[c++20] P0780R2: Support pack-expansion of init-captures.

This permits an init-capture to introduce a new pack:

  template<typename ...T> auto x = [...a = T()] { /* a is a pack */ };

To support this, the mechanism for allowing ParmVarDecls to be packs has
been extended to support arbitrary local VarDecls.

llvm-svn: 361300

5 files changed, 210 insertions, 102 deletions

diff --git a/clang/lib/Sema/SemaLambda.cpp b/clang/lib/Sema/SemaLambda.cpp
index 1966e9e0803..b2055dd650e 100644
--- a/clang/lib/Sema/SemaLambda.cpp
+++ b/clang/lib/Sema/SemaLambda.cpp
@@ -753,11 +753,10 @@ void Sema::deduceClosureReturnType(CapturingScopeInfo &CSI) {
   }
 }
 
-QualType Sema::buildLambdaInitCaptureInitialization(SourceLocation Loc,
-                                                    bool ByRef,
-                                                    IdentifierInfo *Id,
-                                                    bool IsDirectInit,
-                                                    Expr *&Init) {
+QualType Sema::buildLambdaInitCaptureInitialization(
+    SourceLocation Loc, bool ByRef, SourceLocation EllipsisLoc,
+    Optional<unsigned> NumExpansions, IdentifierInfo *Id, bool IsDirectInit,
+    Expr *&Init) {
   // Create an 'auto' or 'auto&' TypeSourceInfo that we can use to
   // deduce against.
   QualType DeductType = Context.getAutoDeductType();
@@ -768,6 +767,18 @@ QualType Sema::buildLambdaInitCaptureInitialization(SourceLocation Loc,
     assert(!DeductType.isNull() && "can't build reference to auto");
     TLB.push<ReferenceTypeLoc>(DeductType).setSigilLoc(Loc);
   }
+  if (EllipsisLoc.isValid()) {
+    if (Init->containsUnexpandedParameterPack()) {
+      Diag(EllipsisLoc, getLangOpts().CPlusPlus2a
+                            ? diag::warn_cxx17_compat_init_capture_pack
+                            : diag::ext_init_capture_pack);
+      DeductType = Context.getPackExpansionType(DeductType, NumExpansions);
+      TLB.push<PackExpansionTypeLoc>(DeductType).setEllipsisLoc(EllipsisLoc);
+    } else {
+      // Just ignore the ellipsis for now and form a non-pack variable. We'll
+      // diagnose this later when we try to capture it.
+    }
+  }
   TypeSourceInfo *TSI = TLB.getTypeSourceInfo(Context, DeductType);
 
   // Deduce the type of the init capture.
@@ -808,10 +819,15 @@ QualType Sema::buildLambdaInitCaptureInitialization(SourceLocation Loc,
 
 VarDecl *Sema::createLambdaInitCaptureVarDecl(SourceLocation Loc,
                                               QualType InitCaptureType,
+                                              SourceLocation EllipsisLoc,
                                               IdentifierInfo *Id,
                                               unsigned InitStyle, Expr *Init) {
-  TypeSourceInfo *TSI = Context.getTrivialTypeSourceInfo(InitCaptureType,
-      Loc);
+  // FIXME: Retain the TypeSourceInfo from buildLambdaInitCaptureInitialization
+  // rather than reconstructing it here.
+  TypeSourceInfo *TSI = Context.getTrivialTypeSourceInfo(InitCaptureType, Loc);
+  if (auto PETL = TSI->getTypeLoc().getAs<PackExpansionTypeLoc>())
+    PETL.setEllipsisLoc(EllipsisLoc);
+
   // Create a dummy variable representing the init-capture. This is not actually
   // used as a variable, and only exists as a way to name and refer to the
   // init-capture.
@@ -1036,8 +1052,6 @@ void Sema::ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
                        ? diag::warn_cxx11_compat_init_capture
                        : diag::ext_init_capture);
 
-      if (C->Init.get()->containsUnexpandedParameterPack())
-        ContainsUnexpandedParameterPack = true;
       // If the initializer expression is usable, but the InitCaptureType
       // is not, then an error has occurred - so ignore the capture for now.
       // for e.g., [n{0}] { }; <-- if no <initializer_list> is included.
@@ -1046,6 +1060,10 @@ void Sema::ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
       if (C->InitCaptureType.get().isNull())
         continue;
 
