[Clang] Implement CWG3005 Function parameters should never be name-independent.

We already attempted to implement this (it was the intent of the paper),
in that parameters were never considered nmame independent.
However, we failed to check that any previously found parameter
declaration was also name independent.

Note that as worded the current resolution is insufficient
(I wrote to CWG with better wording), but there is some
consensus on the design outcome.

Fixes #136373

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -134,6 +134,8 @@ Resolutions to C++ Defect Reports
 - Bumped the ``__cpp_constexpr`` feature-test macro to ``202002L`` in C++20 mode as indicated in
   `P2493R0 <https://wg21.link/P2493R0>`_.
 
+- Implemented `CWG3005 Function parameters should never be name-independent <https://wg21.link/CWG3005>`_.
+
 C Language Changes
 ------------------
 

clang/lib/Sema/SemaDecl.cpp

@@ -7542,16 +7542,20 @@ NamedDecl *Sema::ActOnVariableDeclarator(
 
   DeclSpec::SCS SCSpec = D.getDeclSpec().getStorageClassSpec();
   StorageClass SC = StorageClassSpecToVarDeclStorageClass(D.getDeclSpec());
-
   if (LangOpts.CPlusPlus && (DC->isClosure() || DC->isFunctionOrMethod()) &&
       SC != SC_Static && SC != SC_Extern && II && II->isPlaceholder()) {
+
     IsPlaceholderVariable = true;
+
     if (!Previous.empty()) {
       NamedDecl *PrevDecl = *Previous.begin();
       bool SameDC = PrevDecl->getDeclContext()->getRedeclContext()->Equals(
           DC->getRedeclContext());
-      if (SameDC && isDeclInScope(PrevDecl, CurContext, S, false))
-        DiagPlaceholderVariableDefinition(D.getIdentifierLoc());
+      if (SameDC && isDeclInScope(PrevDecl, CurContext, S, false)) {
+        IsPlaceholderVariable = !isa<ParmVarDecl>(PrevDecl);
+        if (IsPlaceholderVariable)
+          DiagPlaceholderVariableDefinition(D.getIdentifierLoc());
+      }
     }
   }
 

clang/test/CXX/drs/cwg30xx.cpp

@@ -0,0 +1,19 @@
+// RUN: %clang_cc1 -std=c++98 -pedantic-errors -verify=expected,cxx98 %s
+// RUN: %clang_cc1 -std=c++11 -pedantic-errors -verify=expected,since-cxx11,cxx11-23 %s
+// RUN: %clang_cc1 -std=c++14 -pedantic-errors -verify=expected,since-cxx11,cxx11-23 %s
+// RUN: %clang_cc1 -std=c++17 -pedantic-errors -verify=expected,since-cxx11,cxx11-23 %s
+// RUN: %clang_cc1 -std=c++20 -pedantic-errors -verify=expected,since-cxx11,cxx11-23,since-cxx20 %s
+// RUN: %clang_cc1 -std=c++23 -pedantic-errors -verify=expected,since-cxx11,cxx11-23,since-cxx20,since-cxx23 %s
+// RUN: %clang_cc1 -std=c++2c -pedantic-errors -verify=expected,since-cxx11,since-cxx20,since-cxx23,since-cxx26 %s
+
+
+namespace cwg3005 { // cwg3005: 21 open 2025-03-10
+
+void f(int _, // expected-note  {{previous declaration is here}} \
+              // expected-note  {{previous definition is here}}
+       int _) // expected-error {{redefinition of parameter '_'}}
+{
+    int _; // expected-error {{redefinition of '_'}}
+}
+
+}

clang/www/cxx_dr_status.html

@@ -17890,7 +17890,11 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/3005.html">3005</a></td>
     <td>open</td>
     <td>Function parameters should never be name-independent</td>
-    <td align="center">Not resolved</td>
+    <td align="center">
+      <details>
+        <summary>Not resolved</summary>
+        Clang 21 implements 2025-03-10 resolution
+      </details></td>
   </tr>
   <tr class="open" id="3006">
     <td><a href="https://cplusplus.github.io/CWG/issues/3006.html">3006</a></td>