Hello:

I recently found that sometimes an argument of a function is
unexpectedly set to nil when the function is natively compiled.

A recipe to reproduce the behaviour is as follows.

```elisp
(defun test (&optional start)
  "Test the values of START."
  (message "start is %S" start)
  (cond
   ((not (integer-or-marker-p start))
    (message "entered first cond variant")
    (setq start nil))
   ((and
     (markerp start)
     (not (equal (marker-buffer start) (current-buffer))))
    (message "entered second cond variant")
    (setq start nil)))
  (message "start is %S" start))
(native-compile 'test)
(test 12)
```

When I evaluate the above forms in sequence, I got the following two
messages:

start is 12
start is nil

As can be seen from the above, no `cond` variants are triggered, so the
variable START should have the value 12, while it becomes nil at the
end.

Sorry if this example is not minimal: I found this behaviour in my
package, so copied the relevant parts here.  I am not sure what a
minimal example would be.

Hope this report is helpful.
---
In GNU Emacs 29.3.50 (build 1, x86_64-apple-darwin23.4.0, NS
 appkit-2487.50 Version 14.4.1 (Build 23E224)) of 2024-06-12 built on
 MacBook-Pro-de-Severe.local
Repository revision: 59261e6f4fef0ec03c8127b29fe107ed19280a3b
Repository branch: emacs-29
Windowing system distributor 'Apple', version 10.3.2487
System Description:  macOS 14.4.1

Configured using:
 'configure --with-ns --without-tree-sitter
 --with-native-compilation=yes'

Configured features:
ACL DBUS GIF GLIB GMP GNUTLS JPEG JSON LCMS2 LIBXML2 MODULES NATIVE_COMP
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Major mode: Lisp Interaction

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Load-path shadows:
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Memory information:
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 (symbols 48 8233 1)
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 (string-bytes 1 896655)
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---
Durand