[clang] fix transformation of subst constant template parameter nodes

[mizvekov]

This simplifies those transforms a lot, removing a bunch of workarounds which were introducing problems.

The transforms become independent of the template instantiator, so they are moved to TreeTransform instead.

Fixes #131342

[llvmbot]

@llvm/pr-subscribers-clang-modules

@llvm/pr-subscribers-clang

Author: Matheus Izvekov (mizvekov)

Changes

This simplifies those transforms a lot, removing a bunch of workarounds which were introducing problems.

The transforms become independent of the template instantiator, so they are moved to TreeTransform instead.

Fixes #131342

---

Patch is 27.74 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/160777.diff

13 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+1)
• (modified) clang/include/clang/AST/ExprCXX.h (+1-1)
• (modified) clang/include/clang/AST/TypeBase.h (+3-1)
• (modified) clang/include/clang/Sema/Sema.h (+17)
• (modified) clang/lib/AST/ExprCXX.cpp (+2-2)
• (modified) clang/lib/AST/StmtProfile.cpp (+2-1)
• (modified) clang/lib/Sema/SemaTemplate.cpp (+34)
• (modified) clang/lib/Sema/SemaTemplateInstantiate.cpp (+19-184)
• (modified) clang/lib/Sema/SemaTemplateInstantiateDecl.cpp (+10-13)
• (modified) clang/lib/Sema/TreeTransform.h (+55-12)
• (modified) clang/test/SemaCXX/ctad.cpp (+7)
• (modified) clang/test/SemaCXX/cxx20-ctad-type-alias.cpp (+1-1)
• (modified) clang/test/SemaTemplate/temp_arg_nontype_cxx2c.cpp (+11)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 703fe2ab35af3..99ca301f54a57 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -361,6 +361,7 @@ Bug Fixes in This Version
   first parameter. (#GH113323).
 - Fixed a crash with incompatible pointer to integer conversions in designated
   initializers involving string literals. (#GH154046)
+- Fix crash on CTAD for alias template. (#GH131342)
 - Clang now emits a frontend error when a function marked with the `flatten` attribute
   calls another function that requires target features not enabled in the caller. This
   prevents a fatal error in the backend.
diff --git a/clang/include/clang/AST/ExprCXX.h b/clang/include/clang/AST/ExprCXX.h
index 9fedb230ce397..5f16bac94d5e6 100644
--- a/clang/include/clang/AST/ExprCXX.h
+++ b/clang/include/clang/AST/ExprCXX.h
@@ -4714,7 +4714,7 @@ class SubstNonTypeTemplateParmExpr : public Expr {
   // sugared: it doesn't need to be resugared later.
   bool getFinal() const { return Final; }
 
-  NamedDecl *getParameter() const;
+  NonTypeTemplateParmDecl *getParameter() const;
 
   bool isReferenceParameter() const { return AssociatedDeclAndRef.getInt(); }
 
diff --git a/clang/include/clang/AST/TypeBase.h b/clang/include/clang/AST/TypeBase.h
index b02d9c7499fe5..e0d00b82f2b76 100644
--- a/clang/include/clang/AST/TypeBase.h
+++ b/clang/include/clang/AST/TypeBase.h
@@ -3495,7 +3495,9 @@ class AdjustedType : public Type, public llvm::FoldingSetNode {
 
   AdjustedType(TypeClass TC, QualType OriginalTy, QualType AdjustedTy,
                QualType CanonicalPtr)
-      : Type(TC, CanonicalPtr, OriginalTy->getDependence()),
+      : Type(TC, CanonicalPtr,
+             AdjustedTy->getDependence() |
+                 (OriginalTy->getDependence() & ~TypeDependence::Dependent)),
         OriginalTy(OriginalTy), AdjustedTy(AdjustedTy) {}
 
 public:
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index 5edfc29d93781..2bd6be2a32cd5 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -11714,6 +11714,23 @@ class Sema final : public SemaBase {
                                const TemplateArgumentListInfo *TemplateArgs,
                                bool IsAddressOfOperand);
 