+      if (C->Init.get()->containsUnexpandedParameterPack() &&
+          !C->InitCaptureType.get()->getAs<PackExpansionType>())
+        ContainsUnexpandedParameterPack = true;
+
       unsigned InitStyle;
       switch (C->InitKind) {
       case LambdaCaptureInitKind::NoInit:
@@ -1061,7 +1079,8 @@ void Sema::ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
         break;
       }
       Var = createLambdaInitCaptureVarDecl(C->Loc, C->InitCaptureType.get(),
-                                           C->Id, InitStyle, C->Init.get());
+                                           C->EllipsisLoc, C->Id, InitStyle,
+                                           C->Init.get());
       // C++1y [expr.prim.lambda]p11:
       //   An init-capture behaves as if it declares and explicitly
       //   captures a variable [...] whose declarative region is the
@@ -1153,7 +1172,8 @@ void Sema::ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
         EllipsisLoc = C->EllipsisLoc;
       } else {
         Diag(C->EllipsisLoc, diag::err_pack_expansion_without_parameter_packs)
-          << SourceRange(C->Loc);
+            << (C->Init.isUsable() ? C->Init.get()->getSourceRange()
+                                   : SourceRange(C->Loc));
 
         // Just ignore the ellipsis.
       }
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index 4dccf2f459c..263bc3104ef 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -4280,19 +4280,26 @@ Sema::TemplateDeductionResult Sema::DeduceTemplateArguments(
 }
 
 namespace {
+  struct DependentAuto { bool IsPack; };
 
   /// Substitute the 'auto' specifier or deduced template specialization type
   /// specifier within a type for a given replacement type.
   class SubstituteDeducedTypeTransform :
       public TreeTransform<SubstituteDeducedTypeTransform> {
     QualType Replacement;
+    bool ReplacementIsPack;
     bool UseTypeSugar;
 
   public:
+    SubstituteDeducedTypeTransform(Sema &SemaRef, DependentAuto DA)
+        : TreeTransform<SubstituteDeducedTypeTransform>(SemaRef), Replacement(),
+          ReplacementIsPack(DA.IsPack), UseTypeSugar(true) {}
+
     SubstituteDeducedTypeTransform(Sema &SemaRef, QualType Replacement,
-                            bool UseTypeSugar = true)
+                                   bool UseTypeSugar = true)
         : TreeTransform<SubstituteDeducedTypeTransform>(SemaRef),
-          Replacement(Replacement), UseTypeSugar(UseTypeSugar) {}
+          Replacement(Replacement), ReplacementIsPack(false),
+          UseTypeSugar(UseTypeSugar) {}
 
     QualType TransformDesugared(TypeLocBuilder &TLB, DeducedTypeLoc TL) {
       assert(isa<TemplateTypeParmType>(Replacement) &&
@@ -4317,7 +4324,8 @@ namespace {
         return TransformDesugared(TLB, TL);
 
       QualType Result = SemaRef.Context.getAutoType(
-          Replacement, TL.getTypePtr()->getKeyword(), Replacement.isNull());
+          Replacement, TL.getTypePtr()->getKeyword(), Replacement.isNull(),
+          ReplacementIsPack);
       auto NewTL = TLB.push<AutoTypeLoc>(Result);
       NewTL.setNameLoc(TL.getNameLoc());
       return Result;
@@ -4408,9 +4416,12 @@ Sema::DeduceAutoType(TypeLoc Type, Expr *&Init, QualType &Result,
     Init = NonPlaceholder.get();
   }
 
