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

[via cfe-commits]

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

Alias template class template argument deduction is a documented C++20 feature.  C++17 also happens to support it, but there is no message output to indicate the officially supported version. This PR adds that.

Also updated relevant CTAD test cases.

PR for https://github.com/llvm/llvm-project/issues/125913

>From dd978982b2ab41d3d2a55abb448e653a80158ecd Mon Sep 17 00:00:00 2001
From: George Asante <gkasante at gmail.com>
Date: Mon, 31 Mar 2025 17:41:20 -0400
Subject: [PATCH] Add warning message for C++17 alias template CTAD

---
 clang/include/clang/Basic/DiagnosticSemaKinds.td    | 13 ++++++++++---
 clang/lib/Sema/SemaInit.cpp                         |  4 +++-
 clang/test/SemaCXX/cxx17-compat.cpp                 |  2 +-
 .../cxx1z-class-template-argument-deduction.cpp     |  4 ++--
 4 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index c77cde297dc32..6f71628f19a4a 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -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>;
 }
 
 def err_return_in_captured_stmt : Error<
diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index cea121d576c5c..7229cd42f85d0 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -9896,7 +9896,9 @@ QualType Sema::DeduceTemplateSpecializationFromInitializer(
     if (auto *AliasTemplate = dyn_cast_or_null<TypeAliasTemplateDecl>(
             TemplateName.getAsTemplateDecl())) {
       Diag(Kind.getLocation(),
-           diag::warn_cxx17_compat_ctad_for_alias_templates);
+           getLangOpts().CPlusPlus20
+               ? diag::warn_cxx17_compat_ctad_for_alias_templates
+               : diag::ext_ctad_for_alias_templates_cxx20);
       LookupTemplateDecl = AliasTemplate;
       auto UnderlyingType = AliasTemplate->getTemplatedDecl()
                                 ->getUnderlyingType()
diff --git a/clang/test/SemaCXX/cxx17-compat.cpp b/clang/test/SemaCXX/cxx17-compat.cpp
index 54ea3384022d4..81b3e1fde5493 100644
--- a/clang/test/SemaCXX/cxx17-compat.cpp
+++ b/clang/test/SemaCXX/cxx17-compat.cpp
@@ -137,7 +137,7 @@ template<typename T> struct A { A(T); };
 template<typename T> using B = A<T>;
 B b = {1};
 #if __cplusplus <= 201703L
-  // FIXME: diagnose as well
+  // expected-warning at -2 {{class template argument deduction for alias templates is a C++20 extension}}
 #else
   // expected-warning at -4 {{class template argument deduction for alias templates is incompatible with C++ standards before C++20}}
 #endif
diff --git a/clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp b/clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp
index 9aaa13d7ac41a..a7d740e66ba63 100644
--- a/clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp
+++ b/clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp
@@ -113,9 +113,9 @@ namespace dependent {
   };
   template<typename T> void f() {
     typename T::X tx = 0;
-    typename T::Y ty = 0;
+    typename T::Y ty = 0; // expected-warning {{class template argument deduction for alias templates is a C++20 extension}}
   }
-  template void f<B>();
+  template void f<B>(); // expected-note {{in instantiation of function template specialization 'dependent::f<dependent::B>' requested here}}
 
   template<typename T> struct C { C(T); };
   template<typename T> C(T) -> C<T>;