enum SinglePayloadSpareBit {                                                     
  case x(Builtin.Int63)                                                          
  case y                                                                         
  case z                                                                         
} 

diff --git i/lib/IRGen/GenType.cpp w/lib/IRGen/GenType.cpp
index ffdadb8fca..0954d60227 100644
--- i/lib/IRGen/GenType.cpp
+++ w/lib/IRGen/GenType.cpp
@@ -239,13 +239,10 @@ unsigned FixedTypeInfo::getSpareBitExtraInhabitantCount() const {
     return 0;
   // The runtime supports a max of 0x7FFFFFFF extra inhabitants, which ought
   // to be enough for anybody.
-  if (getFixedSize().getValue() >= 4)
-    return 0x7FFFFFFF;
-  unsigned spareBitCount = SpareBits.count();
+  auto spareBitCount = std::min(SpareBits.count(), 31ul);
   assert(spareBitCount <= getFixedSize().getValueInBits()
          && "more spare bits than storage bits?!");
-  unsigned inhabitedBitCount = getFixedSize().getValueInBits() - spareBitCount;
-  return ((1U << spareBitCount) - 1U) << inhabitedBitCount;
+  return ((1U << spareBitCount) - 1U);
 }