Address the feedback

Author: zyn0217

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -719,10 +719,8 @@ def warn_empty_init_statement : Warning<
 def err_keyword_as_parameter : Error <
   "invalid parameter name: '%0' is a keyword">;
 def warn_pre_cxx26_ambiguous_pack_indexing_type : Warning<
-  "parameter packs without specifying a name would become a pack indexing "
-  "declaration in C++2c onwards">, InGroup<CXXPre26Compat>;
-def note_add_a_name_to_pre_cxx26_parameter_packs : Note<
-  "add a name to disambiguate">;
+  "%0 is no longer a pack expansion but a pack "
+  "indexing type; add a name to specify a pack expansion">, InGroup<CXXPre26Compat>;
 
 // C++ derived classes
 def err_dup_virtual : Error<"duplicate 'virtual' in base specifier">;

clang/lib/Parse/ParseExprCXX.cpp

@@ -243,25 +243,7 @@ bool Parser::ParseOptionalCXXScopeSpecifier(
     SourceLocation Start = Tok.getLocation();
     DeclSpec DS(AttrFactory);
     SourceLocation CCLoc;
-    TentativeParsingAction MaybePackIndexing(*this, /*Unannotated=*/true);
     SourceLocation EndLoc = ParsePackIndexingType(DS);
-    // C++ [cpp23.dcl.dcl-2]:
-    //   Previously, T...[n] would declare a pack of function parameters.
-    //   T...[n] is now a pack-index-specifier. [...] Valid C++ 2023 code that
-    //   declares a pack of parameters without specifying a declarator-id
-    //   becomes ill-formed.
-    if (!Tok.is(tok::coloncolon) && !getLangOpts().CPlusPlus26 &&
-        getCurScope()->isFunctionDeclarationScope()) {
-      Diag(DS.getEllipsisLoc(),
-           diag::warn_pre_cxx26_ambiguous_pack_indexing_type);
-      Diag(DS.getEllipsisLoc(),
-           diag::note_add_a_name_to_pre_cxx26_parameter_packs);
-      MaybePackIndexing.Revert();
-      return false;
-    }
-
-    MaybePackIndexing.Commit();
-
     if (DS.getTypeSpecType() == DeclSpec::TST_error)
       return false;
 
@@ -272,6 +254,19 @@ bool Parser::ParseOptionalCXXScopeSpecifier(
     if (Type.isNull())
       return false;
 
+    // C++ [cpp23.dcl.dcl-2]:
+    //   Previously, T...[n] would declare a pack of function parameters.
+    //   T...[n] is now a pack-index-specifier. [...] Valid C++ 2023 code that
+    //   declares a pack of parameters without specifying a declarator-id
+    //   becomes ill-formed.
+    //
+    // However, we still avoid parsing them as pack expansions because this is a
+    // rare use case of packs, despite being partway non-conforming, to ensure
+    // semantic consistency given that we have backported this feature.
+    if (!Tok.is(tok::coloncolon) && !getLangOpts().CPlusPlus26 &&
+        getCurScope()->isFunctionDeclarationScope())
+      Diag(Start, diag::warn_pre_cxx26_ambiguous_pack_indexing_type) << Type;
+
     if (!TryConsumeToken(tok::coloncolon, CCLoc)) {
       AnnotateExistingIndexedTypeNamePack(ParsedType::make(Type), Start,
                                           EndLoc);

clang/test/Parser/cxx0x-decl.cpp

@@ -214,8 +214,12 @@ struct MemberComponentOrder : Base {
 void NoMissingSemicolonHere(struct S
                             [3]);
 template<int ...N> void NoMissingSemicolonHereEither(struct S... [N]);
-// expected-warning@-1 {{parameter packs without specifying a name would become a pack indexing declaration in C++2c onwards}} \
-// expected-note@-1 {{add a name to disambiguate}}
+// expected-warning@-1 {{'S...[N]' is no longer a pack expansion but a pack indexing type; add a name to specify a pack expansion}} \
+// expected-error@-1 {{'S' does not refer to the name of a parameter pack}} \
+// expected-error@-1 {{declaration of anonymous struct must be a definition}} \
+// expected-error@-1 {{expected parameter declarator}} \
+// expected-error@-1 {{pack indexing is a C++2c extension}} \
+
 
 // This must be at the end of the file; we used to look ahead past the EOF token here.
 // expected-error@+1 {{expected unqualified-id}} expected-error@+1{{expected ';'}}

clang/test/SemaCXX/cxx2c-pack-indexing-ext-diags.cpp

@@ -21,8 +21,9 @@ void f(T... t) {
 }
 
 template <typename... T>
-void g(T... [1]); // cxx11-warning {{parameter packs without specifying a name would become a pack indexing declaration in C++2c onwards}} \
-                  // cxx11-note {{add a name to disambiguate}} \
+void g(T... [1]); // cxx11-warning {{'T...[1]' is no longer a pack expansion but a pack indexing type; add a name to specify a pack expansion}} \
+                  // cxx11-warning {{pack indexing is a C++2c extension}} \
+                  // cxx11-note {{candidate function template not viable}} \
                   // cxx26-warning {{pack indexing is incompatible with C++ standards before C++2c}} \
                   // cxx26-note {{candidate function template not viable}}
 
@@ -38,8 +39,14 @@ template <typename... T>
 void h(typename T... [1]::type); // cxx11-warning {{pack indexing is a C++2c extension}} \
                                  // cxx26-warning {{pack indexing is incompatible with C++ standards before C++2c}}
 
+template <typename... T>
+void x(T... [0]); // cxx11-warning {{'T...[0]' is no longer a pack expansion but a pack indexing type; add a name to specify a pack expansion}} \
+                  // cxx26-warning {{pack indexing is incompatible with C++ standards before C++2c}}
+
 void call() {
-  g<int, double>(nullptr, nullptr); // cxx26-error {{no matching function for call to 'g'}}
+  g<int, double>(nullptr, nullptr); // cxx26-error {{no matching function for call to 'g'}} \
+                                    // cxx11-error {{no matching function for call to 'g'}}
   h<int, double>(nullptr, nullptr);
   h<S<int>, S<const char *>>("hello");
+  x<int*>(nullptr);
 }