[Clang][P1061] Add proper C++ compatability warnings

Author: ricejasonf

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -1104,8 +1104,11 @@ def err_binding_multiple_ellipses : Error<
 def note_previous_ellipsis : Note<
   "previous binding pack specified here">;
 def ext_cxx_binding_pack : ExtWarn<
-  "structured binding pack is incompatible with C++ standards before C++2c">,
+  "structured binding packs are a C++2c extension ">,
   InGroup<CXX26>;
+def warn_cxx23_compat_binding_pack : Warning<
+  "structured binding packs are incompatible with C++ standards before C++2c">,
+  InGroup<CXXPre26Compat>, DefaultIgnore;
 def err_capture_default_first : Error<
   "capture default must be first">;
 def ext_decl_attrs_on_lambda : ExtWarn<

clang/lib/Parse/ParseDecl.cpp

@@ -7347,8 +7347,8 @@ void Parser::ParseDecompositionDeclarator(Declarator &D) {
     SourceLocation EllipsisLoc;
 
     if (Tok.is(tok::ellipsis)) {
-      if (!getLangOpts().CPlusPlus26)
-        Diag(Tok, diag::ext_cxx_binding_pack);
+      Diag(Tok, getLangOpts().CPlusPlus26 ? diag::warn_cxx23_compat_binding_pack
+                                          : diag::ext_cxx_binding_pack);
       if (PrevEllipsisLoc.isValid()) {
         Diag(Tok, diag::err_binding_multiple_ellipses);
         Diag(PrevEllipsisLoc, diag::note_previous_ellipsis);

clang/test/SemaCXX/cxx2c-binding-pack-nontemplate.cpp

@@ -1,8 +1,11 @@
-// RUN: %clang_cc1 -fsyntax-only -std=c++26 %s -verify=nontemplate
-// RUN: %clang_cc1 -fsyntax-only %s -verify=nontemplate,compat
+// RUN: %clang_cc1 -std=c++26 -fsyntax-only %s -verify=nontemplate
+// RUN: %clang_cc1 -std=c++2c -verify=cxx26,nontemplate -fsyntax-only -Wpre-c++26-compat %s
+// RUN: %clang_cc1 -std=c++23 -verify=cxx23,nontemplate -fsyntax-only -Wc++26-extensions %s
 
 void decompose_array() {
   int arr[4] = {1, 2, 3, 6};
-  auto [x, ...rest, y] = arr; // nontemplate-error{{pack declaration outside of template}} \
-  // compat-warning{{structured binding pack is incompatible with C++ standards before C++2c}}
+  // cxx26-warning@+3 {{structured binding packs are incompatible with C++ standards before C++2c}}
+  // cxx23-warning@+2 {{structured binding packs are a C++2c extension}}
+  // nontemplate-error@+1 {{pack declaration outside of template}}
+  auto [x, ...rest, y] = arr;
 }