bug#14565: 24.3.50; Error with byte-compiled function using backward-char

[Stephen Berman]

On Thu, 06 Jun 2013 16:24:17 +0200 Stephen Berman <stephen.berman@gmx.net> 
wrote:

> When I evaluate and invoke each of the following functions, they both
> work as expected:
>
> (defun my-test-1 () (forward-char nil))
>
> (defun my-test-2 () (backward-char nil))
>
> But if I byte-compile them, only my-test-1 works; my-test-2 raises a
> Lisp error: (wrong-type-argument number-or-marker-p nil)
>
> Here are the byte codes:
>
> (byte-code "\300\301\302\"\207" [defalias my-test-1 #[nil "\300u\207" [nil] 
> 1]] 3)
>
> (byte-code "\300\301\302\"\207" [defalias my-test-2 #[nil "\300[u\207" [nil] 
> 1]] 3)
>
> and here is the disassembled code:
>
> byte code for my-test-1:
>   args: nil
> 0     constant  nil
> 1     forward-char 
> 2     return    
>
> byte code for my-test-2:
>   args: nil
> 0     constant  nil
> 1     negate    
> 2     forward-char 
> 3     return    

If `negate' in the byte code corresponds to the case Bnegate in
exec_byte_code, then IIUC nil fails the test for INTEGERP so is passed
to Fminus, and from there to arith_driver, where it fails
CHECK_NUMBER_OR_FLOAT_COERCE_MARKER, which via CHECK_TYPE signals the
wrong-type-argument error.  If this is what happens, then the byte code
of backward-char is at odds with its definition via move_point in
cmds.c, which explicitly checks whether the argument is nil, and if so
sets it to 1, which for Fbackward_char is then negated.  Or have I
completely misunderstood and if so, what is the error due to?

Steve Berman