+  DependentAuto DependentResult = {
+      /*.IsPack = */ (bool)Type.getAs<PackExpansionTypeLoc>()};
+
   if (!DependentDeductionDepth &&
       (Type.getType()->isDependentType() || Init->isTypeDependent())) {
-    Result = SubstituteDeducedTypeTransform(*this, QualType()).Apply(Type);
+    Result = SubstituteDeducedTypeTransform(*this, DependentResult).Apply(Type);
     assert(!Result.isNull() && "substituting DependentTy can't fail");
     return DAR_Succeeded;
   }
@@ -4478,7 +4489,8 @@ Sema::DeduceAutoType(TypeLoc Type, Expr *&Init, QualType &Result,
   auto DeductionFailed = [&](TemplateDeductionResult TDK,
                              ArrayRef<SourceRange> Ranges) -> DeduceAutoResult {
     if (Init->isTypeDependent()) {
-      Result = SubstituteDeducedTypeTransform(*this, QualType()).Apply(Type);
+      Result =
+          SubstituteDeducedTypeTransform(*this, DependentResult).Apply(Type);
       assert(!Result.isNull() && "substituting DependentTy can't fail");
       return DAR_Succeeded;
     }
@@ -4559,7 +4571,10 @@ Sema::DeduceAutoType(TypeLoc Type, Expr *&Init, QualType &Result,
 QualType Sema::SubstAutoType(QualType TypeWithAuto,
                              QualType TypeToReplaceAuto) {
   if (TypeToReplaceAuto->isDependentType())
-    TypeToReplaceAuto = QualType();
+    return SubstituteDeducedTypeTransform(
+               *this, DependentAuto{
+                          TypeToReplaceAuto->containsUnexpandedParameterPack()})
+        .TransformType(TypeWithAuto);
   return SubstituteDeducedTypeTransform(*this, TypeToReplaceAuto)
       .TransformType(TypeWithAuto);
 }
@@ -4567,7 +4582,11 @@ QualType Sema::SubstAutoType(QualType TypeWithAuto,
 TypeSourceInfo *Sema::SubstAutoTypeSourceInfo(TypeSourceInfo *TypeWithAuto,
                                               QualType TypeToReplaceAuto) {
   if (TypeToReplaceAuto->isDependentType())
-    TypeToReplaceAuto = QualType();
+    return SubstituteDeducedTypeTransform(
+               *this,
+               DependentAuto{
+                   TypeToReplaceAuto->containsUnexpandedParameterPack()})
+        .TransformType(TypeWithAuto);
   return SubstituteDeducedTypeTransform(*this, TypeToReplaceAuto)
       .TransformType(TypeWithAuto);
 }
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index 55d1d94bd82..ba54d5010ba 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -819,7 +819,19 @@ namespace {
       SemaRef.InstantiateAttrs(TemplateArgs, Old, New);
     }
 
-    void transformedLocalDecl(Decl *Old, Decl *New) {
+    void transformedLocalDecl(Decl *Old, ArrayRef<Decl *> NewDecls) {
+      if (Old->isParameterPack()) {
+        SemaRef.CurrentInstantiationScope->MakeInstantiatedLocalArgPack(Old);
+        for (auto *New : NewDecls)
+          SemaRef.CurrentInstantiationScope->InstantiatedLocalPackArg(
+              Old, cast<VarDecl>(New));
+        return;
+      }
+
+      assert(NewDecls.size() == 1 &&
+             "should only have multiple expansions for a pack");
+      Decl *New = NewDecls.front();
+
       // If we've instantiated the call operator of a lambda or the call
       // operator template of a generic lambda, update the "instantiation of"
       // information.
@@ -888,12 +900,11 @@ namespace {
     ExprResult TransformSubstNonTypeTemplateParmPackExpr(
                                            SubstNonTypeTemplateParmPackExpr *E);
 
-    /// Rebuild a DeclRefExpr for a ParmVarDecl reference.
-    ExprResult RebuildParmVarDeclRefExpr(ParmVarDecl *PD, SourceLocation Loc);
+    /// Rebuild a DeclRefExpr for a VarDecl reference.
+    ExprResult RebuildVarDeclRefExpr(VarDecl *PD, SourceLocation Loc);
 
-    /// Transform a reference to a function parameter pack.
-    ExprResult TransformFunctionParmPackRefExpr(DeclRefExpr *E,
-                                                ParmVarDecl *PD);
+    /// Transform a reference to a function or init-capture parameter pack.
+    ExprResult TransformFunctionParmPackRefExpr(DeclRefExpr *E, VarDecl *PD);
 
     /// Transform a FunctionParmPackExpr which was built when we couldn't
     /// expand a function parameter pack reference which refers to an expanded
@@ -1324,9 +1335,8 @@ TemplateInstantiator::TransformSubstNonTypeTemplateParmPackExpr(
                                          Arg);
 }
 
