bug#51982: Erroneous handling of local variables in byte-compiled nested lambdas

[Paul Pogonyshev <pogonyshev@gmail.com>]

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Steps to reproduce:

1) save attached file as `wtf.el';
2) execute from command line:

    $ emacs --batch --eval "(byte-compile-file \"wtf.el\")"
    $ emacs --batch -L . --eval "(require 'wtf)" --eval "(print (funcall
(wtf 1)))"

Results, of the first command (byte-compilation):

    In wtf:
    wtf.el:9:17:Warning: reference to free variable ‘it’

of the second:

    Symbol’s value as variable is void: it

Expected results: byte compilation succeeds without warnings, second
command prints "1".

Note that if you change line "(let ((fn #'(lambda () it)))" to e.g. "(let
((fn #'(lambda () nil)))", everything works as expected.  However, 'fn' _is
not even used_ when function `wtf' is called with non-nil argument, which
further conforms that the observed behavior is a bug.  Another indication:
when the function is not byte-compiled (e.g. delete file `wtf.elc' after
the first command), it produces expected results.

Function, of course, doesn't make any sense.  It is a result of removing
contents of real failure until it is as small as possible.

Paul

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[-- Attachment #2: wtf.el --]
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; -*- lexical-binding: t -*-

(defun wtf (x)
  (let ((it 0))
    #'(lambda ()
        (let ((fn #'(lambda () it)))
          (if x
              (let ((it x))
                it)
            (funcall fn))))))

(provide 'wtf)