diff --git a/clang-tools-extra/clang-tidy/bugprone/ForwardingReferenceOverloadCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/ForwardingReferenceOverloadCheck.cpp
index 36687a8e761e85..c87b3ea7e26163 100644
--- a/clang-tools-extra/clang-tidy/bugprone/ForwardingReferenceOverloadCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/ForwardingReferenceOverloadCheck.cpp
@@ -54,7 +54,9 @@ AST_MATCHER(QualType, isEnableIf) {
 AST_MATCHER_P(TemplateTypeParmDecl, hasDefaultArgument,
               clang::ast_matchers::internal::Matcher<QualType>, TypeMatcher) {
   return Node.hasDefaultArgument() &&
-         TypeMatcher.matches(Node.getDefaultArgument(), Finder, Builder);
+         TypeMatcher.matches(
+             Node.getDefaultArgument().getArgument().getAsType(), Finder,
+             Builder);
 }
 AST_MATCHER(TemplateDecl, hasAssociatedConstraints) {
   return Node.hasAssociatedConstraints();
diff --git a/clang-tools-extra/clang-tidy/bugprone/IncorrectEnableIfCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/IncorrectEnableIfCheck.cpp
index 09aaf3e31d5dd7..75f1107904fcec 100644
--- a/clang-tools-extra/clang-tidy/bugprone/IncorrectEnableIfCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/IncorrectEnableIfCheck.cpp
@@ -19,10 +19,11 @@ namespace {
 AST_MATCHER_P(TemplateTypeParmDecl, hasUnnamedDefaultArgument,
               ast_matchers::internal::Matcher<TypeLoc>, InnerMatcher) {
   if (Node.getIdentifier() != nullptr || !Node.hasDefaultArgument() ||
-      Node.getDefaultArgumentInfo() == nullptr)
+      Node.getDefaultArgument().getArgument().isNull())
     return false;
 
-  TypeLoc DefaultArgTypeLoc = Node.getDefaultArgumentInfo()->getTypeLoc();
+  TypeLoc DefaultArgTypeLoc =
+      Node.getDefaultArgument().getTypeSourceInfo()->getTypeLoc();
   return InnerMatcher.matches(DefaultArgTypeLoc, Finder, Builder);
 }
 
diff --git a/clang-tools-extra/clang-tidy/modernize/UseConstraintsCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseConstraintsCheck.cpp
index 7a021fe14436a1..ea4d99586c7110 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseConstraintsCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseConstraintsCheck.cpp
@@ -177,9 +177,11 @@ matchTrailingTemplateParam(const FunctionTemplateDecl *FunctionTemplate) {
           dyn_cast<TemplateTypeParmDecl>(LastParam)) {
     if (LastTemplateParam->hasDefaultArgument() &&
         LastTemplateParam->getIdentifier() == nullptr) {
-      return {matchEnableIfSpecialization(
-                  LastTemplateParam->getDefaultArgumentInfo()->getTypeLoc()),
-              LastTemplateParam};
+      return {
+          matchEnableIfSpecialization(LastTemplateParam->getDefaultArgument()
+                                          .getTypeSourceInfo()
+                                          ->getTypeLoc()),
+          LastTemplateParam};
     }
   }
   return {};
diff --git a/clang-tools-extra/clangd/Hover.cpp b/clang-tools-extra/clangd/Hover.cpp
index 06b949bc4a2b55..2ec0994e846e94 100644
--- a/clang-tools-extra/clangd/Hover.cpp
+++ b/clang-tools-extra/clangd/Hover.cpp
@@ -247,8 +247,12 @@ fetchTemplateParameters(const TemplateParameterList *Params,
       if (!TTP->getName().empty())
         P.Name = TTP->getNameAsString();
 
-      if (TTP->hasDefaultArgument())
-        P.Default = TTP->getDefaultArgument().getAsString(PP);
+      if (TTP->hasDefaultArgument()) {
+        P.Default.emplace();
+        llvm::raw_string_ostream Out(*P.Default);
+        TTP->getDefaultArgument().getArgument().print(PP, Out,
+                                                      /*IncludeType=*/false);
+      }
     } else if (const auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(Param)) {
       P.Type = printType(NTTP, PP);
 
diff --git a/clang/include/clang/AST/ASTNodeTraverser.h b/clang/include/clang/AST/ASTNodeTraverser.h
index bf7c204e4ad73a..98db1cb5789909 100644
--- a/clang/include/clang/AST/ASTNodeTraverser.h
+++ b/clang/include/clang/AST/ASTNodeTraverser.h
@@ -695,7 +695,7 @@ class ASTNodeTraverser
     if (const auto *TC = D->getTypeConstraint())
       Visit(TC->getImmediatelyDeclaredConstraint());
     if (D->hasDefaultArgument())
-      Visit(D->getDefaultArgument(), SourceRange(),
+      Visit(D->getDefaultArgument().getArgument(), SourceRange(),
             D->getDefaultArgStorage().getInheritedFrom(),
             D->defaultArgumentWasInherited() ? "inherited from" : "previous");
   }
diff --git a/clang/include/clang/AST/DeclTemplate.h b/clang/include/clang/AST/DeclTemplate.h
index f3d6a321ecf104..07b08b5ed43ca4 100644
--- a/clang/include/clang/AST/DeclTemplate.h
+++ b/clang/include/clang/AST/DeclTemplate.h
@@ -1185,7 +1185,7 @@ class TemplateTypeParmDecl final : public TypeDecl,
 