-ExprResult
-TemplateInstantiator::RebuildParmVarDeclRefExpr(ParmVarDecl *PD,
-                                                SourceLocation Loc) {
+ExprResult TemplateInstantiator::RebuildVarDeclRefExpr(VarDecl *PD,
+                                                       SourceLocation Loc) {
   DeclarationNameInfo NameInfo(PD->getDeclName(), Loc);
   return getSema().BuildDeclarationNameExpr(CXXScopeSpec(), NameInfo, PD);
 }
@@ -1335,11 +1345,11 @@ ExprResult
 TemplateInstantiator::TransformFunctionParmPackExpr(FunctionParmPackExpr *E) {
   if (getSema().ArgumentPackSubstitutionIndex != -1) {
     // We can expand this parameter pack now.
-    ParmVarDecl *D = E->getExpansion(getSema().ArgumentPackSubstitutionIndex);
-    ValueDecl *VD = cast_or_null<ValueDecl>(TransformDecl(E->getExprLoc(), D));
+    VarDecl *D = E->getExpansion(getSema().ArgumentPackSubstitutionIndex);
+    VarDecl *VD = cast_or_null<VarDecl>(TransformDecl(E->getExprLoc(), D));
     if (!VD)
       return ExprError();
-    return RebuildParmVarDeclRefExpr(cast<ParmVarDecl>(VD), E->getExprLoc());
+    return RebuildVarDeclRefExpr(VD, E->getExprLoc());
   }
 
   QualType T = TransformType(E->getType());
@@ -1348,25 +1358,24 @@ TemplateInstantiator::TransformFunctionParmPackExpr(FunctionParmPackExpr *E) {
 
   // Transform each of the parameter expansions into the corresponding
   // parameters in the instantiation of the function decl.
-  SmallVector<ParmVarDecl *, 8> Parms;
-  Parms.reserve(E->getNumExpansions());
+  SmallVector<VarDecl *, 8> Vars;
+  Vars.reserve(E->getNumExpansions());
   for (FunctionParmPackExpr::iterator I = E->begin(), End = E->end();
        I != End; ++I) {
-    ParmVarDecl *D =
-        cast_or_null<ParmVarDecl>(TransformDecl(E->getExprLoc(), *I));
+    VarDecl *D = cast_or_null<VarDecl>(TransformDecl(E->getExprLoc(), *I));
     if (!D)
       return ExprError();
-    Parms.push_back(D);
+    Vars.push_back(D);
   }
 
   return FunctionParmPackExpr::Create(getSema().Context, T,
                                       E->getParameterPack(),
-                                      E->getParameterPackLocation(), Parms);
+                                      E->getParameterPackLocation(), Vars);
 }
 
 ExprResult
 TemplateInstantiator::TransformFunctionParmPackRefExpr(DeclRefExpr *E,
-                                                       ParmVarDecl *PD) {
+                                                       VarDecl *PD) {
   typedef LocalInstantiationScope::DeclArgumentPack DeclArgumentPack;
   llvm::PointerUnion<Decl *, DeclArgumentPack *> *Found
     = getSema().CurrentInstantiationScope->findInstantiationOf(PD);
@@ -1390,8 +1399,7 @@ TemplateInstantiator::TransformFunctionParmPackRefExpr(DeclRefExpr *E,
   }
 
   // We have either an unexpanded pack or a specific expansion.
-  return RebuildParmVarDeclRefExpr(cast<ParmVarDecl>(TransformedDecl),
-                                   E->getExprLoc());
+  return RebuildVarDeclRefExpr(cast<VarDecl>(TransformedDecl), E->getExprLoc());
 }
 
 ExprResult
@@ -1409,7 +1417,7 @@ TemplateInstantiator::TransformDeclRefExpr(DeclRefExpr *E) {
   }
 
   // Handle references to function parameter packs.
-  if (ParmVarDecl *PD = dyn_cast<ParmVarDecl>(D))
+  if (VarDecl *PD = dyn_cast<VarDecl>(D))
     if (PD->isParameterPack())
       return TransformFunctionParmPackRefExpr(E, PD);
 
