[clang] CTAD alias: Fix missing template arg packs during the transformation

[hokein]

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.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Haojian Wu (hokein)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/92535.diff

3 Files Affected:

• (modified) clang/lib/Sema/TreeTransform.h (-8)
• (modified) clang/test/AST/ast-dump-ctad-alias.cpp (+20)
• (modified) clang/test/SemaCXX/cxx20-ctad-type-alias.cpp (+5)

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 {