   /// The default template argument, if any.
   using DefArgStorage =
-      DefaultArgStorage<TemplateTypeParmDecl, TypeSourceInfo *>;
+      DefaultArgStorage<TemplateTypeParmDecl, TemplateArgumentLoc *>;
   DefArgStorage DefaultArgument;
 
   TemplateTypeParmDecl(DeclContext *DC, SourceLocation KeyLoc,
@@ -1225,13 +1225,9 @@ class TemplateTypeParmDecl final : public TypeDecl,
   bool hasDefaultArgument() const { return DefaultArgument.isSet(); }
 
   /// Retrieve the default argument, if any.
-  QualType getDefaultArgument() const {
-    return DefaultArgument.get()->getType();
-  }
-
-  /// Retrieves the default argument's source information, if any.
-  TypeSourceInfo *getDefaultArgumentInfo() const {
-    return DefaultArgument.get();
+  const TemplateArgumentLoc &getDefaultArgument() const {
+    static const TemplateArgumentLoc NoneLoc;
+    return DefaultArgument.isSet() ? *DefaultArgument.get() : NoneLoc;
   }
 
   /// Retrieves the location of the default argument declaration.
@@ -1244,9 +1240,8 @@ class TemplateTypeParmDecl final : public TypeDecl,
   }
 
   /// Set the default argument for this template parameter.
-  void setDefaultArgument(TypeSourceInfo *DefArg) {
-    DefaultArgument.set(DefArg);
-  }
+  void setDefaultArgument(const ASTContext &C,
+                          const TemplateArgumentLoc &DefArg);
 
   /// Set that this default argument was inherited from another
   /// parameter.
diff --git a/clang/include/clang/AST/RecursiveASTVisitor.h b/clang/include/clang/AST/RecursiveASTVisitor.h
index f5cefedb07e0eb..659e4cdd1037b1 100644
--- a/clang/include/clang/AST/RecursiveASTVisitor.h
+++ b/clang/include/clang/AST/RecursiveASTVisitor.h
@@ -1960,7 +1960,7 @@ DEF_TRAVERSE_DECL(TemplateTypeParmDecl, {
     TRY_TO(TraverseType(QualType(D->getTypeForDecl(), 0)));
   TRY_TO(TraverseTemplateTypeParamDeclConstraints(D));
   if (D->hasDefaultArgument() && !D->defaultArgumentWasInherited())
-    TRY_TO(TraverseTypeLoc(D->getDefaultArgumentInfo()->getTypeLoc()));
+    TRY_TO(TraverseTemplateArgumentLoc(D->getDefaultArgument()));
 })
 
 DEF_TRAVERSE_DECL(TypedefDecl, {
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index 01ddba5eaf01dc..9b1c998d4a254f 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -10067,7 +10067,9 @@ class Sema final : public SemaBase {
 
   bool SubstTemplateArgument(const TemplateArgumentLoc &Input,
                              const MultiLevelTemplateArgumentList &TemplateArgs,
-                             TemplateArgumentLoc &Output);
+                             TemplateArgumentLoc &Output,
+                             SourceLocation Loc = {},
+                             const DeclarationName &Entity = {});
   bool
   SubstTemplateArguments(ArrayRef<TemplateArgumentLoc> Args,
                          const MultiLevelTemplateArgumentList &TemplateArgs,
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index 52eab5feb062bc..f9902a978aa34a 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -6494,7 +6494,8 @@ bool ASTContext::isSameDefaultTemplateArgument(const NamedDecl *X,
     if (!TTPX->hasDefaultArgument() || !TTPY->hasDefaultArgument())
       return false;
 
-    return hasSameType(TTPX->getDefaultArgument(), TTPY->getDefaultArgument());
+    return hasSameType(TTPX->getDefaultArgument().getArgument().getAsType(),
+                       TTPY->getDefaultArgument().getArgument().getAsType());
   }
 
   if (auto *NTTPX = dyn_cast<NonTypeTemplateParmDecl>(X)) {
diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
index 9ff8e1ea78d852..a47dd72a5679cd 100644
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -5917,11 +5917,11 @@ ASTNodeImporter::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
   }
 
   if (D->hasDefaultArgument()) {
-    Expected<TypeSourceInfo *> ToDefaultArgOrErr =
-        import(D->getDefaultArgumentInfo());
+    Expected<TemplateArgumentLoc> ToDefaultArgOrErr =
+        import(D->getDefaultArgument());
     if (!ToDefaultArgOrErr)
       return ToDefaultArgOrErr.takeError();
-    ToD->setDefaultArgument(*ToDefaultArgOrErr);
+    ToD->setDefaultArgument(ToD->getASTContext(), *ToDefaultArgOrErr);
   }
 