@@ -2984,14 +2992,14 @@ void LocalInstantiationScope::InstantiatedLocal(const Decl *D, Decl *Inst) {
 #endif
     Stored = Inst;
   } else if (DeclArgumentPack *Pack = Stored.dyn_cast<DeclArgumentPack *>()) {
-    Pack->push_back(cast<ParmVarDecl>(Inst));
+    Pack->push_back(cast<VarDecl>(Inst));
   } else {
     assert(Stored.get<Decl *>() == Inst && "Already instantiated this local");
   }
 }
 
 void LocalInstantiationScope::InstantiatedLocalPackArg(const Decl *D,
-                                                       ParmVarDecl *Inst) {
+                                                       VarDecl *Inst) {
   D = getCanonicalParmVarDecl(D);
   DeclArgumentPack *Pack = LocalDecls[D].get<DeclArgumentPack *>();
   Pack->push_back(Inst);
diff --git a/clang/lib/Sema/SemaTemplateVariadic.cpp b/clang/lib/Sema/SemaTemplateVariadic.cpp
index 8773f45c30a..9b23624a9a8 100644
--- a/clang/lib/Sema/SemaTemplateVariadic.cpp
+++ b/clang/lib/Sema/SemaTemplateVariadic.cpp
@@ -39,11 +39,11 @@ namespace {
     unsigned DepthLimit = (unsigned)-1;
 
     void addUnexpanded(NamedDecl *ND, SourceLocation Loc = SourceLocation()) {
-      if (auto *PVD = dyn_cast<ParmVarDecl>(ND)) {
+      if (auto *VD = dyn_cast<VarDecl>(ND)) {
         // For now, the only problematic case is a generic lambda's templated
         // call operator, so we don't need to look for all the other ways we
         // could have reached a dependent parameter pack.
-        auto *FD = dyn_cast<FunctionDecl>(PVD->getDeclContext());
+        auto *FD = dyn_cast<FunctionDecl>(VD->getDeclContext());
         auto *FTD = FD ? FD->getDescribedFunctionTemplate() : nullptr;
         if (FTD && FTD->getTemplateParameters()->getDepth() >= DepthLimit)
           return;
@@ -313,11 +313,11 @@ Sema::DiagnoseUnexpandedParameterPacks(SourceLocation Loc,
 
     if (auto *LSI = dyn_cast<sema::LambdaScopeInfo>(Func)) {
       if (N == FunctionScopes.size()) {
-        for (auto &Param : Unexpanded) {
-          auto *PD = dyn_cast_or_null<ParmVarDecl>(
-              Param.first.dyn_cast<NamedDecl *>());
-          if (PD && PD->getDeclContext() == LSI->CallOperator)
-            LambdaParamPackReferences.push_back(Param);
+        for (auto &Pack : Unexpanded) {
+          auto *VD = dyn_cast_or_null<VarDecl>(
+              Pack.first.dyn_cast<NamedDecl *>());
+          if (VD && VD->getDeclContext() == LSI->CallOperator)
+            LambdaParamPackReferences.push_back(Pack);
         }
       }
 
@@ -586,11 +586,15 @@ Sema::CheckPackExpansion(TypeSourceInfo *Pattern, SourceLocation EllipsisLoc,
 QualType Sema::CheckPackExpansion(QualType Pattern, SourceRange PatternRange,
                                   SourceLocation EllipsisLoc,
                                   Optional<unsigned> NumExpansions) {
-  // C++0x [temp.variadic]p5:
+  // C++11 [temp.variadic]p5:
   //   The pattern of a pack expansion shall name one or more
   //   parameter packs that are not expanded by a nested pack
   //   expansion.
-  if (!Pattern->containsUnexpandedParameterPack()) {
+  //
+  // A pattern containing a deduced type can't occur "naturally" but arises in
+  // the desugaring of an init-capture pack.
+  if (!Pattern->containsUnexpandedParameterPack() &&
+      !Pattern->getContainedDeducedType()) {
     Diag(EllipsisLoc, diag::err_pack_expansion_without_parameter_packs)
       << PatternRange;
     return QualType();
@@ -641,7 +645,7 @@ bool Sema::CheckParameterPacksForExpansion(
     // Compute the depth and index for this parameter pack.
     unsigned Depth = 0, Index = 0;
     IdentifierInfo *Name;
-    bool IsFunctionParameterPack = false;
+    bool IsVarDeclPack = false;
 
     if (const TemplateTypeParmType *TTP
         = i->first.dyn_cast<const TemplateTypeParmType *>()) {
@@ -650,8 +654,8 @@ bool Sema::CheckParameterPacksForExpansion(
       Name = TTP->getIdentifier();
     } else {
       NamedDecl *ND = i->first.get<NamedDecl *>();
-      if (isa<ParmVarDecl>(ND))
-        IsFunctionParameterPack = true;
+      if (isa<VarDecl>(ND))
+        IsVarDeclPack = true;
       else
         std::tie(Depth, Index) = getDepthAndIndex(ND);
 
