[Clang] Reject this void explicit object parameters (CWG2915) (#108817)

https://cplusplus.github.io/CWG/issues/2915.html

Previously, `struct A { void f(this void); };` was accepted with `A::f`
being a member function with no non-object arguments, but it was still a
little wonky because it was still considered an explicit object member
function. Now, this is rejected immediately.

This applies to any language mode with explicit object parameters as
this is a DR (C++23 and C++26)

Author: MitalAshok

clang/docs/ReleaseNotes.rst

@@ -170,6 +170,9 @@ Resolutions to C++ Defect Reports
   in constant expressions. These comparisons always worked in non-constant expressions.
   (`CWG2749: Treatment of "pointer to void" for relational comparisons <https://cplusplus.github.io/CWG/issues/2749.html>`_).
 
+- Reject explicit object parameters with type ``void`` (``this void``).
+  (`CWG2915: Explicit object parameters of type void <https://cplusplus.github.io/CWG/issues/2915.html>`_).
+
 C Language Changes
 ------------------
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -4687,6 +4687,8 @@ def err_void_only_param : Error<
   "'void' must be the first and only parameter if specified">;
 def err_void_param_qualified : Error<
   "'void' as parameter must not have type qualifiers">;
+def err_void_explicit_object_param : Error<
+  "explicit object parameter cannot have 'void' type">;
 def err_ident_list_in_fn_declaration : Error<
   "a parameter list without types is only allowed in a function definition">;
 def ext_param_not_declared : ExtWarn<

clang/lib/Sema/SemaType.cpp

@@ -5166,6 +5166,14 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
               if (ParamTy.hasQualifiers())
                 S.Diag(DeclType.Loc, diag::err_void_param_qualified);
 
+              // Reject, but continue to parse 'float(this void)' as
+              // 'float(void)'.
+              if (Param->isExplicitObjectParameter()) {
+                S.Diag(Param->getLocation(),
+                       diag::err_void_explicit_object_param);
+                Param->setExplicitObjectParameterLoc(SourceLocation());
+              }
+
               // Do not add 'void' to the list.
               break;
             }

clang/test/CXX/drs/cwg29xx.cpp

@@ -6,6 +6,14 @@
 // RUN: %clang_cc1 -std=c++23 -pedantic-errors -verify=expected %s
 // RUN: %clang_cc1 -std=c++2c -pedantic-errors -verify=expected %s
 
+namespace cwg2915 { // cwg2915: 20 tentatively ready 2024-08-16
+#if __cplusplus >= 202302L
+struct A {
+  void f(this void); // expected-error {{explicit object parameter cannot have 'void' type}}
+};
+#endif
+}
+
 namespace cwg2917 { // cwg2917: 20 review 2024-07-30
 template <typename>
 class Foo;

clang/www/cxx_dr_status.html

@@ -17346,7 +17346,11 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/2915.html">2915</a></td>
     <td>tentatively ready</td>
     <td>Explicit object parameters of type <TT>void</TT></td>
-    <td align="center">Not resolved</td>
+    <td align="center">
+      <details>
+        <summary>Not resolved</summary>
+        Clang 20 implements 2024-08-16 resolution
+      </details></td>
   </tr>
   <tr class="open" id="2916">
     <td><a href="https://cplusplus.github.io/CWG/issues/2916.html">2916</a></td>