diff --git a/clang/include/clang/AST/Type.h b/clang/include/clang/AST/Type.h
--- a/clang/include/clang/AST/Type.h
+++ b/clang/include/clang/AST/Type.h
@@ -1816,6 +1816,8 @@
 
     unsigned : NumTypeBits;
 
+    unsigned HasNonCanonicalUnderlyingType : 1;
+
     /// Represents the index within a pack if this represents a substitution
     /// from a pack expansion. This index starts at the end of the pack and
     /// increments towards the beginning.
@@ -5025,8 +5027,12 @@
 /// been replaced with these.  They are used solely to record that a
 /// type was originally written as a template type parameter;
 /// therefore they are never canonical.
-class SubstTemplateTypeParmType : public Type, public llvm::FoldingSetNode {
+class SubstTemplateTypeParmType final
+    : public Type,
+      public llvm::FoldingSetNode,
+      private llvm::TrailingObjects<SubstTemplateTypeParmType, QualType> {
   friend class ASTContext;
+  friend class llvm::TrailingObjects<SubstTemplateTypeParmType, QualType>;
 
   // The original type parameter.
   const TemplateTypeParmType *Replaced;
@@ -5043,7 +5049,9 @@
   /// Gets the type that was substituted for the template
   /// parameter.
   QualType getReplacementType() const {
-    return getCanonicalTypeInternal();
+    return SubstTemplateTypeParmTypeBits.HasNonCanonicalUnderlyingType
+               ? *getTrailingObjects<QualType>()
+               : getCanonicalTypeInternal();
   }
 
   Optional<unsigned> getPackIndex() const {
@@ -5063,7 +5071,7 @@
                       const TemplateTypeParmType *Replaced,
                       QualType Replacement, Optional<unsigned> PackIndex) {
     ID.AddPointer(Replaced);
-    ID.AddPointer(Replacement.getAsOpaquePtr());
+    Replacement.Profile(ID);
     ID.AddInteger(PackIndex ? *PackIndex - 1 : 0);
   }
 
diff --git a/clang/include/clang/AST/TypeProperties.td b/clang/include/clang/AST/TypeProperties.td
--- a/clang/include/clang/AST/TypeProperties.td
+++ b/clang/include/clang/AST/TypeProperties.td
@@ -738,7 +738,7 @@
     // The call to getCanonicalType here existed in ASTReader.cpp, too.
     return ctx.getSubstTemplateTypeParmType(
         cast<TemplateTypeParmType>(replacedParameter),
-        ctx.getCanonicalType(replacementType), PackIndex);
+        replacementType, PackIndex);
   }]>;
 }
 
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -4782,9 +4782,6 @@
 ASTContext::getSubstTemplateTypeParmType(const TemplateTypeParmType *Parm,
                                          QualType Replacement,
                                          Optional<unsigned> PackIndex) const {
-  assert(Replacement.isCanonical()
-         && "replacement types must always be canonical");
-
   llvm::FoldingSetNodeID ID;
   SubstTemplateTypeParmType::Profile(ID, Parm, Replacement, PackIndex);
   void *InsertPos = nullptr;
@@ -4792,8 +4789,11 @@
     = SubstTemplateTypeParmTypes.FindNodeOrInsertPos(ID, InsertPos);
 
   if (!SubstParm) {
-    SubstParm = new (*this, TypeAlignment)
-        SubstTemplateTypeParmType(Parm, Replacement, PackIndex);
+    void *Mem = Allocate(SubstTemplateTypeParmType::totalSizeToAlloc<QualType>(
+                             !Replacement.isCanonical()),
+                         TypeAlignment);
+    SubstParm =
+        new (Mem) SubstTemplateTypeParmType(Parm, Replacement, PackIndex);
     Types.push_back(SubstParm);
     SubstTemplateTypeParmTypes.InsertNode(SubstParm, InsertPos);
   }
diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -1534,8 +1534,7 @@
     return ToReplacementTypeOrErr.takeError();
 
   return Importer.getToContext().getSubstTemplateTypeParmType(
-      *ReplacedOrErr, ToReplacementTypeOrErr->getCanonicalType(),
-      T->getPackIndex());
+      *ReplacedOrErr, *ToReplacementTypeOrErr, T->getPackIndex());
 }
 
