diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index e73dbf85febc..65708706d743 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -6362,9 +6362,8 @@ public:
                                    QualType InstantiatedParamType, Expr *Arg,
                                    TemplateArgument &Converted,
                                CheckTemplateArgumentKind CTAK = CTAK_Specified);
-  bool CheckTemplateArgument(TemplateTemplateParmDecl *Param,
-                             TemplateArgumentLoc &Arg,
-                             unsigned ArgumentPackIndex);
+  bool CheckTemplateTemplateArgument(TemplateParameterList *Params,
+                                     TemplateArgumentLoc &Arg);
 
   ExprResult
   BuildExpressionFromDeclTemplateArgument(const TemplateArgument &Arg,
@@ -7700,6 +7699,10 @@ public:
   StmtResult SubstStmt(Stmt *S,
                        const MultiLevelTemplateArgumentList &TemplateArgs);
 
+  TemplateParameterList *
+  SubstTemplateParams(TemplateParameterList *Params, DeclContext *Owner,
+                      const MultiLevelTemplateArgumentList &TemplateArgs);
+
   Decl *SubstDecl(Decl *D, DeclContext *Owner,
                   const MultiLevelTemplateArgumentList &TemplateArgs);
 
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 2e8a7061d718..12a74c1b64bc 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -4617,6 +4617,8 @@ bool Sema::CheckTemplateArgument(NamedDecl *Param,
     if (NTTP->isParameterPack() && NTTP->isExpandedParameterPack())
       NTTPType = NTTP->getExpansionType(ArgumentPackIndex);
 
+    // FIXME: Do we need to substitute into parameters here if they're
+    // instantiation-dependent but not dependent?
     if (NTTPType->isDependentType() &&
         !isa<TemplateTemplateParmDecl>(Template) &&
         !Template->getDeclContext()->isDependentContext()) {
@@ -4756,9 +4758,15 @@ bool Sema::CheckTemplateArgument(NamedDecl *Param,
   // Check template template parameters.
   TemplateTemplateParmDecl *TempParm = cast<TemplateTemplateParmDecl>(Param);
 
+  TemplateParameterList *Params = TempParm->getTemplateParameters();
+  if (TempParm->isExpandedParameterPack())
+    Params = TempParm->getExpansionTemplateParameters(ArgumentPackIndex);
+
   // Substitute into the template parameter list of the template
   // template parameter, since previously-supplied template arguments
   // may appear within the template template parameter.
+  //
+  // FIXME: Skip this if the parameters aren't instantiation-dependent.
   {
     // Set up a template instantiation context.
     LocalInstantiationScope Scope(*this);
@@ -4769,10 +4777,9 @@ bool Sema::CheckTemplateArgument(NamedDecl *Param,
       return true;
 
     TemplateArgumentList TemplateArgs(TemplateArgumentList::OnStack, Converted);
-    TempParm = cast_or_null<TemplateTemplateParmDecl>(
-                      SubstDecl(TempParm, CurContext,
-                                MultiLevelTemplateArgumentList(TemplateArgs)));
-    if (!TempParm)
+    Params = SubstTemplateParams(Params, CurContext,
+                                 MultiLevelTemplateArgumentList(TemplateArgs));
+    if (!Params)
       return true;
   }
 
@@ -4793,7 +4800,7 @@ bool Sema::CheckTemplateArgument(NamedDecl *Param,
 
   case TemplateArgument::Template:
   case TemplateArgument::TemplateExpansion:
-    if (CheckTemplateArgument(TempParm, Arg, ArgumentPackIndex))
+    if (CheckTemplateTemplateArgument(Params, Arg))
       return true;
 
     Converted.push_back(Arg.getArgument());
@@ -6536,9 +6543,8 @@ static void DiagnoseTemplateParameterListArityMismatch(
 ///
 /// This routine implements the semantics of C++ [temp.arg.template].
 /// It returns true if an error occurred, and false otherwise.
-bool Sema::CheckTemplateArgument(TemplateTemplateParmDecl *Param,
-                                 TemplateArgumentLoc &Arg,
-                                 unsigned ArgumentPackIndex) {
+bool Sema::CheckTemplateTemplateArgument(TemplateParameterList *Params,
+                                         TemplateArgumentLoc &Arg) {
   TemplateName Name = Arg.getArgument().getAsTemplateOrTemplatePattern();
   TemplateDecl *Template = Name.getAsTemplateDecl();
   if (!Template) {
@@ -6573,10 +6579,6 @@ bool Sema::CheckTemplateArgument(TemplateTemplateParmDecl *Param,
       << Template;
   }
 