   return ToD;
diff --git a/clang/lib/AST/DeclPrinter.cpp b/clang/lib/AST/DeclPrinter.cpp
index c5868256b440d9..bf32aafc3d0af4 100644
--- a/clang/lib/AST/DeclPrinter.cpp
+++ b/clang/lib/AST/DeclPrinter.cpp
@@ -1883,7 +1883,8 @@ void DeclPrinter::VisitTemplateTypeParmDecl(const TemplateTypeParmDecl *TTP) {
 
   if (TTP->hasDefaultArgument()) {
     Out << " = ";
-    Out << TTP->getDefaultArgument().getAsString(Policy);
+    TTP->getDefaultArgument().getArgument().print(Policy, Out,
+                                                  /*IncludeType=*/false);
   }
 }
 
diff --git a/clang/lib/AST/DeclTemplate.cpp b/clang/lib/AST/DeclTemplate.cpp
index 26765a5da1dc64..46bb3e8e51389f 100644
--- a/clang/lib/AST/DeclTemplate.cpp
+++ b/clang/lib/AST/DeclTemplate.cpp
@@ -669,23 +669,30 @@ TemplateTypeParmDecl::CreateDeserialized(const ASTContext &C, GlobalDeclID ID,
 }
 
 SourceLocation TemplateTypeParmDecl::getDefaultArgumentLoc() const {
-  return hasDefaultArgument()
-             ? getDefaultArgumentInfo()->getTypeLoc().getBeginLoc()
-             : SourceLocation();
+  return hasDefaultArgument() ? getDefaultArgument().getLocation()
+                              : SourceLocation();
 }
 
 SourceRange TemplateTypeParmDecl::getSourceRange() const {
   if (hasDefaultArgument() && !defaultArgumentWasInherited())
     return SourceRange(getBeginLoc(),
-                       getDefaultArgumentInfo()->getTypeLoc().getEndLoc());
+                       getDefaultArgument().getSourceRange().getEnd());
   // TypeDecl::getSourceRange returns a range containing name location, which is
   // wrong for unnamed template parameters. e.g:
   // it will return <[[typename>]] instead of <[[typename]]>
-  else if (getDeclName().isEmpty())
+  if (getDeclName().isEmpty())
     return SourceRange(getBeginLoc());
   return TypeDecl::getSourceRange();
 }
 
+void TemplateTypeParmDecl::setDefaultArgument(
+    const ASTContext &C, const TemplateArgumentLoc &DefArg) {
+  if (DefArg.getArgument().isNull())
+    DefaultArgument.set(nullptr);
+  else
+    DefaultArgument.set(new (C) TemplateArgumentLoc(DefArg));
+}
+
 unsigned TemplateTypeParmDecl::getDepth() const {
   return getTypeForDecl()->castAs<TemplateTypeParmType>()->getDepth();
 }
diff --git a/clang/lib/AST/JSONNodeDumper.cpp b/clang/lib/AST/JSONNodeDumper.cpp
index 42608476b1c195..efd3a639d746b3 100644
--- a/clang/lib/AST/JSONNodeDumper.cpp
+++ b/clang/lib/AST/JSONNodeDumper.cpp
@@ -1028,7 +1028,7 @@ void JSONNodeDumper::VisitTemplateTypeParmDecl(const TemplateTypeParmDecl *D) {
 
   if (D->hasDefaultArgument())
     JOS.attributeObject("defaultArg", [=] {
-      Visit(D->getDefaultArgument(), SourceRange(),
+      Visit(D->getDefaultArgument().getArgument(), SourceRange(),
             D->getDefaultArgStorage().getInheritedFrom(),
             D->defaultArgumentWasInherited() ? "inherited from" : "previous");
     });
diff --git a/clang/lib/AST/ODRDiagsEmitter.cpp b/clang/lib/AST/ODRDiagsEmitter.cpp
index 5b1cdc16e2ea2c..97b6c14d9ede61 100644
--- a/clang/lib/AST/ODRDiagsEmitter.cpp
+++ b/clang/lib/AST/ODRDiagsEmitter.cpp
@@ -1409,13 +1409,15 @@ bool ODRDiagsEmitter::diagnoseMismatch(
         }
 
