[racket-dev] Strange issue with identifier-binding being wrong for lexical variables

I agree that this is broken, but I'd like to put it on hold, because
its another basic problem with the way the current macro expander
represents lexical context.

Adjusting the context of the expressions changes the result, because
its the macro-introduced nature of the `main` definition that triggers
the bad result from `identifier-binding`.

Expansions that produce this bad `identifier-binding` result probably
happen up all the time. They don't bother the bytecode compiler,
because the compiler uses `free-identifier=?` to compare bindings in
expanded code, keeping track of all of the bindings that are in the
environment of a given expression. Depending on your use case, that
might be the way to go for now.

At Tue, 21 Oct 2014 22:26:26 -0400, Sam Tobin-Hochstadt wrote:
> I've found what I think is a bug in the expander where lexical
> references can get an `identifier-binding` result that suggests that
> they're module-bound.
> 
> In particular, you need these three files:
> 
> bugtest.rkt:
> 
> (module bugtest "wraptest.rkt")
> 
> bugtest.scm:
> 
> (define (gcbench)
>   (define main #f)
>   main)
> 
> (define main #f)
> 
> wraptest.rkt:
> 
> #lang racket/base
> (provide (rename-out (module-begin #%module-begin)))
> 
> (require racket/include
>          (for-syntax racket/base))
> 
> (define-syntax (module-begin stx)
>   (define name (syntax-property stx 'enclosing-module-name))
>   #`(#%module-begin
>      (include #,(format "~a.scm" name))))
> 
> Then run the macro stepper with macro hiding off on "bugtest.rkt".
> Click on the reference to `main` inside `gcbench`. You'll see that it
> says that it's a module-bound variable named `main.2` which is defined
> in this module.
> 
> Then try changing the name of the top-level definition in bugtest.scm
> to `main2`. Re-run the macro stepper, and you'll see that the
> identifier-binding is now lexical.
> 
> I tried a few things to get this to go away (such as using
> `#%plain-module-begin`) which didn't work. Reducing it to two files,
> ie doing the include directly in bugtest.rkt, made the problem
> disappear.
> 
> Changing the body of the `module-begin` macro to:
> 
> (define-syntax (module-begin stx)
>   (define name (syntax-property stx 'enclosing-module-name))
>   #`(#%module-begin
>      #,(datum->syntax stx `(include ,(format "~a.scm" name)))
>      #,(datum->syntax stx '(main))))
> 
> and then providing a bunch of extra stuff made the issue go away.
> Because there's the workaround, the issue isn't urgent.
> 
> Sam (and Tobias, who found this bug)