 ExpectedType ASTNodeImporter::VisitSubstTemplateTypeParmPackType(
diff --git a/clang/lib/AST/Type.cpp b/clang/lib/AST/Type.cpp
--- a/clang/lib/AST/Type.cpp
+++ b/clang/lib/AST/Type.cpp
@@ -3659,10 +3659,16 @@
 }
 
 SubstTemplateTypeParmType::SubstTemplateTypeParmType(
-    const TemplateTypeParmType *Param, QualType Canon,
+    const TemplateTypeParmType *Param, QualType Replacement,
     Optional<unsigned> PackIndex)
-    : Type(SubstTemplateTypeParm, Canon, Canon->getDependence()),
+    : Type(SubstTemplateTypeParm, Replacement.getCanonicalType(),
+           Replacement->getDependence()),
       Replaced(Param) {
+  SubstTemplateTypeParmTypeBits.HasNonCanonicalUnderlyingType =
+      Replacement != getCanonicalTypeInternal();
+  if (SubstTemplateTypeParmTypeBits.HasNonCanonicalUnderlyingType)
+    *getTrailingObjects<QualType>() = Replacement;
+
   SubstTemplateTypeParmTypeBits.PackIndex = PackIndex ? *PackIndex + 1 : 0;
 }
 
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -3511,8 +3511,7 @@
         0, IndexReplaced, false,
         cast<TemplateTypeParmDecl>(TPL->getParam(IndexReplaced)));
     return SemaRef.Context.getSubstTemplateTypeParmType(
-        cast<TemplateTypeParmType>(TTP), Replacement.getCanonicalType(),
-        PackIndexReplaced);
+        cast<TemplateTypeParmType>(TTP), Replacement, PackIndexReplaced);
   };
 
   switch (BTD->getBuiltinTemplateKind()) {
diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -6408,8 +6408,6 @@
   if (Replacement.isNull())
     return QualType();
 
-  // Always canonicalize the replacement type.
-  Replacement = SemaRef.Context.getCanonicalType(Replacement);
   QualType Result = SemaRef.Context.getSubstTemplateTypeParmType(
       T->getReplacedParameter(), Replacement, T->getPackIndex());
 
diff --git a/clang/test/AST/ast-dump-template-decls.cpp b/clang/test/AST/ast-dump-template-decls.cpp
--- a/clang/test/AST/ast-dump-template-decls.cpp
+++ b/clang/test/AST/ast-dump-template-decls.cpp
@@ -162,3 +162,22 @@
 // CHECK:      SubstTemplateTypeParmType 0x{{[^ ]*}} 'short' sugar pack_index 0
 // CHECK-NEXT: TemplateTypeParmType 0x{{[^ ]*}} 'U' dependent contains_unexpanded_pack depth 0 index 0 pack
 } // namespace PR56099
+
+namespace subst_default_argument {
+template<class A1> class A {};
+template<template<class C1, class C2 = A<C1>> class D1, class D2> using D = D1<D2>;
+
+template<class E1, class E2> class E {};
+using test1 = D<E, int>;
+// CHECK:      TypeAliasDecl 0x{{[^ ]*}} <line:{{[1-9]+}}:1, col:23> col:7 test1 'D<subst_default_argument::E, int>':'subst_default_argument::E<int, subst_default_argument::A<int>>'
+// CHECK:      TemplateSpecializationType 0x{{[^ ]*}} 'A<int>' sugar A
+// CHECK-NEXT: |-TemplateArgument type 'int':'int'
+// CHECK-NEXT: | `-SubstTemplateTypeParmType 0x{{[^ ]*}} 'int' sugar
+// CHECK-NEXT: |   |-TemplateTypeParmType 0x{{[^ ]*}} 'C1' dependent depth 1 index 0
+// CHECK-NEXT: |   | `-TemplateTypeParm 0x{{[^ ]*}} 'C1'
+// CHECK-NEXT: |   `-SubstTemplateTypeParmType 0x{{[^ ]*}} 'int' sugar
+// CHECK-NEXT: |     |-TemplateTypeParmType 0x{{[^ ]*}} 'type-parameter-0-1' dependent depth 0 index 1
+// CHECK-NEXT: |     `-BuiltinType 0x{{[^ ]*}} 'int'
+// CHECK-NEXT: `-RecordType 0x{{[^ ]*}} 'subst_default_argument::A<int>'
+// CHECK-NEXT:   `-ClassTemplateSpecialization 0x{{[^ ]*}} 'A'
+} // namespace subst_default_argument