         if (HasFirstDefaultArgument && HasSecondDefaultArgument) {
-          QualType FirstType = FirstTTPD->getDefaultArgument();
-          QualType SecondType = SecondTTPD->getDefaultArgument();
-          if (computeODRHash(FirstType) != computeODRHash(SecondType)) {
+          TemplateArgument FirstTA =
+              FirstTTPD->getDefaultArgument().getArgument();
+          TemplateArgument SecondTA =
+              SecondTTPD->getDefaultArgument().getArgument();
+          if (computeODRHash(FirstTA) != computeODRHash(SecondTA)) {
             DiagTemplateError(FunctionTemplateParameterDifferentDefaultArgument)
-                << (i + 1) << FirstType;
+                << (i + 1) << FirstTA;
             DiagTemplateNote(FunctionTemplateParameterDifferentDefaultArgument)
-                << (i + 1) << SecondType;
+                << (i + 1) << SecondTA;
             return true;
           }
         }
diff --git a/clang/lib/AST/ODRHash.cpp b/clang/lib/AST/ODRHash.cpp
index 6f04739cf6693d..18b1e17e0fee5d 100644
--- a/clang/lib/AST/ODRHash.cpp
+++ b/clang/lib/AST/ODRHash.cpp
@@ -462,7 +462,7 @@ class ODRDeclVisitor : public ConstDeclVisitor<ODRDeclVisitor> {
         D->hasDefaultArgument() && !D->defaultArgumentWasInherited();
     Hash.AddBoolean(hasDefaultArgument);
     if (hasDefaultArgument) {
-      AddTemplateArgument(D->getDefaultArgument());
+      AddTemplateArgument(D->getDefaultArgument().getArgument());
     }
     Hash.AddBoolean(D->isParameterPack());
 
diff --git a/clang/lib/AST/TypePrinter.cpp b/clang/lib/AST/TypePrinter.cpp
index 87f0a8728d8506..981f09410401b0 100644
--- a/clang/lib/AST/TypePrinter.cpp
+++ b/clang/lib/AST/TypePrinter.cpp
@@ -2273,8 +2273,8 @@ bool clang::isSubstitutedDefaultArgument(ASTContext &Ctx, TemplateArgument Arg,
 
   if (auto *TTPD = dyn_cast<TemplateTypeParmDecl>(Param)) {
     return TTPD->hasDefaultArgument() &&
-           isSubstitutedTemplateArgument(Ctx, Arg, TTPD->getDefaultArgument(),
-                                         Args, Depth);
+           isSubstitutedTemplateArgument(
+               Ctx, Arg, TTPD->getDefaultArgument().getArgument(), Args, Depth);
   } else if (auto *TTPD = dyn_cast<TemplateTemplateParmDecl>(Param)) {
     return TTPD->hasDefaultArgument() &&
            isSubstitutedTemplateArgument(
diff --git a/clang/lib/ExtractAPI/DeclarationFragments.cpp b/clang/lib/ExtractAPI/DeclarationFragments.cpp
index 98b9343924a835..d88e4e0df8ef89 100644
--- a/clang/lib/ExtractAPI/DeclarationFragments.cpp
+++ b/clang/lib/ExtractAPI/DeclarationFragments.cpp
@@ -999,11 +999,11 @@ DeclarationFragmentsBuilder::getFragmentsForTemplateParameters(
             DeclarationFragments::FragmentKind::GenericParameter);
 
       if (TemplateParam->hasDefaultArgument()) {
-        DeclarationFragments After;
+        const auto Default = TemplateParam->getDefaultArgument();
         Fragments.append(" = ", DeclarationFragments::FragmentKind::Text)
-            .append(getFragmentsForType(TemplateParam->getDefaultArgument(),
-                                        TemplateParam->getASTContext(), After));
-        Fragments.append(std::move(After));
+            .append(getFragmentsForTemplateArguments(
+                {Default.getArgument()}, TemplateParam->getASTContext(),
+                {Default}));
       }
     } else if (const auto *NTP =
                    dyn_cast<NonTypeTemplateParmDecl>(ParameterArray[i])) {
diff --git a/clang/lib/Index/IndexDecl.cpp b/clang/lib/Index/IndexDecl.cpp
index 8eb88f5a1e94ee..058f4aef918a8f 100644
--- a/clang/lib/Index/IndexDecl.cpp
+++ b/clang/lib/Index/IndexDecl.cpp
@@ -703,7 +703,8 @@ class IndexingDeclVisitor : public ConstDeclVisitor<IndexingDeclVisitor, bool> {
         IndexCtx.handleDecl(TP);
       if (const auto *TTP = dyn_cast<TemplateTypeParmDecl>(TP)) {
         if (TTP->hasDefaultArgument())
-          IndexCtx.indexTypeSourceInfo(TTP->getDefaultArgumentInfo(), Parent);
+          handleTemplateArgumentLoc(TTP->getDefaultArgument(), Parent,
+                                    TP->getLexicalDeclContext());
         if (auto *C = TTP->getTypeConstraint())
           IndexCtx.handleReference(C->getNamedConcept(), C->getConceptNameLoc(),
                                    Parent, TTP->getLexicalDeclContext());
diff --git a/clang/lib/Sema/HLSLExternalSemaSource.cpp b/clang/lib/Sema/HLSLExternalSemaSource.cpp
index bb283c54b3d29c..fa8c852ea9e9f2 100644
--- a/clang/lib/Sema/HLSLExternalSemaSource.cpp
+++ b/clang/lib/Sema/HLSLExternalSemaSource.cpp
@@ -308,17 +308,18 @@ struct BuiltinTypeDeclBuilder {
     return *this;
   }
 
