From 032ad590d6bac76965fac17b9f2ce41e076a5a0a Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109@gmail.com>
Date: Sat, 22 Mar 2025 22:55:58 +0800
Subject: [Clang] Fix various bugs in alias CTAD transform (#132061)

---
 clang/lib/Sema/SemaTemplateInstantiate.cpp | 46 ++++++++++++++++++++++--------
 1 file changed, 34 insertions(+), 12 deletions(-)

(limited to 'clang/lib/Sema/SemaTemplateInstantiate.cpp')

diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index ad49d6f..00dcadb 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -1348,6 +1348,16 @@ std::optional<TemplateDeductionInfo *> Sema::isSFINAEContext() const {
   return std::nullopt;
 }
 
+static TemplateArgument
+getPackSubstitutedTemplateArgument(Sema &S, TemplateArgument Arg) {
+  assert(S.ArgumentPackSubstitutionIndex >= 0);
+  assert(S.ArgumentPackSubstitutionIndex < (int)Arg.pack_size());
+  Arg = Arg.pack_begin()[S.ArgumentPackSubstitutionIndex];
+  if (Arg.isPackExpansion())
+    Arg = Arg.getPackExpansionPattern();
+  return Arg;
+}
+
 //===----------------------------------------------------------------------===/
 // Template Instantiation for Types
 //===----------------------------------------------------------------------===/
@@ -1467,11 +1477,13 @@ namespace {
       }
     }
 
-    static TemplateArgument
+    TemplateArgument
     getTemplateArgumentPackPatternForRewrite(const TemplateArgument &TA) {
       if (TA.getKind() != TemplateArgument::Pack)
         return TA;
-      assert(TA.pack_size() == 1 &&
+      if (SemaRef.ArgumentPackSubstitutionIndex != -1)
+        return getPackSubstitutedTemplateArgument(SemaRef, TA);
+      assert(TA.pack_size() == 1 && TA.pack_begin()->isPackExpansion() &&
              "unexpected pack arguments in template rewrite");
       TemplateArgument Arg = *TA.pack_begin();
       if (Arg.isPackExpansion())
@@ -1632,6 +1644,9 @@ namespace {
       std::vector<TemplateArgument> TArgs;
       switch (Arg.getKind()) {
       case TemplateArgument::Pack:
+        assert(SemaRef.CodeSynthesisContexts.empty() ||
+               SemaRef.CodeSynthesisContexts.back().Kind ==
+                   Sema::CodeSynthesisContext::BuildingDeductionGuides);
         // Literally rewrite the template argument pack, instead of unpacking
         // it.
         for (auto &pack : Arg.getPackAsArray()) {
@@ -1652,6 +1667,23 @@ namespace {
       return inherited::TransformTemplateArgument(Input, Output, Uneval);
     }
 
+    std::optional<unsigned> ComputeSizeOfPackExprWithoutSubstitution(
+        ArrayRef<TemplateArgument> PackArgs) {
+      // Don't do this when rewriting template parameters for CTAD:
+      //   1) The heuristic needs the unpacked Subst* nodes to figure out the
+      //   expanded size, but this never applies since Subst* nodes are not
+      //   created in rewrite scenarios.
+      //
+      //   2) The heuristic substitutes into the pattern with pack expansion
+      //   suppressed, which does not meet the requirements for argument
+      //   rewriting when template arguments include a non-pack matching against
+      //   a pack, particularly when rewriting an alias CTAD.
+      if (TemplateArgs.isRewrite())
+        return std::nullopt;
+
+      return inherited::ComputeSizeOfPackExprWithoutSubstitution(PackArgs);
+    }
+
     template<typename Fn>
     QualType TransformFunctionProtoType(TypeLocBuilder &TLB,
                                         FunctionProtoTypeLoc TL,
@@ -1871,16 +1903,6 @@ bool TemplateInstantiator::AlreadyTransformed(QualType T) {
   return true;
 }
 
-static TemplateArgument
-getPackSubstitutedTemplateArgument(Sema &S, TemplateArgument Arg) {
-  assert(S.ArgumentPackSubstitutionIndex >= 0);
-  assert(S.ArgumentPackSubstitutionIndex < (int)Arg.pack_size());
-  Arg = Arg.pack_begin()[S.ArgumentPackSubstitutionIndex];
-  if (Arg.isPackExpansion())
-    Arg = Arg.getPackExpansionPattern();
-  return Arg;
-}
-
 Decl *TemplateInstantiator::TransformDecl(SourceLocation Loc, Decl *D) {
   if (!D)
     return nullptr;
-- 
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