+  UnsignedOrNone getPackIndex(TemplateArgument Pack) const {
+    return Pack.pack_size() - 1 - *ArgPackSubstIndex;
+  }
+
+  TemplateArgument
+  getPackSubstitutedTemplateArgument(TemplateArgument Arg) const {
+    Arg = Arg.pack_elements()[*ArgPackSubstIndex];
+    if (Arg.isPackExpansion())
+      Arg = Arg.getPackExpansionPattern();
+    return Arg;
+  }
+
+  ExprResult BuildSubstNonTypeTemplateParmExpr(
+      Decl *AssociatedDecl, const NonTypeTemplateParmDecl *NTTP,
+      SourceLocation loc, TemplateArgument Replacement,
+      UnsignedOrNone PackIndex, bool Final);
+
   /// Form a template name from a name that is syntactically required to name a
   /// template, either due to use of the 'template' keyword or because a name in
   /// this syntactic context is assumed to name a template (C++
diff --git a/clang/lib/AST/ExprCXX.cpp b/clang/lib/AST/ExprCXX.cpp
index 97ae4a07f32aa..95de6a82a5270 100644
--- a/clang/lib/AST/ExprCXX.cpp
+++ b/clang/lib/AST/ExprCXX.cpp
@@ -1725,8 +1725,8 @@ SizeOfPackExpr *SizeOfPackExpr::CreateDeserialized(ASTContext &Context,
   return new (Storage) SizeOfPackExpr(EmptyShell(), NumPartialArgs);
 }
 
-NamedDecl *SubstNonTypeTemplateParmExpr::getParameter() const {
-  return cast<NamedDecl>(
+NonTypeTemplateParmDecl *SubstNonTypeTemplateParmExpr::getParameter() const {
+  return cast<NonTypeTemplateParmDecl>(
       getReplacedTemplateParameterList(getAssociatedDecl())->asArray()[Index]);
 }
 
diff --git a/clang/lib/AST/StmtProfile.cpp b/clang/lib/AST/StmtProfile.cpp
index 37c4d43ec0b2f..8b3af94e1a8bc 100644
--- a/clang/lib/AST/StmtProfile.cpp
+++ b/clang/lib/AST/StmtProfile.cpp
@@ -1353,7 +1353,8 @@ void StmtProfiler::VisitExpr(const Expr *S) {
 }
 
 void StmtProfiler::VisitConstantExpr(const ConstantExpr *S) {
-  VisitExpr(S);
+  // Profile exactly as the sub-expression.
+  Visit(S->getSubExpr());
 }
 
 void StmtProfiler::VisitDeclRefExpr(const DeclRefExpr *S) {
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 91e4dc8a2d039..3ebbb30ae483e 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -775,6 +775,40 @@ Sema::BuildDependentDeclRefExpr(const CXXScopeSpec &SS,
       TemplateArgs);
 }
 
+ExprResult Sema::BuildSubstNonTypeTemplateParmExpr(
+    Decl *AssociatedDecl, const NonTypeTemplateParmDecl *NTTP,
+    SourceLocation Loc, TemplateArgument Arg, UnsignedOrNone PackIndex,
+    bool Final) {
+  // The template argument itself might be an expression, in which case we just
+  // return that expression. This happens when substituting into an alias
+  // template.
+  Expr *Replacement;
+  bool refParam = true;
+  if (Arg.getKind() == TemplateArgument::Expression) {
+    Replacement = Arg.getAsExpr();
+    refParam = Replacement->isLValue();
+    if (refParam && Replacement->getType()->isRecordType()) {
+      QualType ParamType =
+          NTTP->isExpandedParameterPack()
+              ? NTTP->getExpansionType(*SemaRef.ArgPackSubstIndex)
+              : NTTP->getType();
+      if (const auto *PET = dyn_cast<PackExpansionType>(ParamType))
+        ParamType = PET->getPattern();
+      refParam = ParamType->isReferenceType();
+    }
+  } else {
+    ExprResult result =
+        SemaRef.BuildExpressionFromNonTypeTemplateArgument(Arg, Loc);
+    if (result.isInvalid())
+      return ExprError();
+    Replacement = result.get();
+    refParam = Arg.getNonTypeTemplateArgumentType()->isReferenceType();
+  }
+  return new (SemaRef.Context) SubstNonTypeTemplateParmExpr(
+      Replacement->getType(), Replacement->getValueKind(), Loc, Replacement,
+      AssociatedDecl, NTTP->getIndex(), PackIndex, refParam, Final);
+}
+
 bool Sema::DiagnoseUninstantiableTemplate(SourceLocation PointOfInstantiation,
                                           NamedDecl *Instantiation,
                                           bool InstantiatedFromMember,
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index a72c95d6d77cf..1ff94d7ae397f 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -1373,16 +1373,6 @@ std::optional<TemplateDeductionInfo *> Sema::isSFINAEContext() const {
   return std::nullopt;
 }
 