@@ -660,7 +664,7 @@ bool Sema::CheckParameterPacksForExpansion(
 
     // Determine the size of this argument pack.
     unsigned NewPackSize;
-    if (IsFunctionParameterPack) {
+    if (IsVarDeclPack) {
       // Figure out whether we're instantiating to an argument pack or not.
       typedef LocalInstantiationScope::DeclArgumentPack DeclArgumentPack;
 
@@ -694,7 +698,7 @@ bool Sema::CheckParameterPacksForExpansion(
     //   Template argument deduction can extend the sequence of template
     //   arguments corresponding to a template parameter pack, even when the
     //   sequence contains explicitly specified template arguments.
-    if (!IsFunctionParameterPack && CurrentInstantiationScope) {
+    if (!IsVarDeclPack && CurrentInstantiationScope) {
       if (NamedDecl *PartialPack
                     = CurrentInstantiationScope->getPartiallySubstitutedPack()){
         unsigned PartialDepth, PartialIndex;
@@ -778,8 +782,8 @@ Optional<unsigned> Sema::getNumArgumentsInExpansion(QualType T,
       Index = TTP->getIndex();
     } else {
       NamedDecl *ND = Unexpanded[I].first.get<NamedDecl *>();
-      if (isa<ParmVarDecl>(ND)) {
-        // Function parameter pack.
+      if (isa<VarDecl>(ND)) {
+        // Function parameter pack or init-capture pack.
         typedef LocalInstantiationScope::DeclArgumentPack DeclArgumentPack;
 
         llvm::PointerUnion<Decl *, DeclArgumentPack *> *Instantiation
@@ -1084,7 +1088,7 @@ Optional<unsigned> Sema::getFullyPackExpandedSize(TemplateArgument Arg) {
             dyn_cast<SubstNonTypeTemplateParmPackExpr>(Arg.getAsExpr()))
       Pack = Subst->getArgumentPack();
     else if (auto *Subst = dyn_cast<FunctionParmPackExpr>(Arg.getAsExpr()))  {
-      for (ParmVarDecl *PD : *Subst)
+      for (VarDecl *PD : *Subst)
         if (PD->isParameterPack())
           return None;
       return Subst->getNumExpansions();
diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index f8b9b34ae7f..c653fb1d6e2 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -450,8 +450,10 @@ public:
   /// TransformDefinition. However, in some cases (e.g., lambda expressions),
   /// the transformer itself has to transform the declarations. This routine
   /// can be overridden by a subclass that keeps track of such mappings.
-  void transformedLocalDecl(Decl *Old, Decl *New) {
-    TransformedLocalDecls[Old] = New;
+  void transformedLocalDecl(Decl *Old, ArrayRef<Decl *> New) {
+    assert(New.size() == 1 &&
+           "must override transformedLocalDecl if performing pack expansion");
+    TransformedLocalDecls[Old] = New.front();
   }
 
   /// Transform the definition of the given declaration.
@@ -7122,7 +7124,7 @@ TreeTransform<Derived>::TransformCoroutineBodyStmt(CoroutineBodyStmt *S) {
   auto *Promise = SemaRef.buildCoroutinePromise(FD->getLocation());
   if (!Promise)
     return StmtError();
-  getDerived().transformedLocalDecl(S->getPromiseDecl(), Promise);
+  getDerived().transformedLocalDecl(S->getPromiseDecl(), {Promise});
   ScopeInfo->CoroutinePromise = Promise;
 