-  TemplateParameterListBuilder addTemplateArgumentList();
-  BuiltinTypeDeclBuilder &addSimpleTemplateParams(ArrayRef<StringRef> Names);
+  TemplateParameterListBuilder addTemplateArgumentList(Sema &S);
+  BuiltinTypeDeclBuilder &addSimpleTemplateParams(Sema &S,
+                                                  ArrayRef<StringRef> Names);
 };
 
 struct TemplateParameterListBuilder {
   BuiltinTypeDeclBuilder &Builder;
-  ASTContext &AST;
+  Sema &S;
   llvm::SmallVector<NamedDecl *> Params;
 
-  TemplateParameterListBuilder(BuiltinTypeDeclBuilder &RB)
-      : Builder(RB), AST(RB.Record->getASTContext()) {}
+  TemplateParameterListBuilder(Sema &S, BuiltinTypeDeclBuilder &RB)
+      : Builder(RB), S(S) {}
 
   ~TemplateParameterListBuilder() { finalizeTemplateArgs(); }
 
@@ -328,12 +329,15 @@ struct TemplateParameterListBuilder {
       return *this;
     unsigned Position = static_cast<unsigned>(Params.size());
     auto *Decl = TemplateTypeParmDecl::Create(
-        AST, Builder.Record->getDeclContext(), SourceLocation(),
+        S.Context, Builder.Record->getDeclContext(), SourceLocation(),
         SourceLocation(), /* TemplateDepth */ 0, Position,
-        &AST.Idents.get(Name, tok::TokenKind::identifier), /* Typename */ false,
+        &S.Context.Idents.get(Name, tok::TokenKind::identifier),
+        /* Typename */ false,
         /* ParameterPack */ false);
     if (!DefaultValue.isNull())
-      Decl->setDefaultArgument(AST.getTrivialTypeSourceInfo(DefaultValue));
+      Decl->setDefaultArgument(
+          S.Context, S.getTrivialTemplateArgumentLoc(DefaultValue, QualType(),
+                                                     SourceLocation()));
 
     Params.emplace_back(Decl);
     return *this;
@@ -342,11 +346,11 @@ struct TemplateParameterListBuilder {
   BuiltinTypeDeclBuilder &finalizeTemplateArgs() {
     if (Params.empty())
       return Builder;
-    auto *ParamList =
-        TemplateParameterList::Create(AST, SourceLocation(), SourceLocation(),
-                                      Params, SourceLocation(), nullptr);
+    auto *ParamList = TemplateParameterList::Create(S.Context, SourceLocation(),
+                                                    SourceLocation(), Params,
+                                                    SourceLocation(), nullptr);
     Builder.Template = ClassTemplateDecl::Create(
-        AST, Builder.Record->getDeclContext(), SourceLocation(),
+        S.Context, Builder.Record->getDeclContext(), SourceLocation(),
         DeclarationName(Builder.Record->getIdentifier()), ParamList,
         Builder.Record);
     Builder.Record->setDescribedClassTemplate(Builder.Template);
@@ -359,20 +363,22 @@ struct TemplateParameterListBuilder {
     Params.clear();
 
     QualType T = Builder.Template->getInjectedClassNameSpecialization();
-    T = AST.getInjectedClassNameType(Builder.Record, T);
+    T = S.Context.getInjectedClassNameType(Builder.Record, T);
 
     return Builder;
   }
 };
 } // namespace
 
-TemplateParameterListBuilder BuiltinTypeDeclBuilder::addTemplateArgumentList() {
-  return TemplateParameterListBuilder(*this);
+TemplateParameterListBuilder
+BuiltinTypeDeclBuilder::addTemplateArgumentList(Sema &S) {
+  return TemplateParameterListBuilder(S, *this);
 }
 
 BuiltinTypeDeclBuilder &
-BuiltinTypeDeclBuilder::addSimpleTemplateParams(ArrayRef<StringRef> Names) {
-  TemplateParameterListBuilder Builder = this->addTemplateArgumentList();
+BuiltinTypeDeclBuilder::addSimpleTemplateParams(Sema &S,
+                                                ArrayRef<StringRef> Names) {
+  TemplateParameterListBuilder Builder = this->addTemplateArgumentList(S);
   for (StringRef Name : Names)
     Builder.addTypeParameter(Name);
   return Builder.finalizeTemplateArgs();
@@ -426,7 +432,9 @@ void HLSLExternalSemaSource::defineHLSLVectorAlias() {
   auto *TypeParam = TemplateTypeParmDecl::Create(
       AST, HLSLNamespace, SourceLocation(), SourceLocation(), 0, 0,
       &AST.Idents.get("element", tok::TokenKind::identifier), false, false);
-  TypeParam->setDefaultArgument(AST.getTrivialTypeSourceInfo(AST.FloatTy));
+  TypeParam->setDefaultArgument(
+      AST, SemaPtr->getTrivialTemplateArgumentLoc(
+               TemplateArgument(AST.FloatTy), QualType(), SourceLocation()));
 
