Outliner: Add comment explaining that getBridgeTo/FromObjectiveC returns an invalid SILDeclRef to signal that the type does not conform to the bridgeable protocol

Author: aschwaighofer

lib/SILOptimizer/Transforms/Outliner.cpp

@@ -491,6 +491,8 @@ static bool matchSwitch(SwitchInfo &SI, SILInstruction *Inst,
   auto NativeType = Apply->getType().getASTType();
   auto *BridgeFun = FunRef->getInitiallyReferencedFunction();
   auto *SwiftModule = BridgeFun->getModule().getSwiftModule();
+  // Not every type conforms to the ObjectiveCBridgeable protocol in such a case
+  // getBridgeFromObjectiveC returns SILDeclRef().
   auto bridgeWitness = getBridgeFromObjectiveC(NativeType, SwiftModule);
   if (bridgeWitness == SILDeclRef() ||
       BridgeFun->getName() != bridgeWitness.mangle())
@@ -809,6 +811,8 @@ BridgedArgument BridgedArgument::match(unsigned ArgIdx, SILValue Arg,
   auto NativeType = BridgedValue->getType().getASTType();
   auto *BridgeFun = FunRef->getInitiallyReferencedFunction();
   auto *SwiftModule = BridgeFun->getModule().getSwiftModule();
+  // Not every type conforms to the ObjectiveCBridgeable protocol in such a case
+  // getBridgeToObjectiveC returns SILDeclRef().
   auto bridgeWitness = getBridgeToObjectiveC(NativeType, SwiftModule);
   if (bridgeWitness == SILDeclRef() ||
       BridgeFun->getName() != bridgeWitness.mangle())