diff --git a/clang/include/clang/AST/DeclTemplate.h b/clang/include/clang/AST/DeclTemplate.h
index bba72365089f9..058316fa385cf 100644
--- a/clang/include/clang/AST/DeclTemplate.h
+++ b/clang/include/clang/AST/DeclTemplate.h
@@ -3395,9 +3395,10 @@ inline UnsignedOrNone getExpandedPackSize(const NamedDecl *Param) {
   return std::nullopt;
 }
 
-/// Internal helper used by Subst* nodes to retrieve the parameter list
-/// for their AssociatedDecl.
-TemplateParameterList *getReplacedTemplateParameterList(const Decl *D);
+/// Internal helper used by Subst* nodes to retrieve a parameter from the
+/// AssociatedDecl, and the template argument substituted into it, if any.
+std::tuple<NamedDecl *, TemplateArgument>
+getReplacedTemplateParameter(Decl *D, unsigned Index);
 
 } // namespace clang
 
diff --git a/clang/lib/AST/DeclTemplate.cpp b/clang/lib/AST/DeclTemplate.cpp
index b6bb6117d42af..f22f1c9470ed0 100644
--- a/clang/lib/AST/DeclTemplate.cpp
+++ b/clang/lib/AST/DeclTemplate.cpp
@@ -1653,57 +1653,65 @@ void TemplateParamObjectDecl::printAsInit(llvm::raw_ostream &OS,
   getValue().printPretty(OS, Policy, getType(), &getASTContext());
 }
 
-TemplateParameterList *clang::getReplacedTemplateParameterList(const Decl *D) {
+std::tuple<NamedDecl *, TemplateArgument>
+clang::getReplacedTemplateParameter(Decl *D, unsigned Index) {
   switch (D->getKind()) {
-  case Decl::Kind::CXXRecord:
-    return cast<CXXRecordDecl>(D)
-        ->getDescribedTemplate()
-        ->getTemplateParameters();
+  case Decl::Kind::BuiltinTemplate:
   case Decl::Kind::ClassTemplate:
-    return cast<ClassTemplateDecl>(D)->getTemplateParameters();
+  case Decl::Kind::Concept:
+  case Decl::Kind::FunctionTemplate:
+  case Decl::Kind::TemplateTemplateParm:
+  case Decl::Kind::TypeAliasTemplate:
+  case Decl::Kind::VarTemplate:
+    return {cast<TemplateDecl>(D)->getTemplateParameters()->getParam(Index),
+            {}};
   case Decl::Kind::ClassTemplateSpecialization: {
     const auto *CTSD = cast<ClassTemplateSpecializationDecl>(D);
     auto P = CTSD->getSpecializedTemplateOrPartial();
+    TemplateParameterList *TPL;
     if (const auto *CTPSD =
             dyn_cast<ClassTemplatePartialSpecializationDecl *>(P))
-      return CTPSD->getTemplateParameters();
-    return cast<ClassTemplateDecl *>(P)->getTemplateParameters();
+      TPL = CTPSD->getTemplateParameters();
+    else
+      TPL = cast<ClassTemplateDecl *>(P)->getTemplateParameters();
+    return {TPL->getParam(Index), CTSD->getTemplateArgs()[Index]};
+  }
+  case Decl::Kind::VarTemplateSpecialization: {
+    const auto *VTSD = cast<VarTemplateSpecializationDecl>(D);
+    auto P = VTSD->getSpecializedTemplateOrPartial();
+    TemplateParameterList *TPL;
+    if (const auto *VTPSD = dyn_cast<VarTemplatePartialSpecializationDecl *>(P))
+      TPL = VTPSD->getTemplateParameters();
+    else
+      TPL = cast<VarTemplateDecl *>(P)->getTemplateParameters();
+    return {TPL->getParam(Index), VTSD->getTemplateArgs()[Index]};
   }
   case Decl::Kind::ClassTemplatePartialSpecialization:
-    return cast<ClassTemplatePartialSpecializationDecl>(D)
-        ->getTemplateParameters();
-  case Decl::Kind::TypeAliasTemplate:
-    return cast<TypeAliasTemplateDecl>(D)->getTemplateParameters();
-  case Decl::Kind::BuiltinTemplate:
-    return cast<BuiltinTemplateDecl>(D)->getTemplateParameters();
+    return {cast<ClassTemplatePartialSpecializationDecl>(D)
+                ->getTemplateParameters()
+                ->getParam(Index),
+            {}};
+  case Decl::Kind::VarTemplatePartialSpecialization:
+    return {cast<VarTemplatePartialSpecializationDecl>(D)
+                ->getTemplateParameters()
+                ->getParam(Index),
+            {}};
+  // This is used as the AssociatedDecl for placeholder type deduction.
+  case Decl::TemplateTypeParm:
+    return {cast<NamedDecl>(D), {}};
+  // FIXME: Always use the template decl as the AssociatedDecl.
+  case Decl::Kind::CXXRecord:
+    return getReplacedTemplateParameter(
+        cast<CXXRecordDecl>(D)->getDescribedClassTemplate(), Index);
   case Decl::Kind::CXXDeductionGuide:
   case Decl::Kind::CXXConversion:
   case Decl::Kind::CXXConstructor:
   case Decl::Kind::CXXDestructor:
   case Decl::Kind::CXXMethod:
   case Decl::Kind::Function:
-    return cast<FunctionDecl>(D)
-        ->getTemplateSpecializationInfo()
-        ->getTemplate()
-        ->getTemplateParameters();
-  case Decl::Kind::FunctionTemplate:
-    return cast<FunctionTemplateDecl>(D)->getTemplateParameters();
-  case Decl::Kind::VarTemplate:
-    return cast<VarTemplateDecl>(D)->getTemplateParameters();
-  case Decl::Kind::VarTemplateSpecialization: {
-    const auto *VTSD = cast<VarTemplateSpecializationDecl>(D);
-    auto P = VTSD->getSpecializedTemplateOrPartial();
-    if (const auto *VTPSD = dyn_cast<VarTemplatePartialSpecializationDecl *>(P))
-      return VTPSD->getTemplateParameters();
-    return cast<VarTemplateDecl *>(P)->getTemplateParameters();
-  }
-  case Decl::Kind::VarTemplatePartialSpecialization:
-    return cast<VarTemplatePartialSpecializationDecl>(D)
-        ->getTemplateParameters();
-  case Decl::Kind::TemplateTemplateParm:
-    return cast<TemplateTemplateParmDecl>(D)->getTemplateParameters();
-  case Decl::Kind::Concept:
-    return cast<ConceptDecl>(D)->getTemplateParameters();
+    return getReplacedTemplateParameter(
+        cast<FunctionDecl>(D)->getTemplateSpecializationInfo()->getTemplate(),
+        Index);
   default:
     llvm_unreachable("Unhandled templated declaration kind");
   }
diff --git a/clang/lib/AST/ExprCXX.cpp b/clang/lib/AST/ExprCXX.cpp
index 95de6a82a5270..c7f0ff040194d 100644
--- a/clang/lib/AST/ExprCXX.cpp
+++ b/clang/lib/AST/ExprCXX.cpp
@@ -1727,7 +1727,7 @@ SizeOfPackExpr *SizeOfPackExpr::CreateDeserialized(ASTContext &Context,
 
