[Clang] Implement CWG3005 Function parameters should never be name-independent (llvm#138245)

We already attempted to implement this (it was the intent of the paper),
in that parameters were never considered name-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 llvm#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 +1,23 @@
+// RUN: %clang_cc1 -std=c++98 -pedantic-errors -verify=expected %s
+// RUN: %clang_cc1 -std=c++11 -pedantic-errors -verify=expected %s
+// RUN: %clang_cc1 -std=c++14 -pedantic-errors -verify=expected %s
+// RUN: %clang_cc1 -std=c++17 -pedantic-errors -verify=expected %s
+// RUN: %clang_cc1 -std=c++20 -pedantic-errors -verify=expected %s
+// RUN: %clang_cc1 -std=c++23 -pedantic-errors -verify=expected %s
+// RUN: %clang_cc1 -std=c++2c -pedantic-errors -verify=expected %s
+
+
+namespace cwg3005 { // cwg3005: 21 open 2025-03-10
+
+void f(
+    int _, // #cwg3005-first-param
+    int _)
+    // expected-error@-1 {{redefinition of parameter '_'}}
+    //   expected-note@#cwg3005-first-param {{previous definition is here}}
+{
+    int _;
+    // expected-error@-1 {{redefinition of '_'}}
+    // expected-note@#cwg3005-first-param {{previous declaration is here}}
+}
+
+} // namespace cwg3005

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>