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
@@ -1798,8 +1798,10 @@
 
     unsigned : NumTypeBits;
 
+    unsigned HasNonCanonicalUnderlyingType : 1;
+
     // The index of the template parameter this substitution represents.
-    unsigned Index : 16;
+    unsigned Index : 15;
 
     /// 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
@@ -4988,8 +4990,12 @@
 /// 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>;
 
   Decl *ReplacedDecl;
 
@@ -4999,7 +5005,11 @@
 public:
   /// Gets the type that was substituted for the template
   /// parameter.
-  QualType getReplacementType() const { return getCanonicalTypeInternal(); }
+  QualType getReplacementType() const {
+    return SubstTemplateTypeParmTypeBits.HasNonCanonicalUnderlyingType
+               ? *getTrailingObjects<QualType>()
+               : getCanonicalTypeInternal();
+  }
 
   /// Gets the templated entity that was substituted.
   Decl *getReplacedDecl() const { return ReplacedDecl; }
@@ -5026,7 +5036,7 @@
                       const Decl *ReplacedDecl, unsigned Index,
                       Optional<unsigned> PackIndex) {
     ID.AddPointer(ReplacedDecl);
-    ID.AddPointer(Replacement.getAsOpaquePtr());
+    Replacement.Profile(ID);
     ID.AddInteger(Index);
     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
@@ -744,7 +744,7 @@
   // The call to getCanonicalType here existed in ASTReader.cpp, too.
   def : Creator<[{
     return ctx.getSubstTemplateTypeParmType(
-        ctx.getCanonicalType(replacementType), replacedDecl, Index, PackIndex);
+        replacementType, replacedDecl, Index, 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
@@ -4756,9 +4756,6 @@
 ASTContext::getSubstTemplateTypeParmType(QualType Replacement,
                                          Decl *ReplacedDecl, unsigned Index,
                                          Optional<unsigned> PackIndex) const {
-  assert(Replacement.isCanonical()
-         && "replacement types must always be canonical");
-
   llvm::FoldingSetNodeID ID;
   SubstTemplateTypeParmType::Profile(ID, Replacement, ReplacedDecl, Index,
                                      PackIndex);
@@ -4767,7 +4764,10 @@
       SubstTemplateTypeParmTypes.FindNodeOrInsertPos(ID, InsertPos);
 
   if (!SubstParm) {
-    SubstParm = new (*this, TypeAlignment)
+    void *Mem = Allocate(SubstTemplateTypeParmType::totalSizeToAlloc<QualType>(
+                             !Replacement.isCanonical()),
+                         TypeAlignment);
+    SubstParm = new (Mem)
         SubstTemplateTypeParmType(Replacement, ReplacedDecl, Index, 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
@@ -1529,7 +1529,7 @@
     return ToReplacementTypeOrErr.takeError();
 
   return Importer.getToContext().getSubstTemplateTypeParmType(
-      ToReplacementTypeOrErr->getCanonicalType(), *ReplacedOrErr, T->getIndex(),
+      *ToReplacementTypeOrErr, *ReplacedOrErr, T->getIndex(),
       T->getPackIndex());
 }
 
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
@@ -3719,6 +3719,11 @@
     : Type(SubstTemplateTypeParm, Replacement.getCanonicalType(),
            Replacement->getDependence()),
       ReplacedDecl(ReplacedDecl) {
+  SubstTemplateTypeParmTypeBits.HasNonCanonicalUnderlyingType =
+      Replacement != getCanonicalTypeInternal();
+  if (SubstTemplateTypeParmTypeBits.HasNonCanonicalUnderlyingType)
+    *getTrailingObjects<QualType>() = Replacement;
+
   SubstTemplateTypeParmTypeBits.Index = Index;
   SubstTemplateTypeParmTypeBits.PackIndex = PackIndex ? *PackIndex + 1 : 0;
   assert(ReplacedDecl != nullptr);
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
@@ -3508,7 +3508,7 @@
   auto getSubstType = [&](QualType Replacement, unsigned IndexReplaced,
                           Optional<unsigned> PackIndexReplaced) {
     return SemaRef.Context.getSubstTemplateTypeParmType(
-        Replacement.getCanonicalType(), BTD, IndexReplaced, PackIndexReplaced);
+        Replacement, BTD, IndexReplaced, PackIndexReplaced);
   };
 
   switch (BTD->getBuiltinTemplateKind()) {