   // Transform the implicit coroutine statements we built during the initial
@@ -11176,33 +11178,80 @@ TreeTransform<Derived>::TransformLambdaExpr(LambdaExpr *E) {
   // Transform any init-capture expressions before entering the scope of the
   // lambda body, because they are not semantically within that scope.
   typedef std::pair<ExprResult, QualType> InitCaptureInfoTy;
-  SmallVector<InitCaptureInfoTy, 8> InitCaptureExprsAndTypes;
-  InitCaptureExprsAndTypes.resize(E->explicit_capture_end() -
-                                  E->explicit_capture_begin());
+  struct TransformedInitCapture {
+    // The location of the ... if the result is retaining a pack expansion.
+    SourceLocation EllipsisLoc;
+    // Zero or more expansions of the init-capture.
+    SmallVector<InitCaptureInfoTy, 4> Expansions;
+  };
+  SmallVector<TransformedInitCapture, 4> InitCaptures;
+  InitCaptures.resize(E->explicit_capture_end() - E->explicit_capture_begin());
   for (LambdaExpr::capture_iterator C = E->capture_begin(),
                                     CEnd = E->capture_end();
        C != CEnd; ++C) {
     if (!E->isInitCapture(C))
       continue;
-    EnterExpressionEvaluationContext EEEC(
-        getSema(), Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
-    ExprResult NewExprInitResult = getDerived().TransformInitializer(
-        C->getCapturedVar()->getInit(),
-        C->getCapturedVar()->getInitStyle() == VarDecl::CallInit);
-
-    if (NewExprInitResult.isInvalid())
-      return ExprError();
-    Expr *NewExprInit = NewExprInitResult.get();
 
+    TransformedInitCapture &Result = InitCaptures[C - E->capture_begin()];
     VarDecl *OldVD = C->getCapturedVar();
-    QualType NewInitCaptureType =
-        getSema().buildLambdaInitCaptureInitialization(
-            C->getLocation(), OldVD->getType()->isReferenceType(),
-            OldVD->getIdentifier(),
-            C->getCapturedVar()->getInitStyle() != VarDecl::CInit, NewExprInit);
-    NewExprInitResult = NewExprInit;
-    InitCaptureExprsAndTypes[C - E->capture_begin()] =
-        std::make_pair(NewExprInitResult, NewInitCaptureType);
+
+    auto SubstInitCapture = [&](SourceLocation EllipsisLoc,
+                                Optional<unsigned> NumExpansions) {
+      EnterExpressionEvaluationContext EEEC(
+          getSema(), Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
+      ExprResult NewExprInitResult = getDerived().TransformInitializer(
+          OldVD->getInit(), OldVD->getInitStyle() == VarDecl::CallInit);
+
+      if (NewExprInitResult.isInvalid()) {
+        Result.Expansions.push_back(InitCaptureInfoTy(ExprError(), QualType()));
+        return;
+      }
+      Expr *NewExprInit = NewExprInitResult.get();
+
+      QualType NewInitCaptureType =
+          getSema().buildLambdaInitCaptureInitialization(
+              C->getLocation(), OldVD->getType()->isReferenceType(),
+              EllipsisLoc, NumExpansions, OldVD->getIdentifier(),
+              C->getCapturedVar()->getInitStyle() != VarDecl::CInit,
+              NewExprInit);
+      Result.Expansions.push_back(
+          InitCaptureInfoTy(NewExprInit, NewInitCaptureType));
+    };
+
+    // If this is an init-capture pack, consider expanding the pack now.
+    if (OldVD->isParameterPack()) {
+      PackExpansionTypeLoc ExpansionTL = OldVD->getTypeSourceInfo()
+                                             ->getTypeLoc()
+                                             .castAs<PackExpansionTypeLoc>();
+      SmallVector<UnexpandedParameterPack, 2> Unexpanded;
+      SemaRef.collectUnexpandedParameterPacks(OldVD->getInit(), Unexpanded);
+
+      // Determine whether the set of unexpanded parameter packs can and should
+      // be expanded.
+      bool Expand = true;
+      bool RetainExpansion = false;
+      Optional<unsigned> OrigNumExpansions =
+          ExpansionTL.getTypePtr()->getNumExpansions();
+      Optional<unsigned> NumExpansions = OrigNumExpansions;
+      if (getDerived().TryExpandParameterPacks(
+              ExpansionTL.getEllipsisLoc(),
+              OldVD->getInit()->getSourceRange(), Unexpanded, Expand,
+              RetainExpansion, NumExpansions))
+        return ExprError();
+      if (Expand) {
+        for (unsigned I = 0; I != *NumExpansions; ++I) {
+          Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), I);
+          SubstInitCapture(SourceLocation(), None);
+        }
+      }
+      if (!Expand || RetainExpansion) {
+        ForgetPartiallySubstitutedPackRAII Forget(getDerived());
+        SubstInitCapture(ExpansionTL.getEllipsisLoc(), NumExpansions);
+        Result.EllipsisLoc = ExpansionTL.getEllipsisLoc();
+      }
+    } else {
+      SubstInitCapture(SourceLocation(), None);
+    }
   }
 