 NonTypeTemplateParmDecl *SubstNonTypeTemplateParmExpr::getParameter() const {
   return cast<NonTypeTemplateParmDecl>(
-      getReplacedTemplateParameterList(getAssociatedDecl())->asArray()[Index]);
+      std::get<0>(getReplacedTemplateParameter(getAssociatedDecl(), Index)));
 }
 
 PackIndexingExpr *PackIndexingExpr::Create(
@@ -1793,7 +1793,7 @@ SubstNonTypeTemplateParmPackExpr::SubstNonTypeTemplateParmPackExpr(
 NonTypeTemplateParmDecl *
 SubstNonTypeTemplateParmPackExpr::getParameterPack() const {
   return cast<NonTypeTemplateParmDecl>(
-      getReplacedTemplateParameterList(getAssociatedDecl())->asArray()[Index]);
+      std::get<0>(getReplacedTemplateParameter(getAssociatedDecl(), Index)));
 }
 
 TemplateArgument SubstNonTypeTemplateParmPackExpr::getArgumentPack() const {
diff --git a/clang/lib/AST/TemplateName.cpp b/clang/lib/AST/TemplateName.cpp
index 2b8044e4188cd..797a354c5d0fa 100644
--- a/clang/lib/AST/TemplateName.cpp
+++ b/clang/lib/AST/TemplateName.cpp
@@ -64,16 +64,14 @@ SubstTemplateTemplateParmPackStorage::getArgumentPack() const {
 
 TemplateTemplateParmDecl *
 SubstTemplateTemplateParmPackStorage::getParameterPack() const {
-  return cast<TemplateTemplateParmDecl>(
-      getReplacedTemplateParameterList(getAssociatedDecl())
-          ->asArray()[Bits.Index]);
+  return cast<TemplateTemplateParmDecl>(std::get<0>(
+      getReplacedTemplateParameter(getAssociatedDecl(), Bits.Index)));
 }
 
 TemplateTemplateParmDecl *
 SubstTemplateTemplateParmStorage::getParameter() const {
-  return cast<TemplateTemplateParmDecl>(
-      getReplacedTemplateParameterList(getAssociatedDecl())
-          ->asArray()[Bits.Index]);
+  return cast<TemplateTemplateParmDecl>(std::get<0>(
+      getReplacedTemplateParameter(getAssociatedDecl(), Bits.Index)));
 }
 
 void SubstTemplateTemplateParmStorage::Profile(llvm::FoldingSetNodeID &ID) {
diff --git a/clang/lib/AST/Type.cpp b/clang/lib/AST/Type.cpp
index 9794314a98f81..ee7a68ee8ba7e 100644
--- a/clang/lib/AST/Type.cpp
+++ b/clang/lib/AST/Type.cpp
@@ -4436,14 +4436,6 @@ IdentifierInfo *TemplateTypeParmType::getIdentifier() const {
   return isCanonicalUnqualified() ? nullptr : getDecl()->getIdentifier();
 }
 
