Also check witness table devirtualization

Author: aschwaighofer

lib/SILOptimizer/Utils/Devirtualize.cpp

@@ -984,6 +984,15 @@ static bool canDevirtualizeWitnessMethod(ApplySite AI) {
       return false;
   }
 
+  // devirtualizeWitnessMethod below does not support this case. It currently
+  // assumes it can try_apply call the target.
+  if (!F->getLoweredFunctionType()->hasErrorResult() &&
+      isa<TryApplyInst>(AI.getInstruction())) {
+    LLVM_DEBUG(llvm::dbgs() << "        FAIL: Trying to devirtualize a "
+          "try_apply but wtable entry has no error result.\n");
+    return false;
+  }
+
   return true;
 }