-  TemplateParameterList *Params = Param->getTemplateParameters();
-  if (Param->isExpandedParameterPack())
-    Params = Param->getExpansionTemplateParameters(ArgumentPackIndex);
-
   // C++1z [temp.arg.template]p3: (DR 150)
   //   A template-argument matches a template template-parameter P when P
   //   is at least as specialized as the template-argument A.
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 800956dd535d..67204356080a 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -3118,6 +3118,13 @@ TemplateDeclInstantiator::SubstTemplateParams(TemplateParameterList *L) {
   return InstL;
 }
 
+TemplateParameterList *
+Sema::SubstTemplateParams(TemplateParameterList *Params, DeclContext *Owner,
+                          const MultiLevelTemplateArgumentList &TemplateArgs) {
+  TemplateDeclInstantiator Instantiator(*this, Owner, TemplateArgs);
+  return Instantiator.SubstTemplateParams(Params);
+}
+
 /// \brief Instantiate the declaration of a class template partial
 /// specialization.
 ///
diff --git a/clang/test/CXX/drs/dr10xx.cpp b/clang/test/CXX/drs/dr10xx.cpp
index e1db9ef54adf..7f5fd8c7ec27 100644
--- a/clang/test/CXX/drs/dr10xx.cpp
+++ b/clang/test/CXX/drs/dr10xx.cpp
@@ -31,9 +31,8 @@ namespace dr1004 { // dr1004: 5
 
   // This example (from the standard) is actually ill-formed, because
   // name lookup of "T::template A" names the constructor.
-  // FIXME: Only issue one diagnostic for this case.
-  template<class T, template<class> class U = T::template A> struct Third { }; // expected-error 2{{is a constructor name}}
-  Third<A<int> > t; // expected-note {{in instantiation of}} expected-note {{while substituting}} expected-note {{while checking}}
+  template<class T, template<class> class U = T::template A> struct Third { }; // expected-error {{is a constructor name}}
+  Third<A<int> > t; // expected-note {{in instantiation of default argument}}
 }
 
 namespace dr1048 { // dr1048: 3.6
diff --git a/clang/test/SemaTemplate/temp-param-subst-linear.cpp b/clang/test/SemaTemplate/temp-param-subst-linear.cpp
new file mode 100644
index 000000000000..fd93aa568553
--- /dev/null
+++ b/clang/test/SemaTemplate/temp-param-subst-linear.cpp
@@ -0,0 +1,56 @@
+// RUN: %clang_cc1 -std=c++17 %s -verify
+// expected-no-diagnostics
+
+// This test attempts to ensure that the below template parameter pack
+// splitting technique executes in linear time in the number of template
+// parameters. The size of the list below is selected so as to execute
+// relatively quickly on a "good" compiler and to time out otherwise.
+
+template<typename...> struct TypeList;
+
+namespace detail {
+template<unsigned> using Unsigned = unsigned;
+template<typename T, T ...N> using ListOfNUnsignedsImpl = TypeList<Unsigned<N>...>;
+template<unsigned N> using ListOfNUnsigneds =
+  __make_integer_seq<ListOfNUnsignedsImpl, unsigned, N>;
+
+template<typename T> struct TypeWrapper {
+  template<unsigned> using AsTemplate = T;
+};
+
+template<typename ...N> struct Splitter {
+  template<template<N> class ...L,
+           template<unsigned> class ...R> struct Split {
+    using Left = TypeList<L<0>...>;
+    using Right = TypeList<R<0>...>;
+  };
+};
+}
+
+template<typename TypeList, unsigned N, typename = detail::ListOfNUnsigneds<N>>
+struct SplitAtIndex;
+
+template<typename ...T, unsigned N, typename ...NUnsigneds>
+struct SplitAtIndex<TypeList<T...>, N, TypeList<NUnsigneds...>> :
+  detail::Splitter<NUnsigneds...>::
+    template Split<detail::TypeWrapper<T>::template AsTemplate...> {};
+
+template<typename T, int N> struct Rep : Rep<typename Rep<T, N-1>::type, 1> {};
+template<typename ...T> struct Rep<TypeList<T...>, 1> { typedef TypeList<T..., T...> type; };
+
+using Ints = Rep<TypeList<int>, 14>::type;
+
+template<typename T> extern int Size;
+template<typename ...T> constexpr int Size<TypeList<T...>> = sizeof...(T);
+
+using Left = SplitAtIndex<Ints, Size<Ints> / 2>::Left;
+using Right = SplitAtIndex<Ints, Size<Ints> / 2>::Right;
+static_assert(Size<Left> == 8192);
+static_assert(Size<Right> == 8192);
+
+template<typename L, typename R> struct Concat;
+template<typename ...L, typename ...R> struct Concat<TypeList<L...>, TypeList<R...>> {
+  using type = TypeList<L..., R...>;
+};
+
+using Ints = Concat<Left, Right>::type;