-static const TemplateTypeParmDecl *getReplacedParameter(Decl *D,
-                                                        unsigned Index) {
-  if (const auto *TTP = dyn_cast<TemplateTypeParmDecl>(D))
-    return TTP;
-  return cast<TemplateTypeParmDecl>(
-      getReplacedTemplateParameterList(D)->getParam(Index));
-}
-
 SubstTemplateTypeParmType::SubstTemplateTypeParmType(QualType Replacement,
                                                      Decl *AssociatedDecl,
                                                      unsigned Index,
@@ -4466,7 +4458,8 @@ SubstTemplateTypeParmType::SubstTemplateTypeParmType(QualType Replacement,
 
 const TemplateTypeParmDecl *
 SubstTemplateTypeParmType::getReplacedParameter() const {
-  return ::getReplacedParameter(getAssociatedDecl(), getIndex());
+  return cast<TemplateTypeParmDecl>(std::get<0>(
+      getReplacedTemplateParameter(getAssociatedDecl(), getIndex())));
 }
 
 void SubstTemplateTypeParmType::Profile(llvm::FoldingSetNodeID &ID,
@@ -4532,7 +4525,8 @@ bool SubstTemplateTypeParmPackType::getFinal() const {
 
 const TemplateTypeParmDecl *
 SubstTemplateTypeParmPackType::getReplacedParameter() const {
-  return ::getReplacedParameter(getAssociatedDecl(), getIndex());
+  return cast<TemplateTypeParmDecl>(std::get<0>(
+      getReplacedTemplateParameter(getAssociatedDecl(), getIndex())));
 }
 
 IdentifierInfo *SubstTemplateTypeParmPackType::getIdentifier() const {
diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 51b55b82f4208..940324bbc5e4d 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -16364,16 +16364,21 @@ ExprResult TreeTransform<Derived>::TransformSubstNonTypeTemplateParmExpr(
       AssociatedDecl == E->getAssociatedDecl())
     return E;
 
+  auto getParamAndType = [Index = E->getIndex()](Decl *AssociatedDecl)
+      -> std::tuple<NonTypeTemplateParmDecl *, QualType> {
+    auto [PDecl, Arg] = getReplacedTemplateParameter(AssociatedDecl, Index);
+    auto *Param = cast<NonTypeTemplateParmDecl>(PDecl);
+    return {Param, Arg.isNull() ? Param->getType()
+                                : Arg.getNonTypeTemplateArgumentType()};
+  };
+
   // If the replacement expression did not change, and the parameter type
   // did not change, we can skip the semantic action because it would
   // produce the same result anyway.
-  auto *Param = cast<NonTypeTemplateParmDecl>(
-      getReplacedTemplateParameterList(AssociatedDecl)
-          ->asArray()[E->getIndex()]);
-  if (QualType ParamType = Param->getType();
-      !SemaRef.Context.hasSameType(ParamType, E->getParameter()->getType()) ||
+  if (auto [Param, ParamType] = getParamAndType(AssociatedDecl);
+      !SemaRef.Context.hasSameType(
+          ParamType, std::get<1>(getParamAndType(E->getAssociatedDecl()))) ||
       Replacement.get() != OrigReplacement) {
-
     // When transforming the replacement expression previously, all Sema
     // specific annotations, such as implicit casts, are discarded. Calling the
     // corresponding sema action is necessary to recover those. Otherwise,
diff --git a/clang/test/CodeGenCXX/template-cxx20.cpp b/clang/test/CodeGenCXX/template-cxx20.cpp
new file mode 100644
index 0000000000000..aeb1cc915145f
--- /dev/null
+++ b/clang/test/CodeGenCXX/template-cxx20.cpp
@@ -0,0 +1,24 @@
+// RUN: %clang_cc1 %s -O0 -disable-llvm-passes -triple=x86_64 -std=c++20 -emit-llvm -o - | FileCheck %s
+
+namespace GH161029_regression1 {
+  template <class _Fp> auto f(int) { _Fp{}(0); }
+  template <class _Fp, int... _Js> void g() {
+    (..., f<_Fp>(_Js));
+  }
+  enum E { k };
+  template <int, E> struct ElementAt;
+  template <E First> struct ElementAt<0, First> {
+    static int value;
+  };
+  template <typename T, T Item> struct TagSet {
+    template <int Index> using Tag = ElementAt<Index, Item>;
+  };
+  template <typename TagSet> struct S {
+    void U() { (void)TagSet::template Tag<0>::value; }
+  };
+  S<TagSet<E, k>> s;
+  void h() {
+    g<decltype([](auto) -> void { s.U(); }), 0>();
+  }
+  // CHECK: call void @_ZN20GH161029_regression11SINS_6TagSetINS_1EELS2_0EEEE1UEv
+}