   TemplateParams.emplace_back(TypeParam);
 
@@ -492,7 +500,7 @@ static BuiltinTypeDeclBuilder setupBufferType(CXXRecordDecl *Decl, Sema &S,
 void HLSLExternalSemaSource::defineHLSLTypesWithForwardDeclarations() {
   CXXRecordDecl *Decl;
   Decl = BuiltinTypeDeclBuilder(*SemaPtr, HLSLNamespace, "RWBuffer")
-             .addSimpleTemplateParams({"element_type"})
+             .addSimpleTemplateParams(*SemaPtr, {"element_type"})
              .Record;
   onCompletion(Decl, [this](CXXRecordDecl *Decl) {
     setupBufferType(Decl, *SemaPtr, ResourceClass::UAV,
@@ -503,7 +511,7 @@ void HLSLExternalSemaSource::defineHLSLTypesWithForwardDeclarations() {
 
   Decl =
       BuiltinTypeDeclBuilder(*SemaPtr, HLSLNamespace, "RasterizerOrderedBuffer")
-          .addSimpleTemplateParams({"element_type"})
+          .addSimpleTemplateParams(*SemaPtr, {"element_type"})
           .Record;
   onCompletion(Decl, [this](CXXRecordDecl *Decl) {
     setupBufferType(Decl, *SemaPtr, ResourceClass::UAV,
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 02d9b64c2b14b1..0b26cb4a872b1f 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -1071,7 +1071,8 @@ NamedDecl *Sema::ActOnTypeParameter(Scope *S, bool Typename,
       return Param;
     }
 
-    Param->setDefaultArgument(DefaultTInfo);
+    Param->setDefaultArgument(
+        Context, TemplateArgumentLoc(DefaultTInfo->getType(), DefaultTInfo));
   }
 
   return Param;
@@ -2318,11 +2319,11 @@ transformTemplateTypeParam(Sema &SemaRef, DeclContext *DC,
     SemaRef.SubstTypeConstraint(NewTTP, TC, Args,
                                 /*EvaluateConstraint=*/true);
   if (TTP->hasDefaultArgument()) {
-    TypeSourceInfo *InstantiatedDefaultArg =
-        SemaRef.SubstType(TTP->getDefaultArgumentInfo(), Args,
-                          TTP->getDefaultArgumentLoc(), TTP->getDeclName());
-    if (InstantiatedDefaultArg)
-      NewTTP->setDefaultArgument(InstantiatedDefaultArg);
+    TemplateArgumentLoc InstantiatedDefaultArg;
+    if (!SemaRef.SubstTemplateArgument(
+            TTP->getDefaultArgument(), Args, InstantiatedDefaultArg,
+            TTP->getDefaultArgumentLoc(), TTP->getDeclName()))
+      NewTTP->setDefaultArgument(SemaRef.Context, InstantiatedDefaultArg);
   }
   SemaRef.CurrentInstantiationScope->InstantiatedLocal(TTP, NewTTP);
   return NewTTP;
@@ -3575,10 +3576,9 @@ bool Sema::CheckTemplateParameterList(TemplateParameterList *NewParams,
           = dyn_cast<TemplateTypeParmDecl>(*NewParam)) {
       // Check the presence of a default argument here.
       if (NewTypeParm->hasDefaultArgument() &&
-          DiagnoseDefaultTemplateArgument(*this, TPC,
-                                          NewTypeParm->getLocation(),
-               NewTypeParm->getDefaultArgumentInfo()->getTypeLoc()
-                                                       .getSourceRange()))
+          DiagnoseDefaultTemplateArgument(
+              *this, TPC, NewTypeParm->getLocation(),
+              NewTypeParm->getDefaultArgument().getSourceRange()))
         NewTypeParm->removeDefaultArgument();
 
       // Merge default arguments for template type parameters.
@@ -6040,22 +6040,26 @@ bool Sema::CheckTemplateTypeArgument(
 ///
 /// \param Converted the list of template arguments provided for template
 /// parameters that precede \p Param in the template parameter list.
-/// \returns the substituted template argument, or NULL if an error occurred.
-static TypeSourceInfo *SubstDefaultTemplateArgument(
+///
+/// \param Output the resulting substituted template argument.
+///
+/// \returns true if an error occurred.
+static bool SubstDefaultTemplateArgument(
     Sema &SemaRef, TemplateDecl *Template, SourceLocation TemplateLoc,
     SourceLocation RAngleLoc, TemplateTypeParmDecl *Param,
     ArrayRef<TemplateArgument> SugaredConverted,
-    ArrayRef<TemplateArgument> CanonicalConverted) {
-  TypeSourceInfo *ArgType = Param->getDefaultArgumentInfo();
+    ArrayRef<TemplateArgument> CanonicalConverted,
+    TemplateArgumentLoc &Output) {
+  Output = Param->getDefaultArgument();
 
