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

[cor3ntin]

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 #136373

[llvmbot]

@llvm/pr-subscribers-clang

Author: cor3ntin (cor3ntin)

Changes

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 #136373

---

Full diff: https://github.com/llvm/llvm-project/pull/138245.diff

4 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+2)
• (modified) clang/lib/Sema/SemaDecl.cpp (+7-3)
• (added) clang/test/CXX/drs/cwg30xx.cpp (+19)
• (modified) clang/www/cxx_dr_status.html (+5-1)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index f5cd1fbeabcfe..3d83aeb46ad72 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/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
 ------------------
 
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 46933c5c43168..eb1f2c172c427 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -7532,16 +7532,20 @@ 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());
+      }
     }
   }
 
diff --git a/clang/test/CXX/drs/cwg30xx.cpp b/clang/test/CXX/drs/cwg30xx.cpp
new file mode 100644
index 0000000000000..2f65e8b9bebbe
--- /dev/null
+++ b/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 '_'}}
+}
+
+}
diff --git a/clang/www/cxx_dr_status.html b/clang/www/cxx_dr_status.html
index 8fe53ad46aca9..f8783c6858de1 100755
--- a/clang/www/cxx_dr_status.html
+++ b/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>

[Endilll]

I'm not going to bother you with the description of what I want to see in this file. Here you go:

// 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 declaration is here}}
{
  int _;
  // expected-error@-1 {{redefinition of '_'}}
  //   expected-note@#cwg3005-first-param {{previous declaration is here}}
}

} // namespace cwg3005

[erichkeane]

THIS example should also be ill-formed by CWG3005:

struct S3 {
   union {
     int _;
     };
    int _; // error, the one in the union isn't type independent
};

[cor3ntin]

THIS example should also be ill-formed by CWG3005:

struct S3 {
   union {
     int _;
     };
    int _; // error, the one in the union isn't type independent
};

This is CWG2764 - I'll do that in a separate PR (and I need to open another issue because I think the wording fails to implement the intent)

[erichkeane]

THIS example should also be ill-formed by CWG3005:

struct S3 {
   union {
     int _;
     };
    int _; // error, the one in the union isn't type independent
};

This is CWG2764 - I'll do that in a separate PR (and I need to open another issue because I think the wording fails to implement the intent)

Ah! SGTM!