[Clang][Parser] Add a warning to ambiguous uses of T...[N] types

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -718,6 +718,11 @@ def warn_empty_init_statement : Warning<
   "has no effect">, InGroup<EmptyInitStatement>, DefaultIgnore;
 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">;
 
 // C++ derived classes
 def err_dup_virtual : Error<"duplicate 'virtual' in base specifier">;

clang/lib/Parse/ParseExprCXX.cpp

@@ -242,7 +242,25 @@ 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;
 

clang/test/CXX/dcl.decl/dcl.meaning/dcl.fct/p13.cpp

@@ -58,8 +58,9 @@ void b(T[] ...);
 
 template<typename T>
 void c(T ... []); // expected-error {{expected expression}} \
-                  // expected-error {{'T' does not refer to the name of a parameter pack}} \
-                  // expected-warning {{pack indexing is a C++2c extension}}
+                  // expected-error {{type 'T[]' of function parameter pack does not contain any unexpanded parameter packs}} \
+                  // expected-warning {{would become a pack indexing declaration in C++2c onwards}} \
+                  // expected-note {{add a name to disambiguate}}
 
 template<typename T>
 void d(T ... x[]); // expected-error{{type 'T[]' of function parameter pack does not contain any unexpanded parameter packs}}

clang/test/Parser/cxx0x-decl.cpp

@@ -214,12 +214,8 @@ struct MemberComponentOrder : Base {
 void NoMissingSemicolonHere(struct S
                             [3]);
 template<int ...N> void NoMissingSemicolonHereEither(struct S... [N]);
-// 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}} \
-
-
+// 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}}
 
 // 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

@@ -19,3 +19,27 @@ void f(T... t) {
     // cxx11-warning@+1 {{pack indexing is a C++2c extension}}
     T...[0] c;
 }
+
+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}} \
+                  // cxx26-warning {{pack indexing is incompatible with C++ standards before C++2c}} \
+                  // cxx26-note {{candidate function template not viable}}
+
+template <typename... T>
+void h(T... param[1]);
+
+template <class T>
+struct S {
+  using type = T;
+};
+
+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}}
+
+void call() {
+  g<int, double>(nullptr, nullptr); // cxx26-error {{no matching function for call to 'g'}}
+  h<int, double>(nullptr, nullptr);
+  h<S<int>, S<const char *>>("hello");
+}