diff --git a/include/swift/SIL/SILBuilder.h b/include/swift/SIL/SILBuilder.h
index 32d1389d4d990..f56a62293009f 100644
--- a/include/swift/SIL/SILBuilder.h
+++ b/include/swift/SIL/SILBuilder.h
@@ -2102,13 +2102,20 @@ class SILBuilder {
   SingleValueInstruction *
   createUncheckedReinterpretCast(SILLocation Loc, SILValue Op, SILType Ty);
 
-  /// Create an appropriate cast instruction based on result type.
+  /// Create an appropriate cast instruction based on result type. This cast
+  /// forwards ownership from the operand to the result.
   ///
-  /// NOTE: This assumes that the input and the result cast are layout
-  /// compatible. Reduces to createUncheckedReinterpretCast when ownership is
-  /// disabled.
-  SingleValueInstruction *createUncheckedBitCast(SILLocation Loc, SILValue Op,
-                                                 SILType Ty);
+  /// WARNING: Because it forwards ownership, this cast is only valid with the
+  /// source and destination types are layout equivalent. The destination type
+  /// must include all the same references in the same positions.
+  ///
+  /// Note: Forwarding casts do not exist outside of OSSA. When ownership is
+  /// disabled, this reduces to createUncheckedReinterpretCast, which may
+  /// fall-back to unchecked_bitwise_cast. It is the caller's responsibility to
+  /// emit the correct retains and releases.
+  SingleValueInstruction *createUncheckedForwardingCast(SILLocation Loc,
+                                                        SILValue Op,
+                                                        SILType Ty);
 
   //===--------------------------------------------------------------------===//
   // Runtime failure
diff --git a/lib/SIL/IR/SILBuilder.cpp b/lib/SIL/IR/SILBuilder.cpp
index d8782af1158d2..05e4ed2baf1ad 100644
--- a/lib/SIL/IR/SILBuilder.cpp
+++ b/lib/SIL/IR/SILBuilder.cpp
@@ -174,7 +174,8 @@ SILBuilder::createUncheckedReinterpretCast(SILLocation Loc, SILValue Op,
 
 // Create the appropriate cast instruction based on result type.
 SingleValueInstruction *
-SILBuilder::createUncheckedBitCast(SILLocation Loc, SILValue Op, SILType Ty) {
+SILBuilder::createUncheckedForwardingCast(SILLocation Loc, SILValue Op,
+                                          SILType Ty) {
   // Without ownership, delegate to unchecked reinterpret cast.
   if (!hasOwnership())
     return createUncheckedReinterpretCast(Loc, Op, Ty);
diff --git a/lib/SILOptimizer/SILCombiner/SILCombinerApplyVisitors.cpp b/lib/SILOptimizer/SILCombiner/SILCombinerApplyVisitors.cpp
index e52a4df73b347..655678e7840c0 100644
--- a/lib/SILOptimizer/SILCombiner/SILCombinerApplyVisitors.cpp
+++ b/lib/SILOptimizer/SILCombiner/SILCombinerApplyVisitors.cpp
@@ -169,7 +169,7 @@ SILCombiner::optimizeApplyOfConvertFunctionInst(FullApplySite AI,
       Args.push_back(UAC);
     } else if (OldOpType.getASTType() != NewOpType.getASTType()) {
       auto URC =
-          Builder.createUncheckedBitCast(AI.getLoc(), Op, NewOpType);
+          Builder.createUncheckedForwardingCast(AI.getLoc(), Op, NewOpType);
       Args.push_back(URC);
     } else {
       Args.push_back(Op);
@@ -221,8 +221,8 @@ SILCombiner::optimizeApplyOfConvertFunctionInst(FullApplySite AI,
       for (auto e = newOpResultTypes.end(); newRetI != e;
            ++oldRetI, ++newRetI, ++origArgI) {
         auto arg = normalBB->createPhiArgument(*newRetI, (*origArgI)->getOwnershipKind());
-        auto converted = Builder.createUncheckedBitCast(AI.getLoc(),
-                                                                arg, *oldRetI);
+        auto converted =
+          Builder.createUncheckedForwardingCast(AI.getLoc(), arg, *oldRetI);
         branchArgs.push_back(converted);
       }
       
@@ -246,7 +246,8 @@ SILCombiner::optimizeApplyOfConvertFunctionInst(FullApplySite AI,
   SILInstruction *result = NAI;
   
   if (oldResultTy != newResultTy) {
-    result = Builder.createUncheckedBitCast(AI.getLoc(), NAI, oldResultTy);
+    result =
+      Builder.createUncheckedForwardingCast(AI.getLoc(), NAI, oldResultTy);
   }
   
   return result;
diff --git a/lib/SILOptimizer/Transforms/EagerSpecializer.cpp b/lib/SILOptimizer/Transforms/EagerSpecializer.cpp
index 0793facbd0a15..dd9ef9af2c373 100644
--- a/lib/SILOptimizer/Transforms/EagerSpecializer.cpp
+++ b/lib/SILOptimizer/Transforms/EagerSpecializer.cpp
@@ -438,7 +438,7 @@ void EagerDispatch::emitDispatchTo(SILFunction *NewFunc) {
     auto GenResultTy = GenericFunc->mapTypeIntoContext(resultTy);
 
     SILValue CastResult =
-        Builder.createUncheckedBitCast(Loc, Result, GenResultTy);
+        Builder.createUncheckedForwardingCast(Loc, Result, GenResultTy);
 
     addReturnValue(Builder.getInsertionBB(), OldReturnBB, CastResult);
   }
@@ -640,7 +640,7 @@ SILValue EagerDispatch::emitArgumentCast(CanSILFunctionType CalleeSubstFnTy,
   if (CastTy.isAddress())
     return Builder.createUncheckedAddrCast(Loc, OrigArg, CastTy);
 
-  return Builder.createUncheckedBitCast(Loc, OrigArg, CastTy);
+  return Builder.createUncheckedForwardingCast(Loc, OrigArg, CastTy);
 }
 
 /// Converts each generic function argument into a SILValue that can be passed