   // If the argument type is dependent, instantiate it now based
   // on the previously-computed template arguments.
-  if (ArgType->getType()->isInstantiationDependentType()) {
+  if (Output.getArgument().isInstantiationDependent()) {
     Sema::InstantiatingTemplate Inst(SemaRef, TemplateLoc, Param, Template,
                                      SugaredConverted,
                                      SourceRange(TemplateLoc, RAngleLoc));
     if (Inst.isInvalid())
-      return nullptr;
+      return true;
 
     // Only substitute for the innermost template argument list.
     MultiLevelTemplateArgumentList TemplateArgLists(Template, SugaredConverted,
@@ -6068,12 +6072,14 @@ static TypeSourceInfo *SubstDefaultTemplateArgument(
       ForLambdaCallOperator = Rec->isLambda();
     Sema::ContextRAII SavedContext(SemaRef, Template->getDeclContext(),
                                    !ForLambdaCallOperator);
-    ArgType =
-        SemaRef.SubstType(ArgType, TemplateArgLists,
-                          Param->getDefaultArgumentLoc(), Param->getDeclName());
+
+    if (SemaRef.SubstTemplateArgument(Output, TemplateArgLists, Output,
+                                      Param->getDefaultArgumentLoc(),
+                                      Param->getDeclName()))
+      return true;
   }
 
-  return ArgType;
+  return false;
 }
 
 /// Substitute template arguments into the default template argument for
@@ -6196,13 +6202,12 @@ TemplateArgumentLoc Sema::SubstDefaultTemplateArgumentIfAvailable(
       return TemplateArgumentLoc();
 
     HasDefaultArg = true;
-    TypeSourceInfo *DI = SubstDefaultTemplateArgument(
-        *this, Template, TemplateLoc, RAngleLoc, TypeParm, SugaredConverted,
-        CanonicalConverted);
-    if (DI)
-      return TemplateArgumentLoc(TemplateArgument(DI->getType()), DI);
-
-    return TemplateArgumentLoc();
+    TemplateArgumentLoc Output;
+    if (SubstDefaultTemplateArgument(*this, Template, TemplateLoc, RAngleLoc,
+                                     TypeParm, SugaredConverted,
+                                     CanonicalConverted, Output))
+      return TemplateArgumentLoc();
+    return Output;
   }
 
   if (NonTypeTemplateParmDecl *NonTypeParm
@@ -6785,14 +6790,10 @@ bool Sema::CheckTemplateArgumentList(
         return diagnoseMissingArgument(*this, TemplateLoc, Template, TTP,
                                        NewArgs);
 
-      TypeSourceInfo *ArgType = SubstDefaultTemplateArgument(
-          *this, Template, TemplateLoc, RAngleLoc, TTP, SugaredConverted,
-          CanonicalConverted);
-      if (!ArgType)
+      if (SubstDefaultTemplateArgument(*this, Template, TemplateLoc, RAngleLoc,
+                                       TTP, SugaredConverted,
+                                       CanonicalConverted, Arg))
         return true;
-
-      Arg = TemplateArgumentLoc(TemplateArgument(ArgType->getType()),
-                                ArgType);
     } else if (NonTypeTemplateParmDecl *NTTP
                  = dyn_cast<NonTypeTemplateParmDecl>(*Param)) {
       if (!hasReachableDefaultArgument(NTTP))
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index 41fd210f29d097..0c348633576d64 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -519,18 +519,14 @@ static NamedDecl *getTemplateParameterWithDefault(Sema &S, NamedDecl *A,
   switch (A->getKind()) {
   case Decl::TemplateTypeParm: {
     auto *T = cast<TemplateTypeParmDecl>(A);
-    // FIXME: A TemplateTypeParmDecl's DefaultArgument can't hold a full
-    // TemplateArgument, so there is currently no way to specify a pack as a
-    // default argument for these.
-    if (T->isParameterPack())
-      return A;
     auto *R = TemplateTypeParmDecl::Create(
         S.Context, A->getDeclContext(), SourceLocation(), SourceLocation(),
         T->getDepth(), T->getIndex(), T->getIdentifier(),
-        T->wasDeclaredWithTypename(), /*ParameterPack=*/false,
+        T->wasDeclaredWithTypename(), T->isParameterPack(),
         T->hasTypeConstraint());
     R->setDefaultArgument(
-        S.Context.getTrivialTypeSourceInfo(Default.getAsType()));
+        S.Context,
+        S.getTrivialTemplateArgumentLoc(Default, QualType(), SourceLocation()));
     if (R->hasTypeConstraint()) {
       auto *C = R->getTypeConstraint();
       R->setTypeConstraint(C->getConceptReference(),
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index 07626058c79776..abb8a260faab9d 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -1619,11 +1619,6 @@ namespace {
       case TemplateArgument::Pack:
         // Literally rewrite the template argument pack, instead of unpacking
         // it.
-        assert(
-            SemaRef.CodeSynthesisContexts.back().Kind ==
-                Sema::CodeSynthesisContext::BuildingDeductionGuides &&
-            "Transforming a template argument pack is only allowed in building "
-            "deduction guide");
         for (auto &pack : Arg.getPackAsArray()) {
           TemplateArgumentLoc Input = SemaRef.getTrivialTemplateArgumentLoc(
               pack, QualType(), SourceLocation{});
@@ -4375,9 +4370,9 @@ Sema::SubstStmt(Stmt *S, const MultiLevelTemplateArgumentList &TemplateArgs) {
 bool Sema::SubstTemplateArgument(
     const TemplateArgumentLoc &Input,
     const MultiLevelTemplateArgumentList &TemplateArgs,
-    TemplateArgumentLoc &Output) {
-  TemplateInstantiator Instantiator(*this, TemplateArgs, SourceLocation(),
-                                    DeclarationName());
+    TemplateArgumentLoc &Output, SourceLocation Loc,
+    const DeclarationName &Entity) {
+  TemplateInstantiator Instantiator(*this, TemplateArgs, Loc, Entity);
   return Instantiator.TransformTemplateArgument(Input, Output);
 }
 
