Index: include/clang/AST/Type.h
===================================================================
--- include/clang/AST/Type.h
+++ include/clang/AST/Type.h
@@ -1638,6 +1638,21 @@
     unsigned NumArgs;
   };
 
+  class PackExpansionTypeBitfields {
+    friend class PackExpansionType;
+
+    unsigned : NumTypeBits;
+
+    /// The number of expansions that this pack expansion will
+    /// generate when substituted (+1).
+    ///
+    /// This field will only have a non-zero value when some of the parameter
+    /// packs that occur within the pattern have been substituted but others
+    /// have not. Intentionally not a bitfield since we have plenty
+    /// of space left.
+    unsigned NumExpansions;
+  };
+
   union {
     TypeBitfields TypeBits;
     ArrayTypeBitfields ArrayTypeBits;
@@ -1650,6 +1665,7 @@
     TypeWithKeywordBitfields TypeWithKeywordBits;
     VectorTypeBitfields VectorTypeBits;
     TemplateSpecializationTypeBitfields TemplateSpecializationTypeBits;
+    PackExpansionTypeBitfields PackExpansionTypeBits;
 
     static_assert(sizeof(TypeBitfields) <= 8,
                   "TypeBitfields is larger than 8 bytes!");
@@ -1674,6 +1690,8 @@
     static_assert(sizeof(TemplateSpecializationTypeBitfields) <= 8,
                   "TemplateSpecializationTypeBitfields is larger"
                   " than 8 bytes!");
+    static_assert(sizeof(PackExpansionTypeBitfields) <= 8,
+                  "PackExpansionTypeBitfields is larger than 8 bytes");
   };
 
 private:
@@ -5189,22 +5207,16 @@
   /// The pattern of the pack expansion.
   QualType Pattern;
 
-  /// The number of expansions that this pack expansion will
-  /// generate when substituted (+1), or indicates that
-  ///
-  /// This field will only have a non-zero value when some of the parameter
-  /// packs that occur within the pattern have been substituted but others have
-  /// not.
-  unsigned NumExpansions;
-
   PackExpansionType(QualType Pattern, QualType Canon,
                     Optional<unsigned> NumExpansions)
       : Type(PackExpansion, Canon, /*Dependent=*/Pattern->isDependentType(),
              /*InstantiationDependent=*/true,
              /*VariablyModified=*/Pattern->isVariablyModifiedType(),
              /*ContainsUnexpandedParameterPack=*/false),
-        Pattern(Pattern),
-        NumExpansions(NumExpansions ? *NumExpansions + 1 : 0) {}
+        Pattern(Pattern) {
+    PackExpansionTypeBits.NumExpansions =
+        NumExpansions ? *NumExpansions + 1 : 0;
+  }
 
 public:
   /// Retrieve the pattern of this pack expansion, which is the
@@ -5215,9 +5227,8 @@
   /// Retrieve the number of expansions that this pack expansion will
   /// generate, if known.
   Optional<unsigned> getNumExpansions() const {
-    if (NumExpansions)
-      return NumExpansions - 1;
-
+    if (PackExpansionTypeBits.NumExpansions)
+      return PackExpansionTypeBits.NumExpansions - 1;
     return None;
   }