bug#16048: 24.3.50; String compare surprise

[Josh <josh@foxtail.org>]

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On Wed, Dec 4, 2013 at 9:29 AM, Eli Zaretskii <eliz@gnu.org> wrote:
> > From: Josh <josh@foxtail.org>
> > Date: Wed, 4 Dec 2013 06:00:46 -0800
> > Cc: Michael Albinus <michael.albinus@gmx.de>, 16048@debbugs.gnu.org
> > On Wed, Dec 4, 2013 at 5:07 AM, Andreas Schwab <schwab@linux-m68k.org
>wrote:
> > > michael.albinus@gmx.de writes:
> > >
> > > > The following form evals to nil:
> > > >
> > > >   (string-equal "\377" "ÿ")
> > >
> > > "\377" is a unibyte string.  When converted to multibyte it yields
> > > "\x3fffff".
> >
> >
> > At least as of 24.3, the manual[0] suggests that such a conversion
> > should not occur in this case:
> And it doesn't occur, indeed:
>
>   (multibyte-string-p "\377")
>
>     => nil
>
> >     You can also use hexadecimal escape sequences (`\xN') and octal
> >     escape sequences (`\N') in string constants.  *But beware:* If a
> >     string constant contains hexadecimal or octal escape sequences,
> >     and these escape sequences all specify unibyte characters (i.e.,
> >     less than 256), and there are no other literal non-ASCII
> >     characters or Unicode-style escape sequences in the string, then
> >     Emacs automatically assumes that it is a unibyte string.  That is
> >     to say, it assumes that all non-ASCII characters occurring in the
> >     string are 8-bit raw bytes.
> >
> > [0] (info "(elisp) Non-ASCII in Strings")
> Best citation contest? you're on!

No, thanks.  I haven't entered such contests in many years.

>    -- Function: string= string1 string2
>        This function returns `t' if the characters of the two strings
>        match exactly.  Symbols are also allowed as arguments, in which
>        case the symbol names are used.  Case is always significant,
>        regardless of `case-fold-search'.
>
>    [...]
>
>        For technical reasons, a unibyte and a multibyte string are
>        `equal' if and only if they contain the same sequence of character
>        codes and all these codes are either in the range 0 through 127
>        (ASCII) or 160 through 255 (`eight-bit-graphic').  However, when a
>        unibyte string is converted to a multibyte string, all characters
>        with codes in the range 160 through 255 are converted to
>        characters with higher codes, whereas ASCII characters remain
>        unchanged.  Thus, a unibyte string and its conversion to multibyte
>        are only `equal' if the string is all ASCII.
>
> Note the last sentence.

Yes, I must have misunderstood Andreas' meaning; I believed he was
suggesting that the two strings compared differently due to "\377"
having been converted to a multibyte string and therefore miscomparing
with the unibyte (or so I thought) string "ÿ".  I see now that I had
it exactly backwards.  Thanks for setting me straight.

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