bug#26486: 25.1.91; spurious warnings from cconv.el

[John Williams <jrw@pobox.com>]

When I byte compile a file where a macro expansion introduces an unused
lexical veriable, I get warnings from cconv.el, even if I wrap the
expansion in (with-no-warnings) and set byte-compile-warnings to nil.
IMHO warnings of this type should never be issued for code that comes
from a macro expansion, but it should at least be possible to suppress
them with the usual mechanisms.  Here's a file that demonstrates the
problem when it's compiled with byte-compile-file:

========================================================================
;; -*- lexical-binding: t; byte-compile-warnings: nil  -*-

(defmacro test ()
  `(let (unused)))

(with-no-warnings
  (test))
========================================================================



In GNU Emacs 25.1.91.1 (x86_64-pc-linux-gnu, GTK+ Version 3.10.8),
modified by Debian
Windowing system distributor 'The X.Org Foundation', version 11.0.11803000
System Description:     Ubuntu 14.04.5 LTS

Configured using:
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 'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro
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Major mode: Emacs-Lisp

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Recent messages:
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Type "q" in help window to restore its previous buffer.
Undo! [22 times]
Saving file /usr/local/google/home/jrw/homedir/test.el...
Wrote /usr/local/google/home/jrw/homedir/test.el
Compiling /usr/local/google/home/jrw/homedir/test.el...done
Wrote /usr/local/google/home/jrw/homedir/test.elc
Quit

Load-path shadows:
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Memory information:
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