[Clang] Remove the wrong assumption when rebuilding SizeOfPackExprs for constraint normalization (#115120)

In 463a4f1, we assumed that all the template argument packs are of
size 1 when normalizing a constraint expression because I mistakenly
thought those packs were obtained from their injected template
parameters. This was wrong because we might be checking constraints when
instantiating a friend declaration within a class template
specialization, where the parent class template is specialized with
non-dependent template arguments.

In that sense, we shouldn't assume any pack size nor expand anything in
such a scenario. Moreover, there are no intermediate (substituted but
unexpanded) AST nodes for template template parameters, so we have to
special-case their transformations by looking into the instantiation
scope instead of extracting anything from template arguments.

Fixes #115098

Author: zyn0217

clang/include/clang/AST/ExprCXX.h

@@ -4326,6 +4326,8 @@ 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

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -1749,31 +1749,21 @@ namespace {
       return inherited::TransformLambdaBody(E, Body);
     }
 
-    ExprResult RebuildSizeOfPackExpr(SourceLocation OperatorLoc,
-                                     NamedDecl *Pack, SourceLocation PackLoc,
-                                     SourceLocation RParenLoc,
-                                     std::optional<unsigned> Length,
-                                     ArrayRef<TemplateArgument> PartialArgs) {
-      if (SemaRef.CodeSynthesisContexts.back().Kind !=
-          Sema::CodeSynthesisContext::ConstraintNormalization)
-        return inherited::RebuildSizeOfPackExpr(OperatorLoc, Pack, PackLoc,
-                                                RParenLoc, Length, PartialArgs);
-
-#ifndef NDEBUG
-      for (auto *Iter = TemplateArgs.begin(); Iter != TemplateArgs.end();
-           ++Iter)
-        for (const TemplateArgument &TA : Iter->Args)
-          assert(TA.getKind() != TemplateArgument::Pack || TA.pack_size() == 1);
-#endif
-      Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(
-          SemaRef, /*NewSubstitutionIndex=*/0);
-      Decl *NewPack = TransformDecl(PackLoc, Pack);
-      if (!NewPack)
-        return ExprError();
-
-      return inherited::RebuildSizeOfPackExpr(OperatorLoc,
-                                              cast<NamedDecl>(NewPack), PackLoc,
-                                              RParenLoc, Length, PartialArgs);
+    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) {
@@ -1899,6 +1889,15 @@ Decl *TemplateInstantiator::TransformDecl(SourceLocation Loc, Decl *D) {
       TemplateArgument Arg = TemplateArgs(TTP->getDepth(), TTP->getPosition());
 
       if (TTP->isParameterPack()) {
+        // We might not have an index for pack expansion when normalizing
+        // constraint expressions. In that case, resort to instantiation scopes
+        // for the transformed declarations.
+        if (SemaRef.ArgumentPackSubstitutionIndex == -1 &&
+            SemaRef.CodeSynthesisContexts.back().Kind ==
+                Sema::CodeSynthesisContext::ConstraintNormalization) {
+          return SemaRef.FindInstantiatedDecl(Loc, cast<NamedDecl>(D),
+                                              TemplateArgs);
+        }
         assert(Arg.getKind() == TemplateArgument::Pack &&
                "Missing argument pack");
         Arg = getPackSubstitutedTemplateArgument(getSema(), Arg);

clang/test/SemaTemplate/concepts-out-of-line-def.cpp

@@ -703,6 +703,25 @@ C v;
 
 } // namespace GH93099
 
+namespace GH115098 {
+
+template <typename... Ts> struct c {
+  template <typename T>
+    requires(sizeof...(Ts) > 0)
+  friend bool operator==(c, c);
+};
+
+template <typename... Ts> struct d {
+  template <typename T>
+    requires(sizeof...(Ts) > 0)
+  friend bool operator==(d, d);
+};
+
+template struct c<int>;
+template struct d<int, int>;
+
+} // namespace GH115098
+
 namespace GH114685 {
 
 template <typename T> struct ptr {