[es6] Handle conflicts for sloppy let

src/preparser.h

 // Copyright 2012 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.
 
 #ifndef V8_PREPARSER_H
 #define V8_PREPARSER_H
 
 #include "src/v8.h"
 
 #include "src/bailout-reason.h"
@@ skipping 3747 matching lines @@
     // that duplicates are not allowed.  Of course, the arrow function may
     // itself be strict as well.
     const bool allow_duplicate_parameters = false;
     this->ValidateFormalParameters(&formals_classifier, language_mode(),
                                    allow_duplicate_parameters, CHECK_OK);
 
     // Validate strict mode.
     if (is_strict(language_mode())) {
       CheckStrictOctalLiteral(formal_parameters.scope->start_position(),
                               scanner()->location().end_pos, CHECK_OK);
+    }
+    if (is_strict(language_mode()) || allow_harmony_sloppy()) {

[Dan Ehrenberg, 2015/07/08 23:42:13]

A tangent, but it looks like CheckConflictingVarDeclarations is implemented only
in the parser, not the preparser, so the errors aren't quite as early as we'd
optimally like. Any particular reason for this? It looks like the preparser
maintains the whole scope thing anyway.

[rossberg, 2015/07/10 12:13:17]

No, the preparser doesn't maintain scopes. This is a long-standing issue. We
would love to do that, but experiments showed that it is fairly costly and
causes significant regressions in e.g. CodeLoad. It's still an open problem how
to diagnose binding errors early in all cases, and we have punted so far.

(It's a particular pain point for strong mode, btw.)

[arv (Not doing code reviews), 2015/07/10 13:00:48]

This is not entirely true. We do set up the scopes but we do not create the
bindings in them in the preparser.

       this->CheckConflictingVarDeclarations(formal_parameters.scope, CHECK_OK);
     }
   }
 
   FunctionLiteralT function_literal = factory()->NewFunctionLiteral(
       this->EmptyIdentifierString(), ast_value_factory(),
       formal_parameters.scope, body, materialized_literal_count,
       expected_property_count, num_parameters,
       FunctionLiteral::kNoDuplicateParameters,
       FunctionLiteral::ANONYMOUS_EXPRESSION, FunctionLiteral::kIsFunction,
@@ skipping 195 matching lines @@
       *ok = false;
       return;
     }
     has_seen_constructor_ = true;
     return;
   }
 }
 } }  // v8::internal
 
 #endif  // V8_PREPARSER_H