[Webkit-unassigned] [Bug 145132] [ES6] Arrow function syntax. Arrow function specific features. Lexical bind "arguments"

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=145132

--- Comment #52 from Saam Barati <sbarati at apple.com> ---
Comment on attachment 268326
  --> https://bugs.webkit.org/attachment.cgi?id=268326
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=268326&action=review

> Source/JavaScriptCore/bytecompiler/BytecodeGenerator.cpp:83
> +            if (m_codeBlock->isStrictMode())
> +                emitTDZCheck(argumentsRegister);

why would we ever need a TDZ check here?

> Source/JavaScriptCore/bytecompiler/BytecodeGenerator.cpp:4123
> +RegisterID* BytecodeGenerator::emitPutArgumentsToArrowFunctionContextBlockScope()

I see what you're going for here in this function, but this seems a bit hacky to me.
I think there might be an alternate solution that might be cleaner:

We currently don't allow functions to declare the variable named "arguments"
as being captured (you can see this in the "collectFreeVariables" function inside Parser.h).
But, in the case of an arrow function, we really do want "arguments" to act like a captured variable.
If we teach the parser we can really capture "arguments", then we could probably do away
with a lot of this specialized code. We wouldn't need this function (because "arguments" would already
be in whatever environment is represented by scopeRegister()). We probably wouldn't need a specialized
argumentsLocalPrivateName either. We would need some BytecodeGenerator code just to teach it
that we don't have a local "arguments" that we created, but that's about it. I think most of this change
could be a parser change.

You would basically have to teach the parser that any arrow function implicitly captures the name "arguments".
Or we could be smart about it, and only capture "arguments" if we use "arguments" or "eval" inside the
arrow function.

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