[strong] Disallow implicit conversions for add

src/compiler/js-typed-lowering.cc

Index: src/compiler/js-typed-lowering.cc
diff --git a/src/compiler/js-typed-lowering.cc b/src/compiler/js-typed-lowering.cc
index c0ed7c46797660ca0e036caa012b8e2874703970..628172a3029f3f8a8ee3326b3734799acbc2e41f 100644
--- a/src/compiler/js-typed-lowering.cc
+++ b/src/compiler/js-typed-lowering.cc
@@ -320,7 +320,8 @@ Reduction JSTypedLowering::ReduceJSAdd(Node* node) {
     // JSAdd(x:number, y:number) => NumberAdd(x, y)
     return r.ChangeToPureOperator(simplified()->NumberAdd(), Type::Number());
   }
-  if (r.NeitherInputCanBe(Type::StringOrReceiver())) {
+  if (r.NeitherInputCanBe(Type::StringOrReceiver()) &&
+      !is_strong(OpParameter<LanguageMode>(node))) {
     // JSAdd(x:-string, y:-string) => NumberAdd(ToNumber(x), ToNumber(y))
     Node* frame_state = NodeProperties::GetFrameStateInput(node, 1);
     r.ConvertInputsToNumber(frame_state);
@@ -331,7 +332,9 @@ Reduction JSTypedLowering::ReduceJSAdd(Node* node) {
   //   a) The inserted ToString operation screws up valueOf vs. toString order.
   //   b) Deoptimization at ToString doesn't have corresponding bailout id.
   //   c) Our current StringAddStub is actually non-pure and requires context.
-  if (r.OneInputIs(Type::String())) {
+  if (r.OneInputIs(Type::String()) &&
+      (!is_strong(OpParameter<LanguageMode>(node)) ||
+       r.BothInputsAre(Type::String()))) {
     // JSAdd(x:string, y:string) => StringAdd(x, y)
     // JSAdd(x:string, y) => StringAdd(x, ToString(y))
     // JSAdd(x, y:string) => StringAdd(ToString(x), y)