Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -18878,14 +18878,24 @@
     const llvm::APSInt &InitVal = ECD->getInitVal();
 
     // Keep track of the size of positive and negative values.
-    if (InitVal.isUnsigned() || InitVal.isNonNegative())
-      NumPositiveBits = std::max(NumPositiveBits,
-                                 (unsigned)InitVal.getActiveBits());
-    else
+    if (InitVal.isUnsigned() || InitVal.isNonNegative()) {
+      // If the enumerator is zero that should still be counted as a positive
+      // bit since we need a bit to store the value zero
+      const unsigned ActiveBits =
+          InitVal.getActiveBits() ? InitVal.getActiveBits() : 1;
+      NumPositiveBits = std::max(NumPositiveBits, ActiveBits);
+    } else
       NumNegativeBits = std::max(NumNegativeBits,
                                  (unsigned)InitVal.getMinSignedBits());
   }
 
+  // If we have have a empty set of enumerators we still need one bit.
+  // From [dcl.enum]p8
+  // If the enumerator-list is empty, the values of the enumeration are as if
+  // the enumeration had a single enumerator with value 0
+  if (!NumPositiveBits && !NumNegativeBits)
+    NumPositiveBits = 1;
+
   // Figure out the type that should be used for this enum.
   QualType BestType;
   unsigned BestWidth;
Index: clang/test/CodeGenCXX/pr12251.cpp
===================================================================
--- clang/test/CodeGenCXX/pr12251.cpp
+++ clang/test/CodeGenCXX/pr12251.cpp
@@ -18,14 +18,14 @@
   return *x;
 }
 // CHECK-LABEL: define{{.*}} i32 @_Z2g1P2e1
-// CHECK: ret i32 0
+// CHECK: ret i32 %0
 
 enum e2 { e2_a = 0 };
 e2 g2(e2 *x) {
   return *x;
 }
 // CHECK-LABEL: define{{.*}} i32 @_Z2g2P2e2
-// CHECK: ret i32 0
+// CHECK: ret i32 %0
 
 enum e3 { e3_a = 16 };
 e3 g3(e3 *x) {