-static TemplateArgument
-getPackSubstitutedTemplateArgument(Sema &S, TemplateArgument Arg) {
-  assert(S.ArgPackSubstIndex);
-  assert(*S.ArgPackSubstIndex < Arg.pack_size());
-  Arg = Arg.pack_begin()[*S.ArgPackSubstIndex];
-  if (Arg.isPackExpansion())
-    Arg = Arg.getPackExpansionPattern();
-  return Arg;
-}
-
 //===----------------------------------------------------------------------===/
 // Template Instantiation for Types
 //===----------------------------------------------------------------------===/
@@ -1449,13 +1439,6 @@ namespace {
       return TemplateArgs.getNewDepth(Depth);
     }
 
-    UnsignedOrNone getPackIndex(TemplateArgument Pack) {
-      UnsignedOrNone Index = getSema().ArgPackSubstIndex;
-      if (!Index)
-        return std::nullopt;
-      return Pack.pack_size() - 1 - *Index;
-    }
-
     bool TryExpandParameterPacks(SourceLocation EllipsisLoc,
                                  SourceRange PatternRange,
                                  ArrayRef<UnexpandedParameterPack> Unexpanded,
@@ -1537,7 +1520,7 @@ namespace {
       if (TA.getKind() != TemplateArgument::Pack)
         return TA;
       if (SemaRef.ArgPackSubstIndex)
-        return getPackSubstitutedTemplateArgument(SemaRef, TA);
+        return SemaRef.getPackSubstitutedTemplateArgument(TA);
       assert(TA.pack_size() == 1 && TA.pack_begin()->isPackExpansion() &&
              "unexpected pack arguments in template rewrite");
       TemplateArgument Arg = *TA.pack_begin();
@@ -1643,10 +1626,6 @@ namespace {
 
     ExprResult TransformTemplateParmRefExpr(DeclRefExpr *E,
                                             NonTypeTemplateParmDecl *D);
-    ExprResult TransformSubstNonTypeTemplateParmPackExpr(
-                                           SubstNonTypeTemplateParmPackExpr *E);
-    ExprResult TransformSubstNonTypeTemplateParmExpr(
-                                           SubstNonTypeTemplateParmExpr *E);
 
     /// Rebuild a DeclRefExpr for a VarDecl reference.
     ExprResult RebuildVarDeclRefExpr(ValueDecl *PD, SourceLocation Loc);
@@ -1933,12 +1912,6 @@ namespace {
         SmallVectorImpl<QualType> &PTypes,
         SmallVectorImpl<ParmVarDecl *> &TransParams,
         Sema::ExtParameterInfoBuilder &PInfos);
-
-  private:
-    ExprResult
-    transformNonTypeTemplateParmRef(Decl *AssociatedDecl, const NamedDecl *parm,
-                                    SourceLocation loc, TemplateArgument arg,
-                                    UnsignedOrNone PackIndex, bool Final);
   };
 }
 
@@ -1975,7 +1948,7 @@ Decl *TemplateInstantiator::TransformDecl(SourceLocation Loc, Decl *D) {
       if (TTP->isParameterPack()) {
         assert(Arg.getKind() == TemplateArgument::Pack &&
                "Missing argument pack");
-        Arg = getPackSubstitutedTemplateArgument(getSema(), Arg);
+        Arg = SemaRef.getPackSubstitutedTemplateArgument(Arg);
       }
 
       TemplateName Template = Arg.getAsTemplate();
@@ -2079,7 +2052,7 @@ TemplateInstantiator::TransformFirstQualifierInScope(NamedDecl *D,
         if (!getSema().ArgPackSubstIndex)
           return nullptr;
 
-        Arg = getPackSubstitutedTemplateArgument(getSema(), Arg);
+        Arg = SemaRef.getPackSubstitutedTemplateArgument(Arg);
       }
 
