Fix lower bound in JSLoadContext typer.

src/compiler/typer.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/graph-inl.h"
 #include "src/compiler/js-operator.h"
 #include "src/compiler/node.h"
 #include "src/compiler/node-properties-inl.h"
 #include "src/compiler/node-properties.h"
 #include "src/compiler/simplified-operator.h"
@@ skipping 467 matching lines @@
 
 
 Bounds Typer::Visitor::TypeJSInstanceOf(Node* node) {
   return Bounds(Type::Boolean(zone()));
 }
 
 
 // JS context operators.
 
 Bounds Typer::Visitor::TypeJSLoadContext(Node* node) {
-// TODO(rossberg): Fix this once we actually use the lower bound anywhere.
-#if 0
   Bounds outer = OperandType(node, 0);
   DCHECK(outer.upper->Maybe(Type::Internal()));
   // TODO(rossberg): More precisely, instead of the above assertion, we should
   // back-propagate the constraint that it has to be a subtype of Internal.
 
   ContextAccess access = OpParameter<ContextAccess>(node);
   Type* context_type = outer.upper;
   MaybeHandle<Context> context;
   if (context_type->IsConstant()) {
     context = Handle<Context>::cast(context_type->AsConstant()->Value());
   }
   // Walk context chain (as far as known), mirroring dynamic lookup.
   // Since contexts are mutable, the information is only useful as a lower
   // bound.
   // TODO(rossberg): Could use scope info to fix upper bounds for constant
   // bindings if we know that this code is never shared.
   for (int i = access.depth(); i > 0; --i) {
     if (context_type->IsContext()) {
       context_type = context_type->AsContext()->Outer();
       if (context_type->IsConstant()) {
         context = Handle<Context>::cast(context_type->AsConstant()->Value());
       }
-    } else {
+    } else if (!context.is_null()) {
       context = handle(context.ToHandleChecked()->previous(), isolate());
     }
   }
   if (context.is_null()) {
     return Bounds::Unbounded(zone());
   } else {
     Handle<Object> value =
         handle(context.ToHandleChecked()->get(access.index()), isolate());
     Type* lower = TypeConstant(value);
     return Bounds(lower, Type::Any(zone()));
   }
-#else
-  return Bounds::Unbounded(zone());
-#endif
 }
 
 
 Bounds Typer::Visitor::TypeJSStoreContext(Node* node) {
   return Bounds(Type::None(zone()));
 }
 
 
 Bounds Typer::Visitor::TypeJSCreateFunctionContext(Node* node) {
   Type* outer = ContextType(node);
@@ skipping 309 matching lines @@
 }
 
 
 void Typer::DecorateGraph(Graph* graph) {
   graph->AddDecorator(new (zone()) TyperDecorator(this));
 }
 
 }
 }
 }  // namespace v8::internal::compiler