[clang] Add warning message for C++17 alias template CTAD (PR #133806)

[via cfe-commits]

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@@ -8444,9 +8444,16 @@ let CategoryName = "Lambda Issue" in {
     "C++ standards before C++20">, InGroup<CXXPre20Compat>, DefaultIgnore;
 
   // C++20 class template argument deduction for alias templates.
-  def warn_cxx17_compat_ctad_for_alias_templates : Warning<
-  "class template argument deduction for alias templates is incompatible with "
-  "C++ standards before C++20">, InGroup<CXXPre20Compat>, DefaultIgnore;
+  def warn_cxx17_compat_ctad_for_alias_templates
+      : Warning<"class template argument deduction for alias templates is "
+                "incompatible with "
+                "C++ standards before C++20">,
+        InGroup<CXXPre20Compat>,
+        DefaultIgnore;
+  def ext_ctad_for_alias_templates_cxx20
+      : ExtWarn<"class template argument deduction for alias templates is a "
+                "C++20 extension">,
+        InGroup<CXX20>;
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Sirraide wrote:

Can you use `CXX20Compat` to define these (there are some examples at the top of this file)

https://github.com/llvm/llvm-project/pull/133806