[clang] [Clang] Remove unnecessary Decl transform & profiles for SizeOfPackExpr (PR #124533)

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 created https://github.com/llvm/llvm-project/pull/124533

We used to always transform the pattern declaration for SizeOfPackExpr to ensure the constraint expression's profile produced the desired result. However, this approach failed to handle pack expansions when the pack referred to function parameters. In such cases, the function parameters were formerly expanded to 1 to avoid building Subst* nodes (see e6974daa7). That workaround caused us to transform a pack without a proper ArgumentPackSubstitutionIndex, leading to crashes when transforming the pattern.

It turns out that profiling the pattern for partially substituted SizeOfPackExprs is unnecessary because their transformed forms are also profiled within the partial arguments.

Fixes https://github.com/llvm/llvm-project/issues/124161

>From b195bfb253df84ea315eb92a59bbae2401b0248f Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Mon, 27 Jan 2025 19:48:43 +0800
Subject: [PATCH] [Clang] Remove unnecessary Decl transform & profiles for
 SizeOfPackExpr

We used to always transform the pattern declaration for SizeOfPackExpr
to ensure the constraint expression's profile produced the desired
result. However, this approach failed to handle pack expansions when
the pack referred to function parameters. In such cases, the function
parameters were previously expanded to 1 to avoid building Subst* nodes
(see e6974daa7). That workaround caused us to transform a pack without
a proper ArgumentPackSubstitutionIndex, leading to crashes when
transforming the pattern.

It turns out that profiling the pattern for partially substituted
SizeOfPackExprs is unnecessary because their transformed forms are
already profiled within the partial arguments. Moreover, we always
end up with a partially transformed SizeOfPackExpr for constraint
comparison.
---
 clang/include/clang/AST/ExprCXX.h             |  2 --
 clang/lib/AST/StmtProfile.cpp                 |  2 +-
 clang/lib/Sema/SemaTemplateInstantiate.cpp    | 17 -----------
 .../SemaTemplate/concepts-out-of-line-def.cpp | 28 +++++++++++++++++++
 4 files changed, 29 insertions(+), 20 deletions(-)

diff --git a/clang/include/clang/AST/ExprCXX.h b/clang/include/clang/AST/ExprCXX.h
index 4cec89c979f775..04529fa616d703 100644
--- a/clang/include/clang/AST/ExprCXX.h
+++ b/clang/include/clang/AST/ExprCXX.h
@@ -4326,8 +4326,6 @@ class SizeOfPackExpr final
   /// Retrieve the parameter pack.
   NamedDecl *getPack() const { return Pack; }
 
-  void setPack(NamedDecl *NewPack) { Pack = NewPack; }
-
   /// Retrieve the length of the parameter pack.
   ///
   /// This routine may only be invoked when the expression is not
diff --git a/clang/lib/AST/StmtProfile.cpp b/clang/lib/AST/StmtProfile.cpp
index 0f1ebc68a4f762..089d91d2736459 100644
--- a/clang/lib/AST/StmtProfile.cpp
+++ b/clang/lib/AST/StmtProfile.cpp
@@ -2266,13 +2266,13 @@ void StmtProfiler::VisitPackExpansionExpr(const PackExpansionExpr *S) {
 
 void StmtProfiler::VisitSizeOfPackExpr(const SizeOfPackExpr *S) {
   VisitExpr(S);
-  VisitDecl(S->getPack());
   if (S->isPartiallySubstituted()) {
     auto Args = S->getPartialArguments();
     ID.AddInteger(Args.size());
     for (const auto &TA : Args)
       VisitTemplateArgument(TA);
   } else {
+    VisitDecl(S->getPack());
     ID.AddInteger(0);
   }
 }
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index 839c4e8a28220b..98bc630fb69cc5 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -1747,23 +1747,6 @@ namespace {
       return inherited::TransformLambdaBody(E, Body);
     }
 
-    ExprResult TransformSizeOfPackExpr(SizeOfPackExpr *E) {
-      ExprResult Transformed = inherited::TransformSizeOfPackExpr(E);
-      if (!Transformed.isUsable())
-        return Transformed;
-      auto *TransformedExpr = cast<SizeOfPackExpr>(Transformed.get());
-      if (SemaRef.CodeSynthesisContexts.back().Kind ==
-              Sema::CodeSynthesisContext::ConstraintNormalization &&
-          TransformedExpr->getPack() == E->getPack()) {
-        Decl *NewPack =
-            TransformDecl(E->getPackLoc(), TransformedExpr->getPack());
-        if (!NewPack)
-          return ExprError();
-        TransformedExpr->setPack(cast<NamedDecl>(NewPack));
-      }
-      return TransformedExpr;
-    }
-
     ExprResult TransformRequiresExpr(RequiresExpr *E) {
       LocalInstantiationScope Scope(SemaRef, /*CombineWithOuterScope=*/true);
       ExprResult TransReq = inherited::TransformRequiresExpr(E);
diff --git a/clang/test/SemaTemplate/concepts-out-of-line-def.cpp b/clang/test/SemaTemplate/concepts-out-of-line-def.cpp
index 7cb5cfc10b9a7b..16779ba184296d 100644
--- a/clang/test/SemaTemplate/concepts-out-of-line-def.cpp
+++ b/clang/test/SemaTemplate/concepts-out-of-line-def.cpp
@@ -720,6 +720,34 @@ template <typename... Ts> struct d {
 template struct c<int>;
 template struct d<int, int>;
 
+namespace Regression_123441 {
+
+struct buf {
+  constexpr buf(auto&&... initList) requires (sizeof...(initList) <= 8);
+};
+
+constexpr buf::buf(auto&&... initList) requires (sizeof...(initList) <= 8) {}
+
+template <class>
+struct buffer {
+  constexpr buffer(auto&&... initList) requires (sizeof...(initList) <= 8);
+};
+
+template <class T>
+constexpr buffer<T>::buffer(auto&&... initList) requires (sizeof...(initList) <= 8) {}
+
+template <class...>
+struct foo { // expected-note {{foo defined here}}
+  constexpr foo(auto&&... initList)
+    requires (sizeof...(initList) <= 8);
+};
+
+template <class... T>
+constexpr foo<T...>::foo(auto&&... initList) // expected-error {{does not match any declaration}}
+  requires (sizeof...(T) <= 8) {}
+
+}
+
 } // namespace GH115098
 
 namespace GH114685 {