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 381d79b2fcd466..38a300332068fa 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -2956,11 +2956,10 @@ Decl *TemplateDeclInstantiator::VisitTemplateTypeParmDecl(
     }
   }
   if (D->hasDefaultArgument() && !D->defaultArgumentWasInherited()) {
-    TypeSourceInfo *InstantiatedDefaultArg =
-        SemaRef.SubstType(D->getDefaultArgumentInfo(), TemplateArgs,
-                          D->getDefaultArgumentLoc(), D->getDeclName());
-    if (InstantiatedDefaultArg)
-      Inst->setDefaultArgument(InstantiatedDefaultArg);
+    TemplateArgumentLoc Output;
+    if (!SemaRef.SubstTemplateArgument(D->getDefaultArgument(), TemplateArgs,
+                                       Output))
+      Inst->setDefaultArgument(SemaRef.getASTContext(), Output);
   }
 
   // Introduce this template parameter's instantiation into the instantiation
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index a6254b70560c36..d7a9e31b477f88 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -2695,7 +2695,8 @@ void ASTDeclReader::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
   }
 
   if (Record.readInt())
-    D->setDefaultArgument(readTypeSourceInfo());
+    D->setDefaultArgument(Reader.getContext(),
+                          Record.readTemplateArgumentLoc());
 }
 
 void ASTDeclReader::VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D) {
diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
index c2f1d1b44241cc..b2a214e935aadd 100644
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -1899,7 +1899,7 @@ void ASTDeclWriter::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
                         !D->defaultArgumentWasInherited();
   Record.push_back(OwnsDefaultArg);
   if (OwnsDefaultArg)
-    Record.AddTypeSourceInfo(D->getDefaultArgumentInfo());
+    Record.AddTemplateArgumentLoc(D->getDefaultArgument());
 
   if (!TC && !OwnsDefaultArg &&
       D->getDeclContext() == D->getLexicalDeclContext() &&
diff --git a/clang/tools/libclang/CIndex.cpp b/clang/tools/libclang/CIndex.cpp
index f00ba9e3acfc84..d0d654568d8475 100644
--- a/clang/tools/libclang/CIndex.cpp
+++ b/clang/tools/libclang/CIndex.cpp
@@ -776,10 +776,9 @@ bool CursorVisitor::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
   }
 
   // Visit the default argument.
-  if (D->hasDefaultArgument() && !D->defaultArgumentWasInherited())
-    if (TypeSourceInfo *DefArg = D->getDefaultArgumentInfo())
-      if (Visit(DefArg->getTypeLoc()))
-        return true;
+  if (D->hasDefaultArgument() && !D->defaultArgumentWasInherited() &&
+      VisitTemplateArgumentLoc(D->getDefaultArgument()))
+    return true;
 
   return false;
 }
diff --git a/clang/unittests/AST/ASTImporterTest.cpp b/clang/unittests/AST/ASTImporterTest.cpp
index 4ee64de697d37a..ba630002c055a3 100644
--- a/clang/unittests/AST/ASTImporterTest.cpp
+++ b/clang/unittests/AST/ASTImporterTest.cpp
@@ -1188,7 +1188,7 @@ TEST_P(ASTImporterOptionSpecificTestBase, TemplateTypeParmDeclDefaultArg) {
       FromTU, templateTypeParmDecl(hasName("T")));
   TemplateTypeParmDecl *To = Import(From, Lang_CXX03);
   ASSERT_TRUE(To->hasDefaultArgument());
-  QualType ToArg = To->getDefaultArgument();
+  QualType ToArg = To->getDefaultArgument().getArgument().getAsType();
   ASSERT_EQ(ToArg, QualType(To->getASTContext().IntTy));
 }