       QualType T = Arg.getAsType();
@@ -2165,8 +2138,8 @@ TemplateName TemplateInstantiator::TransformTemplateName(
               Arg, AssociatedDecl, TTP->getIndex(), Final);
         }
 
-        PackIndex = getPackIndex(Arg);
-        Arg = getPackSubstitutedTemplateArgument(getSema(), Arg);
+        PackIndex = SemaRef.getPackIndex(Arg);
+        Arg = SemaRef.getPackSubstitutedTemplateArgument(Arg);
       }
 
       TemplateName Template = Arg.getAsTemplate();
@@ -2183,10 +2156,10 @@ TemplateName TemplateInstantiator::TransformTemplateName(
 
     TemplateArgument Pack = SubstPack->getArgumentPack();
     TemplateName Template =
-        getPackSubstitutedTemplateArgument(getSema(), Pack).getAsTemplate();
+        SemaRef.getPackSubstitutedTemplateArgument(Pack).getAsTemplate();
     return getSema().Context.getSubstTemplateTemplateParm(
         Template, SubstPack->getAssociatedDecl(), SubstPack->getIndex(),
-        getPackIndex(Pack), SubstPack->getFinal());
+        SemaRef.getPackIndex(Pack), SubstPack->getFinal());
   }
 
   return inherited::TransformTemplateName(
@@ -2252,11 +2225,11 @@ TemplateInstantiator::TransformTemplateParmRefExpr(DeclRefExpr *E,
           ExprType, TargetType->isReferenceType() ? VK_LValue : VK_PRValue,
           E->getLocation(), Arg, AssociatedDecl, NTTP->getPosition(), Final);
     }
-    PackIndex = getPackIndex(Arg);
-    Arg = getPackSubstitutedTemplateArgument(getSema(), Arg);
+    PackIndex = SemaRef.getPackIndex(Arg);
+    Arg = SemaRef.getPackSubstitutedTemplateArgument(Arg);
   }
-  return transformNonTypeTemplateParmRef(AssociatedDecl, NTTP, E->getLocation(),
-                                         Arg, PackIndex, Final);
+  return SemaRef.BuildSubstNonTypeTemplateParmExpr(
+      AssociatedDecl, NTTP, E->getLocation(), Arg, PackIndex, Final);
 }
 
 const AnnotateAttr *
@@ -2344,144 +2317,6 @@ TemplateInstantiator::TransformOpenACCRoutineDeclAttr(
                    "applies to a Function Decl (and a few places for VarDecl)");
 }
 
