[clang] [clang] CTAD alias: Fix missing template arg packs during the transformation (PR #92535)

[Haojian Wu via cfe-commits]

https://github.com/hokein created https://github.com/llvm/llvm-project/pull/92535

clang rejects some valid code (see testcases) because of an incorrect transformed deduction guides. This patch fixes it.

We miss the template argument packs during the transformation (`auto (type-parameter-0-0...) -> Foo<>`). In `TreeTransform::TransformTemplateArguments `, we have a logic of handling template argument packs which were originally added to support CTAD alias, it doesn't seem to be needed, we need to unpack them. 

>From 21ff4048fdee4f76d7d73ae9f2bfaf18614a4a3b Mon Sep 17 00:00:00 2001
From: Haojian Wu <hokein.wu at gmail.com>
Date: Fri, 17 May 2024 14:12:04 +0200
Subject: [PATCH] [clang] CTAD alias: Fix missing template arg packs during the
 transformation.

---
 clang/lib/Sema/TreeTransform.h               |  8 --------
 clang/test/AST/ast-dump-ctad-alias.cpp       | 20 ++++++++++++++++++++
 clang/test/SemaCXX/cxx20-ctad-type-alias.cpp |  5 +++++
 3 files changed, 25 insertions(+), 8 deletions(-)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index b10e5ba65eb1c..f9fec21bf5bb6 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -4818,14 +4818,6 @@ bool TreeTransform<Derived>::TransformTemplateArguments(
     TemplateArgumentLoc In = *First;
 
     if (In.getArgument().getKind() == TemplateArgument::Pack) {
-      // When building the deduction guides, we rewrite the argument packs
-      // instead of unpacking.
-      if (getSema().CodeSynthesisContexts.back().Kind ==
-          Sema::CodeSynthesisContext::BuildingDeductionGuides) {
-        if (getDerived().TransformTemplateArgument(In, Out, Uneval))
-          return true;
-        continue;
-      }
       // Unpack argument packs, which we translate them into separate
       // arguments.
       // FIXME: We could do much better if we could guarantee that the
diff --git a/clang/test/AST/ast-dump-ctad-alias.cpp b/clang/test/AST/ast-dump-ctad-alias.cpp
index 7fe6c05621eee..9382558393e4c 100644
--- a/clang/test/AST/ast-dump-ctad-alias.cpp
+++ b/clang/test/AST/ast-dump-ctad-alias.cpp
@@ -48,3 +48,23 @@ Out2<double>::AInner t(1.0);
 // CHECK-NEXT: |       |-TemplateArgument type 'double'
 // CHECK-NEXT: |       | `-BuiltinType {{.*}} 'double'
 // CHECK-NEXT: |       `-ParmVarDecl {{.*}} 'double'
+
+template <typename... T1>
+struct Foo {
+  Foo(T1...);
+};
+
+template <typename...T2>
+using AFoo = Foo<T2...>;
+AFoo a(1, 2);
+// CHECK:      |-CXXDeductionGuideDecl {{.*}} implicit <deduction guide for AFoo> 'auto (type-parameter-0-0...) -> Foo<type-parameter-0-0...>'
+// CHECK-NEXT: | | `-ParmVarDecl {{.*}} 'type-parameter-0-0...' pack
+// CHECK-NEXT: | `-CXXDeductionGuideDecl {{.*}} implicit used <deduction guide for AFoo> 'auto (int, int) -> Foo<int, int>' implicit_instantiation
+
+template <typename T>
+using BFoo = Foo<T, T>;
+BFoo b2(1.0, 2.0);
+// CHECK:      |-CXXDeductionGuideDecl {{.*}} implicit <deduction guide for BFoo> 'auto (type-parameter-0-0, type-parameter-0-0) -> Foo<type-parameter-0-0, type-parameter-0-0>'
+// CHECK-NEXT: | | |-ParmVarDecl {{.*}} 'type-parameter-0-0'
+// CHECK-NEXT: | | `-ParmVarDecl {{.*}} 'type-parameter-0-0'
+// CHECK-NEXT: | `-CXXDeductionGuideDecl {{.*}} implicit used <deduction guide for BFoo> 'auto (double, double) -> Foo<double, double>' implicit_instantiation
diff --git a/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp b/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
index 7c186dc379c7b..479b0e3606721 100644
--- a/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
+++ b/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
@@ -173,6 +173,11 @@ template <typename... Ts>
 using AFoo = Foo<Ts...>;
 
 auto b = AFoo{};
+AFoo a(1, 2);
+
+template <typename T>
+using BFoo = Foo<T, T>;
+BFoo b2(1.0, 2.0);
 } // namespace test13
 
 namespace test14 {