[clang] Implement P3176R1: The Oxford variadic comma (#117524)

Emit a deprecation warning when a variadic parameter in a
parameter-declaration-clause is not preceded by a comma for C++26.

Author: antangelo

clang/docs/ReleaseNotes.rst

@@ -256,6 +256,8 @@ C++2c Feature Support
 - Added the ``__builtin_is_within_lifetime`` builtin, which supports
   `P2641R4 Checking if a union alternative is active <https://wg21.link/p2641r4>`_
 
+- Implemented `P3176R1 The Oxford variadic comma <https://wg21.link/P3176R1>`_
+
 C++23 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 - Removed the restriction to literal types in constexpr functions in C++23 mode.

clang/include/clang/Basic/DiagnosticGroups.td

@@ -211,6 +211,7 @@ def DeprecatedWritableStr : DiagGroup<"deprecated-writable-strings",
                                       [CXX11CompatDeprecatedWritableStr]>;
 def DeprecatedPragma : DiagGroup<"deprecated-pragma">;
 def DeprecatedType : DiagGroup<"deprecated-type">;
+def DeprecatedMissingCommaVariadicParam : DiagGroup<"deprecated-missing-comma-variadic-parameter">;
 // FIXME: Why is DeprecatedImplementations not in this group?
 def Deprecated : DiagGroup<"deprecated", [DeprecatedAnonEnumEnumConversion,
                                           DeprecatedArrayCompare,
@@ -235,7 +236,8 @@ def Deprecated : DiagGroup<"deprecated", [DeprecatedAnonEnumEnumConversion,
                                           DeprecatedType,
                                           DeprecatedVolatile,
                                           DeprecatedWritableStr,
-                                          DeprecatedRedundantConstexprStaticDef
+                                          DeprecatedRedundantConstexprStaticDef,
+                                          DeprecatedMissingCommaVariadicParam
                                           ]>,
                  DiagCategory<"Deprecations">;
 

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -413,6 +413,9 @@ def err_function_scope_depth_exceeded : Error<
   "function scope depth exceeded maximum of %0">, DefaultFatal;
 def err_missing_comma_before_ellipsis : Error<
   "C requires a comma prior to the ellipsis in a variadic function type">;
+def warn_deprecated_missing_comma_before_ellipsis : Warning<
+  "declaration of a variadic function without a comma before '...' is deprecated">,
+  InGroup<DeprecatedMissingCommaVariadicParam>;
 def err_unexpected_typedef_ident : Error<
   "unexpected type name %0: expected identifier">;
 def warn_cxx98_compat_decltype : Warning<

clang/lib/Parse/ParseDecl.cpp

@@ -8119,6 +8119,14 @@ void Parser::ParseParameterDeclarationClause(
     }
 
     if (TryConsumeToken(tok::ellipsis, EllipsisLoc)) {
+      if (getLangOpts().CPlusPlus26) {
+        // C++26 [dcl.dcl.fct]p3:
+        //   A parameter-declaration-clause of the form
+        //   parameter-list '...' is deprecated.
+        Diag(EllipsisLoc, diag::warn_deprecated_missing_comma_before_ellipsis)
+            << FixItHint::CreateInsertion(EllipsisLoc, ", ");
+      }
+
       if (!getLangOpts().CPlusPlus) {
         // We have ellipsis without a preceding ',', which is ill-formed
         // in C. Complain and provide the fix.

clang/test/CXX/drs/cwg722.cpp

@@ -14,7 +14,7 @@ namespace std {
   using nullptr_t = decltype(nullptr);
 }
 
-void f(std::nullptr_t...);
+void f(std::nullptr_t, ...);
 std::nullptr_t g();
 void h() {
   std::nullptr_t np;

clang/test/Parser/cxx2c-oxford-variadic-comma.cpp

@@ -0,0 +1,54 @@
+// RUN: %clang_cc1 -std=c++2c -fsyntax-only -fblocks -verify %s
+
+void a(...);
+
+void b(auto...);
+void c(auto, ...);
+
+void d(auto......); // expected-warning {{declaration of a variadic function without a comma before '...' is deprecated}} \
+                    // expected-warning {{'...' in this location creates a C-style varargs function}} \
+                    // expected-note {{preceding '...' declares a function parameter pack}} \
+                    // expected-note {{insert ',' before '...' to silence this warning}}
+void e(auto..., ...);
+
+void f(auto x...); // expected-warning {{declaration of a variadic function without a comma before '...' is deprecated}}
+void g(auto x, ...);
+
+void h(auto... x...); // expected-warning {{declaration of a variadic function without a comma before '...' is deprecated}} \
+                      // expected-warning {{'...' in this location creates a C-style varargs function}} \
+                      // expected-note {{preceding '...' declares a function parameter pack}} \
+                      // expected-note {{insert ',' before '...' to silence this warning}}
+void i(auto... x, ...);
+
+template<class ...Ts>
+void j(Ts... t...) {}; // expected-warning {{declaration of a variadic function without a comma before '...' is deprecated}} \
+                       // expected-warning {{'...' in this location creates a C-style varargs function}} \
+                       // expected-note {{preceding '...' declares a function parameter pack}} \
+                       // expected-note {{insert ',' before '...' to silence this warning}}
+template<class ...Ts>
+void k(Ts... t, ...) {}
+
+void l(int...); // expected-warning {{declaration of a variadic function without a comma before '...' is deprecated}}
+void m(int, ...);
+
+void n(int x...); // expected-warning {{declaration of a variadic function without a comma before '...' is deprecated}}
+void o(int x, ...);
+
+struct S {
+  void p(this S...) {} // expected-warning {{declaration of a variadic function without a comma before '...' is deprecated}}
+};
+
+template<class ...Ts>
+void q(Ts......) {} // expected-warning {{declaration of a variadic function without a comma before '...' is deprecated}} \
+                    // expected-warning {{'...' in this location creates a C-style varargs function}} \
+                    // expected-note {{preceding '...' declares a function parameter pack}} \
+                    // expected-note {{insert ',' before '...' to silence this warning}}
+
+template<class T>
+void r(T...) {} // expected-warning {{declaration of a variadic function without a comma before '...' is deprecated}}
+
+auto type_specifier = (void (*)(int...)) nullptr; // expected-warning {{declaration of a variadic function without a comma before '...' is deprecated}}
+
+auto lambda = [](int...) {}; // expected-warning {{declaration of a variadic function without a comma before '...' is deprecated}}
+
+auto block = ^(int...){}; // expected-warning {{declaration of a variadic function without a comma before '...' is deprecated}}

clang/www/cxx_status.html

@@ -249,7 +249,7 @@ <h2 id="cxx26">C++2c implementation status</h2>
  <tr>
   <td>The Oxford variadic comma</td>
   <td><a href="https://wg21.link/P3176R1">P3176R1</a></td>
-  <td class="none" align="center">No</td>
+  <td class="unreleased" align="center">Clang 20</td>
  </tr>
 </table>
 </details>