[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 updated https://github.com/llvm/llvm-project/pull/90195

>From f708694fc2686684589dca7b8f3738a117fc047e Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Fri, 26 Apr 2024 19:06:57 +0800
Subject: [PATCH 1/2] [Sema] Avoid an undesired pack expansion while
 transforming PackIndexingType

---
 clang/lib/Sema/TreeTransform.h             |  3 +++
 clang/test/SemaCXX/cxx2c-pack-indexing.cpp | 19 +++++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 9404be5a46f3f7..abc4a16c004a9f 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -6649,6 +6649,9 @@ TreeTransform<Derived>::TransformPackIndexingType(TypeLocBuilder &TLB,
     }
   }
 
+  // We may be doing this in the context of expanding the Pattern. Forget that
+  // because it has been handled above.
+  Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
   QualType Result = getDerived().TransformType(TLB, TL.getPatternLoc());
 
   QualType Out = getDerived().RebuildPackIndexingType(
diff --git a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
index 606715e6aacffd..2fd0dbfed294a5 100644
--- a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
+++ b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
@@ -160,3 +160,22 @@ namespace GH88929 {
     using E = P...[0]; // expected-error {{unknown type name 'P'}} \
                        // expected-error {{expected ';' after alias declaration}}
 }
+
+namespace GH88925 {
+template <typename...> struct S {};
+
+template <int...> struct sequence {};
+
+template <typename... Args, int... indices> auto f(sequence<indices...>) {
+  return S<Args...[indices]...>(); // #use
+}
+
+void g() {
+  static_assert(__is_same(decltype(f<int>(sequence<0, 0>())), S<int, int>));
+  static_assert(__is_same(decltype(f<int, long>(sequence<0, 0>())), S<int, int>));
+  static_assert(__is_same(decltype(f<int, long>(sequence<0, 1>())), S<int, long>));
+  f<int, long>(sequence<3>());
+  // expected-error@#use {{invalid index 3 for pack 'Args' of size 2}}}
+  // expected-note-re at -2 {{function template specialization '{{.*}}' requested here}}
+}
+}

>From 7b0ae16b6777c6e98df64cd2366434972fc68164 Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Sat, 27 Apr 2024 12:07:30 +0800
Subject: [PATCH 2/2] Clarify comments

---
 clang/lib/Sema/TreeTransform.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index abc4a16c004a9f..b50fdab8bfd05e 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -6649,8 +6649,9 @@ TreeTransform<Derived>::TransformPackIndexingType(TypeLocBuilder &TLB,
     }
   }
 
-  // We may be doing this in the context of expanding the Pattern. Forget that
-  // because it has been handled above.
+  // A pack indexing type can appear in a larger pack expansion,
+  // e.g. `Pack...[pack_of_indexes]...`
+  // so we need to temporarily disable substitution of pack elements
   Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
   QualType Result = getDerived().TransformType(TLB, TL.getPatternLoc());