-ExprResult TemplateInstantiator::transformNonTypeTemplateParmRef(
-    Decl *AssociatedDecl, const NamedDecl *parm, SourceLocation loc,
-    TemplateArgument arg, UnsignedOrNone PackIndex, bool Final) {
-  ExprResult result;
-
-  // Determine the substituted parameter type. We can usually infer this from
-  // the template argument, but not always.
-  auto SubstParamType = [&] {
-    if (const auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(parm)) {
-      QualType T;
-      if (NTTP->isExpandedParameterPack())
-        T = NTTP->getExpansionType(*SemaRef.ArgPackSubstIndex);
-      else
-        T = NTTP->getType();
-      if (parm->isParameterPack() && isa<PackExpansionType>(T))
-        T = cast<PackExpansionType>(T)->getPattern();
-      return SemaRef.SubstType(T, TemplateArgs, loc, parm->getDeclName());
-    }
-    return SemaRef.SubstType(arg.getAsExpr()->getType(), TemplateArgs, loc,
-                             parm->getDeclName());
-  };
-
-  bool refParam = false;
-
-  // The template argument itself might be an expression, in which case we just
-  // return that expression. This happens when substituting into an alias
-  // template.
-  if (arg.getKind() == TemplateArgument::Expression) {
-    Expr *argExpr = arg.getAsExpr();
-    result = argExpr;
-    if (argExpr->isLValue()) {
-      if (argExpr->getType()->isRecordType()) {
-        // Check whether the parameter was actually a reference.
-        QualType paramType = SubstParamType();
-        if (paramType.isNull())
-          return ExprError();
-        refParam = paramType->isReferenceType();
-      } else {
-        refParam = true;
-      }
-    }
-  } else if (arg.getKind() == TemplateArgument::Declaration ||
-             arg.getKind() == TemplateArgument::NullPtr) {
-    if (arg.getKind() == TemplateArgument::Declaration) {
-      ValueDecl *VD = arg.getAsDecl();
-
-      // Find the instantiation of the template argument.  This is
-      // required for nested templates.
-      VD = cast_or_null<ValueDecl>(
-             getSema().FindInstantiatedDecl(loc, VD, TemplateArgs));
-      if (!VD)
-        return ExprError();
-    }
-
-    QualType paramType = arg.getNonTypeTemplateArgumentType();
-    assert(!paramType.isNull() && "type substitution failed for param type");
-    assert(!paramType->isDependentType() && "param type still dependent");
-    result = SemaRef.BuildExpressionFromDeclTemplateArgument(arg, paramType, loc);
-    refParam = paramType->isReferenceType();
-  } else {
-    QualType paramType = arg.getNonTypeTemplateArgumentType();
-    result = SemaRef.BuildExpressionFromNonTypeTemplateArgument(arg, loc);
-    refParam = paramType->isReferenceType();
-    assert(result.isInvalid() ||
-           SemaRef.Context.hasSameType(result.get()->getType(),
-                                       paramType.getNonReferenceType()));
-  }
-
-  if (result.isInvalid())
-    return ExprError();
-
-  Expr *resultExpr = result.get();
-  return new (SemaRef.Context) SubstNonTypeTemplateParmExpr(
-      resultExpr->getType(), resultExpr->getValueKind(), loc, resultExpr,
-      AssociatedDecl,
-      clang::getDepthAndIndex(const_cast<NamedDecl *>(parm)).second, PackIndex,
-      refParam, Final);
-}
-
-ExprResult
-TemplateInstantiator::TransformSubstNonTypeTemplateParmPackExpr(
-                                          SubstNonTypeTemplateParmPackExpr *E) {
-  if (!getSema().ArgPackSubstIndex) {
-    // We aren't expanding the parameter pack, so just return ourselves.
-    return E;
-  }
-
-  TemplateArgument Pack = E->getArgumentPack();
-  TemplateArgument Arg = getPackSubstitutedTemplateArgument(getSema(), Pack);
-  return transformNonTypeTemplateParmRef(
-      E->getAssociatedDecl(), E->getParameterPack(),
-      E->getParameterPackLocation(), Arg, getPackIndex(Pack), E->getFinal());
-}
-
-ExprResult
-TemplateInstantiator::TransformSubstNonTypeTemplateParmExpr(
-                                          SubstNonTypeTemplateParmExpr *E) {
-  ExprResult SubstReplacement = E->getReplacement();
-  if (!isa<ConstantExpr>(SubstReplacement.get()))
-    SubstReplacement = TransformExpr(E->getReplacement());
-  if (SubstReplacement.isInvalid())
-    return true;
-  QualType SubstType = TransformType(E->getParameterType(getSema().Context));
-  if (SubstType.isNull())
-    return true;
-  // The type may have been previously dependent and not now, which means we
-  // might have to implicit cast the argument to the new type, for example:
-  // template<auto T, decltype(T) U>
-  // concept C = sizeof(U) == 4;
-  // void foo() requires C<2, 'a'> { }
-  // When normalizing foo(), we first form the normalized constraints of C:
-  // AtomicExpr(sizeof(U) == 4,
-  //            U=SubstNonTypeTemplateParmExpr(Param=U,
-  //                                           Expr=DeclRef(U),
-  //                                           Type=decltype(T)))
-  // Then we substitute T = 2, U = 'a' into the parameter mapping, and need to
-  // produce:
-  // AtomicExpr(sizeof(U) == 4,
-  //            U=SubstNonTypeTemplateParmExpr(Param=U,
-  //                                           Expr=ImpCast(
-  //                                               decltype(2),
-  //                                               SubstNTTPE(Param=U, Expr='a',
-  //                                                          Type=char)),
-  //                                           Type=decltype(2)))
-  // The call to CheckTemplateArgument here produces the ImpCast.
-  TemplateArgument SugaredConverted, CanonicalConverted;
-  if (SemaRef
-          .CheckTemplateArgument(E->getParameter(), SubstType,
-                                 SubstReplacement.get(), SugaredConverted,
-                                 CanonicalConverted,
-                                 /*StrictCheck=*/false, Sema::CTAK_Specified)
-          .isInvalid())
-    return true;
-  return transformNonTypeTemplateParmRef(
-      E->getAssociatedDecl(), E->getParameter(), E->getExprLoc(),
-      SugaredConverted, E->getPackIndex(), E->getFinal());
-}
-
 ExprResult TemplateInstantiator::RebuildVarDeclRefExpr(ValueDecl *PD,
                                                        SourceLocation Loc) {
   DeclarationNameInfo NameInfo(PD->getDeclName(), Loc);
@@ -2701,8 +2536,8 @@ TemplateInstantiator::TransformTemplateTypeParmType(TypeLocBuilder &TLB,
       }
 