   // Transform the template parameters, and add them to the current
@@ -11245,7 +11294,7 @@ TreeTransform<Derived>::TransformLambdaExpr(LambdaExpr *E) {
                                         NewCallOpTSI,
                                         /*KnownDependent=*/false,
                                         E->getCaptureDefault());
-  getDerived().transformedLocalDecl(E->getLambdaClass(), Class);
+  getDerived().transformedLocalDecl(E->getLambdaClass(), {Class});
 
   // Build the call operator.
   CXXMethodDecl *NewCallOperator = getSema().startLambdaDefinition(
@@ -11270,7 +11319,7 @@ TreeTransform<Derived>::TransformLambdaExpr(LambdaExpr *E) {
   }
 
   getDerived().transformAttrs(E->getCallOperator(), NewCallOperator);
-  getDerived().transformedLocalDecl(E->getCallOperator(), NewCallOperator);
+  getDerived().transformedLocalDecl(E->getCallOperator(), {NewCallOperator});
 
   // Introduce the context of the call operator.
   Sema::ContextRAII SavedContext(getSema(), NewCallOperator,
@@ -11313,24 +11362,33 @@ TreeTransform<Derived>::TransformLambdaExpr(LambdaExpr *E) {
 
     // Rebuild init-captures, including the implied field declaration.
     if (E->isInitCapture(C)) {
-      InitCaptureInfoTy InitExprTypePair =
-          InitCaptureExprsAndTypes[C - E->capture_begin()];
-      ExprResult Init = InitExprTypePair.first;
-      QualType InitQualType = InitExprTypePair.second;
-      if (Init.isInvalid() || InitQualType.isNull()) {
-        Invalid = true;
-        continue;
-      }
+      TransformedInitCapture &NewC = InitCaptures[C - E->capture_begin()];
+
       VarDecl *OldVD = C->getCapturedVar();
-      VarDecl *NewVD = getSema().createLambdaInitCaptureVarDecl(
-          OldVD->getLocation(), InitExprTypePair.second, OldVD->getIdentifier(),
-          OldVD->getInitStyle(), Init.get());
-      if (!NewVD)
-        Invalid = true;
-      else {
-        getDerived().transformedLocalDecl(OldVD, NewVD);
+      llvm::SmallVector<Decl*, 4> NewVDs;
+
+      for (InitCaptureInfoTy &Info : NewC.Expansions) {
+        ExprResult Init = Info.first;
+        QualType InitQualType = Info.second;
+        if (Init.isInvalid() || InitQualType.isNull()) {
+          Invalid = true;
+          break;
+        }
+        VarDecl *NewVD = getSema().createLambdaInitCaptureVarDecl(
+            OldVD->getLocation(), InitQualType, NewC.EllipsisLoc,
+            OldVD->getIdentifier(), OldVD->getInitStyle(), Init.get());
+        if (!NewVD) {
+          Invalid = true;
+          break;
+        }
+        NewVDs.push_back(NewVD);
+        getSema().buildInitCaptureField(LSI, NewVD);
       }
-      getSema().buildInitCaptureField(LSI, NewVD);
+
+      if (Invalid)
+        break;
+
+      getDerived().transformedLocalDecl(OldVD, NewVDs);
       continue;
     }
 
@@ -12471,8 +12529,7 @@ TreeTransform<Derived>::TransformBlockExpr(BlockExpr *E) {
       VarDecl *oldCapture = I.getVariable();
 
       // Ignore parameter packs.
-      if (isa<ParmVarDecl>(oldCapture) &&
-          cast<ParmVarDecl>(oldCapture)->isParameterPack())
+      if (oldCapture->isParameterPack())
         continue;
 
       VarDecl *newCapture =