[ast] Mark temporaries as maybe-assigned by default.

This is in order to prevent accidental bugs in desugarings.

R=adamk@chromium.org
BUG=v8:5636

Review-Url: https://codereview.chromium.org/2693313002
Cr-Commit-Position: refs/heads/master@{#43237}

src/ast/scopes.cc

@@ -708,7 +708,8 @@ Variable* DeclarationScope::DeclareGeneratorObjectVar(
   DCHECK(is_function_scope() || is_module_scope());
   DCHECK_NULL(generator_object_var());
 
-  Variable* result = EnsureRareData()->generator_object = NewTemporary(name);
+  Variable* result = EnsureRareData()->generator_object =
+      NewTemporary(name, kNotAssigned);
   result->set_is_used();
   return result;
 }
@@ -1205,10 +1206,16 @@ bool Scope::RemoveUnresolved(VariableProxy* var) {
 }
 
 Variable* Scope::NewTemporary(const AstRawString* name) {
+  return NewTemporary(name, kMaybeAssigned);
+}
+
+Variable* Scope::NewTemporary(const AstRawString* name,
+                              MaybeAssignedFlag maybe_assigned) {
   DeclarationScope* scope = GetClosureScope();
   Variable* var = new (zone())
       Variable(scope, name, TEMPORARY, NORMAL_VARIABLE, kCreatedInitialized);
   scope->AddLocal(var);
+  if (maybe_assigned == kMaybeAssigned) var->set_maybe_assigned();
   return var;
 }
 

src/ast/scopes.h

@@ -483,6 +483,8 @@ class V8_EXPORT_PRIVATE Scope : public NON_EXPORTED_BASE(ZoneObject) {
   // should also be invoked after resolution.
   bool NeedsScopeInfo() const;
 
+  Variable* NewTemporary(const AstRawString* name,
+                         MaybeAssignedFlag maybe_assigned);
   Zone* zone_;
 
   // Scope tree.

src/parsing/parser.cc

@@ -3005,7 +3005,7 @@ Block* Parser::BuildRejectPromiseOnException(Block* inner_block) {
         Context::ASYNC_FUNCTION_PROMISE_CREATE_INDEX,
         new (zone()) ZoneList<Expression*>(0, zone()), kNoSourcePosition);
     Assignment* assign_promise = factory()->NewAssignment(
-        Token::INIT, factory()->NewVariableProxy(PromiseVariable()),
+        Token::ASSIGN, factory()->NewVariableProxy(PromiseVariable()),
         create_promise, kNoSourcePosition);
     set_promise =
         factory()->NewExpressionStatement(assign_promise, kNoSourcePosition);