[clang] [clang] Do not substitute parameter pack while retaining the pack expansion (PR #108197)

[Utkarsh Saxena via cfe-commits]

https://github.com/usx95 updated https://github.com/llvm/llvm-project/pull/108197

>From 5901d82ea0543074853b963f7dc9106a6fe3bcee Mon Sep 17 00:00:00 2001
From: Utkarsh Saxena <usx at google.com>
Date: Wed, 11 Sep 2024 11:33:45 +0000
Subject: [PATCH 1/3] [clang] Do not expand pack while retaining expansion

---
 clang/lib/Sema/TreeTransform.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 0daf620b4123e4..a40673b04764da 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -4361,7 +4361,7 @@ bool TreeTransform<Derived>::TransformExprs(Expr *const *Inputs,
       // forgetting the partially-substituted parameter pack.
       if (RetainExpansion) {
         ForgetPartiallySubstitutedPackRAII Forget(getDerived());
-
+        Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
         ExprResult Out = getDerived().TransformExpr(Pattern);
         if (Out.isInvalid())
           return true;

>From 97fbf34c3edd09348fb48b4dc66f1d854516e8ef Mon Sep 17 00:00:00 2001
From: Utkarsh Saxena <usx at google.com>
Date: Wed, 11 Sep 2024 11:59:58 +0000
Subject: [PATCH 2/3] Add comment

---
 clang/lib/Sema/TreeTransform.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index a40673b04764da..0de43d2127b12f 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -4361,6 +4361,7 @@ bool TreeTransform<Derived>::TransformExprs(Expr *const *Inputs,
       // forgetting the partially-substituted parameter pack.
       if (RetainExpansion) {
         ForgetPartiallySubstitutedPackRAII Forget(getDerived());
+        // Simple transform producing another pack expansion.
         Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
         ExprResult Out = getDerived().TransformExpr(Pattern);
         if (Out.isInvalid())

>From ab756525ab4a5f71ee5d3703238a1f9188a376e1 Mon Sep 17 00:00:00 2001
From: Utkarsh Saxena <usx at google.com>
Date: Wed, 11 Sep 2024 15:45:40 +0000
Subject: [PATCH 3/3] Add tests

---
 clang/test/SemaCXX/GH107560.cpp | 14 ++++++++++++++
 1 file changed, 14 insertions(+)
 create mode 100644 clang/test/SemaCXX/GH107560.cpp

diff --git a/clang/test/SemaCXX/GH107560.cpp b/clang/test/SemaCXX/GH107560.cpp
new file mode 100644
index 00000000000000..faaa7004e06cf9
--- /dev/null
+++ b/clang/test/SemaCXX/GH107560.cpp
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+// expected-no-diagnostics
+
+int bar(...);
+
+template <int> struct Int {};
+
+template <class ...T>
+consteval auto foo(T... x) -> decltype(bar(T(x)...)) { return 10; }
+
+template <class ...T>
+constexpr auto baz(Int<foo<T>(T())>... x) -> int { return 1; }
+
+static_assert(baz<Int<1>, Int<2>, Int<3>>(Int<10>(), Int<10>(), Int<10>()) == 1);