[clang] Add C++26 diagnostics to compatibility diagnosic groups (#97806)

This patch adds `CXXPre26Compat` and `CXXPre26CompatPedantic` groups
(which are concerned with new features not available in older language
modes) to `CXX98Compat`, etc. This way, if user has `-Wc++20-compat` and
they use pack indexing, they will be warned.

Ideally this should have been done when C++26 groups were created, but
we shipped two releases of Clang since then.

Author: Endilll

clang/docs/ReleaseNotes.rst

@@ -439,8 +439,11 @@ New Compiler Flags
   Matches MSVC behaviour by defining ``__STDC__`` to ``1`` when
   MSVC compatibility mode is used. It has no effect for C++ code.
 
+- ``-Wc++23-compat`` group was added to help migrating existing codebases
+  to C++23.
+
 - ``-Wc++2c-compat`` group was added to help migrating existing codebases
-  to C++26.
+  to upcoming C++26.
 
 Deprecated Compiler Flags
 -------------------------
@@ -480,6 +483,10 @@ Modified Compiler Flags
   evaluating to ``true`` and an empty body such as ``while(1);``)
   are considered infinite, even when the ``-ffinite-loop`` flag is set.
 
+- Diagnostics groups about compatibility with a particular C++ Standard version
+  now include dianostics about C++26 features that are not present in older
+  versions.
+
 Removed Compiler Flags
 -------------------------
 

clang/include/clang/Basic/DiagnosticGroups.td

@@ -344,7 +344,8 @@ def CXX98Compat : DiagGroup<"c++98-compat",
                              CXXPre14Compat,
                              CXXPre17Compat,
                              CXXPre20Compat,
-                             CXXPre23Compat]>;
+                             CXXPre23Compat,
+                             CXXPre26Compat]>;
 // Warnings for C++11 features which are Extensions in C++98 mode.
 def CXX98CompatPedantic : DiagGroup<"c++98-compat-pedantic",
                                     [CXX98Compat,
@@ -353,7 +354,8 @@ def CXX98CompatPedantic : DiagGroup<"c++98-compat-pedantic",
                                      CXXPre14CompatPedantic,
                                      CXXPre17CompatPedantic,
                                      CXXPre20CompatPedantic,
-                                     CXXPre23CompatPedantic]>;
+                                     CXXPre23CompatPedantic,
+                                     CXXPre26CompatPedantic]>;
 
 def CXX11NarrowingConstReference : DiagGroup<"c++11-narrowing-const-reference">;
 def CXX11Narrowing : DiagGroup<"c++11-narrowing", [CXX11NarrowingConstReference]>;
@@ -384,42 +386,52 @@ def CXX11Compat : DiagGroup<"c++11-compat",
                              CXXPre14Compat,
                              CXXPre17Compat,
                              CXXPre20Compat,
-                             CXXPre23Compat]>;
+                             CXXPre23Compat,
+                             CXXPre26Compat]>;
 def : DiagGroup<"c++0x-compat", [CXX11Compat]>;
 def CXX11CompatPedantic : DiagGroup<"c++11-compat-pedantic",
                                     [CXX11Compat,
                                      CXXPre14CompatPedantic,
                                      CXXPre17CompatPedantic,
                                      CXXPre20CompatPedantic,
-                                     CXXPre23CompatPedantic]>;
+                                     CXXPre23CompatPedantic,
+                                     CXXPre26CompatPedantic]>;
 
 def CXX14Compat : DiagGroup<"c++14-compat", [CXXPre17Compat,
                                              CXXPre20Compat,
-                                             CXXPre23Compat]>;
+                                             CXXPre23Compat,
+                                             CXXPre26Compat]>;
 def CXX14CompatPedantic : DiagGroup<"c++14-compat-pedantic",
                                     [CXX14Compat,
                                      CXXPre17CompatPedantic,
                                      CXXPre20CompatPedantic,
-                                     CXXPre23CompatPedantic]>;
+                                     CXXPre23CompatPedantic,
+                                     CXXPre26CompatPedantic]>;
 
 def CXX17Compat : DiagGroup<"c++17-compat", [DeprecatedRegister,
                                              DeprecatedIncrementBool,
                                              CXX17CompatMangling,
                                              CXXPre20Compat,
-                                             CXXPre23Compat]>;
+                                             CXXPre23Compat,
+                                             CXXPre26Compat]>;
 def CXX17CompatPedantic : DiagGroup<"c++17-compat-pedantic",
                                     [CXX17Compat,
                                      CXXPre20CompatPedantic,
-                                     CXXPre23CompatPedantic]>;
+                                     CXXPre23CompatPedantic,
+                                     CXXPre26CompatPedantic]>;
 def : DiagGroup<"c++1z-compat", [CXX17Compat]>;
 
-def CXX20Compat : DiagGroup<"c++20-compat", [CXXPre23Compat]>;
+def CXX20Compat : DiagGroup<"c++20-compat", [CXXPre23Compat,
+                                             CXXPre26Compat]>;
 def CXX20CompatPedantic : DiagGroup<"c++20-compat-pedantic",
                                     [CXX20Compat,
-                                     CXXPre23CompatPedantic]>;
+                                     CXXPre23CompatPedantic,
+                                     CXXPre26CompatPedantic]>;
 def : DiagGroup<"c++2a-compat", [CXX20Compat]>;
 def : DiagGroup<"c++2a-compat-pedantic", [CXX20CompatPedantic]>;
 
+def CXX23Compat : DiagGroup<"c++23-compat", [CXXPre26Compat]>;
+
 def CXX26Compat : DiagGroup<"c++2c-compat", [DeleteIncomplete]>;
 
 def ExitTimeDestructors : DiagGroup<"exit-time-destructors">;