // expected-warning {{attribute 'lifetimebound' cannot be applied to a 'void' parameter}}
+void d([[clang::annotate("a", "b", 1)]] void) {} // expected-warning {{attribute 'annotate' cannot be applied to a 'void' parameter}}
+
+struct S {
+  void e([[maybe_unused]] void) {} // expected-warning {{attribute 'maybe_unused' cannot be applied to a 'void' parameter}} \
+                                   // expected-warning {{use of the 'maybe_unused' attribute is a C++17 extension}}
+};
+
+template <typename T>
+void f([[maybe_unused]] void) {} // expected-warning {{attribute 'maybe_unused' cannot be applied to a 'void' parameter}} \
+                                 // expected-warning {{use of the 'maybe_unused' attribute is a C++17 extension}}
+
+auto g = []([[maybe_unused]] void) { }; // expected-warning {{attribute 'maybe_unused' cannot be applied to a 'void' parameter}} \
+                                        // expected-warning {{use of the 'maybe_unused' attribute is a C++17 extension}}
+}