       // PackIndex starts from last element.
-      PackIndex = getPackIndex(Arg);
-      Arg = getPackSubstitutedTemplateArgument(getSema(), Arg);
+      PackIndex = SemaRef.getPackIndex(Arg);
+      Arg = SemaRef.getPackSubstitutedTemplateArgument(Arg);
     }
 
     assert(Arg.getKind() == TemplateArgument::Type &&
@@ -2749,20 +2584,20 @@ QualType TemplateInstantiator::TransformSubstTemplateTypeParmPackType(
   }
 
   TemplateArgument Pack = T->getArgumentPack();
-  TemplateArgument Arg = getPackSubstitutedTemplateArgument(getSema(), Pack);
+  TemplateArgument Arg = SemaRef.getPackSubstitutedTemplateArgument(Pack);
   return BuildSubstTemplateTypeParmType(
       TLB, SuppressObjCLifetime, T->getFinal(), NewReplaced, T->getIndex(),
-      getPackIndex(Pack), Arg, TL.getNameLoc());
+      SemaRef.getPackIndex(Pack), Arg, TL.getNameLoc());
 }
 
 QualType TemplateInstantiator::TransformSubstBuiltinTemplatePackType(
     TypeLocBuilder &TLB, SubstBuiltinTemplatePackTypeLoc TL) {
   if (!getSema().ArgPackSubstIndex)
     return TreeTransform::TransformSubstBuiltinTemplatePackType(TLB, TL);
-  auto &Sema = getSema();
-  TemplateArgument Result = getPackSubstitutedTemplateArgument(
-      Sema, TL.getTypePtr()->getArgumentPack());
-  TLB.pushTrivial(Sema.getASTContext(), Result.getAsType(), TL.getBeginLoc());
+  TemplateArgument Result = SemaRef.getPackSubstitutedTemplateArgument(
+      TL.getTypePtr()->getArgumentPack());
+  TLB.pushTrivial(SemaRef.getASTContext(), Result.getAsType(),
+                  TL.getBeginLoc());
   return Result.getAsType();
 }
 
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index adac3dff5b2b4..e2dc70360506e 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -3742,7 +3742,7 @@ TemplateDeclInstantiator::VisitTemplateTemplateParmDecl(
     ExpandedParams.reserve(D->getNumExpansionTemplateParameters());
     for (unsigned I = 0, N = D->getNumExpansionTemplateParameters();
          I != N; ++I) {
-      LocalInstantiationScope Scope(SemaRef);
+      LocalInstantiationScope Scope(SemaRef, /*CombineWithOuterScope=*/true);
       TemplateParameterList *Expansion ...
[truncated]

[zyn0217]

I think we can still leave it as-is, and leave the refactor in NFC patches.

[mizvekov]

I can't, as that is used from multiple translation units now.

[zyn0217]

It would be better to have a Rebuild function call the Build function

[mizvekov]

I can't think of a single use case for customizing this behavior, and Rebuild functions are rarely customized.
If someone finds one, it'd be a simple change to do that.

[cor3ntin]

isn't that a (very slight) regression?

[erichkeane]

Isn't it the opposite? It goes from the value to a slightly sugared value?

[mizvekov]

It goes from a canonicalized form of an expression into a real expression.

So yes this gained sugar.

[mizvekov]

Now this increases the amount of sugar that we can preserve.

Now whether this extra sugar is supposed to be printed in this case is a separate question, related to the pre-existing problem that we don't preserve whether the replacement expression for the Subst node was canonicalized or not.

[cor3ntin]

LGTM but give some time to @zyn0217

[zyn0217]

Thanks

[mizvekov]

Ops, I screwed up, merged this into another PR instead of main.