GNU bug report logs - #50720
unnamed &rest broken

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Package: emacs; Reported by: Mattias Engdegård <mattiase@HIDDEN>; merged with #50268; dated Tue, 21 Sep 2021 11:17:01 UTC; Maintainer for emacs is bug-gnu-emacs@HIDDEN.
Merged 50268 50720. Request was from Andrea Corallo <akrl@HIDDEN> to control <at> debbugs.gnu.org. Full text available.

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Subject: unnamed &rest broken
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The interpreter and compiler allow &rest to be used without a variable =
name following but the generated byte code is completely broken:

(funcall (byte-compile (lambda (&rest) 'ta)))

crashes, and

(defun boo (a &rest)
  (if a a (list 1 2 3 4)))

(boo 'hiss)
=3D> hiss        ; interpreted
=3D> (1 2 3 4)   ; compiled

The reason is that the compiler generates code from the argument =
variable list but the byte-code interpreter will only look at the =
signature code which was generated from the actual signature:

(byte-compile (lambda (&rest) 'ta))
=3D> #[128 "\300\207" [ta] 1 "..."]

The 128 indicates zero positional parameters and a &rest argument, and =
the 1 is the maximum stack size required which is wrong; 2 stack slots =
are needed and that's what we get if naming the argument:

(byte-compile (lambda (&rest _r) 'ta))
=3D> #[128 "\300\207" [ta] 2 "..."]

In the `boo` case above, it is clear that the compiler doesn't expect =
any &rest param to have been pushed at all so the stack offsets are =
wrong.

Now, either we fix this bug or we stop pretending that unnamed &rest =
arguments work at all and signal an error, because it's clear from the =
above that they can't have seen much use.





Acknowledgement sent to Mattias Engdegård <mattiase@HIDDEN>:
New bug report received and forwarded. Copy sent to bug-gnu-emacs@HIDDEN. Full text available.
Report forwarded to bug-gnu-emacs@HIDDEN:
bug#50720; Package emacs. Full text available.
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Last modified: Tue, 21 Sep 2021 16:00:02 UTC

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