[Clang][Parser] Add a warning to ambiguous uses of T...[N] types (#116332)

`void f(T... [N])` is no longer treated as a function with a parameter
of pack expansion type after the implementation of the pack indexing
feature. This patch introduces a warning to clarify such cases while
maintaining it as a pack indexing type in all language modes.

Closes #115222

Author: zyn0217

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -721,6 +721,9 @@ 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<
+  "%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

@@ -238,7 +238,8 @@ bool Parser::ParseOptionalCXXScopeSpecifier(
 
   else if (!HasScopeSpecifier && Tok.is(tok::identifier) &&
            GetLookAheadToken(1).is(tok::ellipsis) &&
-           GetLookAheadToken(2).is(tok::l_square)) {
+           GetLookAheadToken(2).is(tok::l_square) &&
+           !GetLookAheadToken(3).is(tok::r_square)) {
     SourceLocation Start = Tok.getLocation();
     DeclSpec DS(AttrFactory);
     SourceLocation CCLoc;
@@ -253,6 +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 treat it as a pack indexing type because the use case
+    // is fairly rare, to ensure semantic consistency given that we have
+    // backported this feature to pre-C++26 modes.
+    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/CXX/dcl.decl/dcl.meaning/dcl.fct/p13.cpp

@@ -57,9 +57,11 @@ template<typename T>
 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}}
+void c(T ... []); // expected-error {{type 'T[]' of function parameter pack does not contain any unexpanded parameter packs}}
+
+// A function that takes pointers to each type T as arguments, after any decay.
+template <typename ...T>
+void g(T...[]);
 
 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

@@ -214,6 +214,7 @@ struct MemberComponentOrder : Base {
 void NoMissingSemicolonHere(struct S
                             [3]);
 template<int ...N> void NoMissingSemicolonHereEither(struct S... [N]);
+// 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}} \

clang/test/Parser/cxx2c-pack-indexing.cpp

@@ -12,8 +12,7 @@ struct S {
             // expected-note {{to match this '['}} \
            // expected-warning{{declaration does not declare anything}}
 
-    T...[]; // expected-error{{expected expression}} \
-           // expected-warning{{declaration does not declare anything}}
+    T...[]; // expected-error{{expected member name or ';' after declaration specifiers}}
 
     void f(auto... v) {
         decltype(v...[1]) a = v...[1];
@@ -65,7 +64,7 @@ int main() {
 }
 
 
-namespace GH11460 {
+namespace GH111460 {
 template <typename... T>
 requires( ); // expected-error {{expected expression}}
 struct SS {

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

@@ -19,3 +19,35 @@ 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 {{'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}}
+
+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}}
+
+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}} \
+                  // 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'}} \
+                                    // cxx11-error {{no matching function for call to 'g'}}
+  h<int, double>(nullptr, nullptr);
+  h<S<int>, S<const char *>>("hello");
+  x<int*>(nullptr);
+}