xetex (TeXLive 2015) doesn't like @node with ß

[Werner LEMBERG]

Compiling

  \input texinfo.tex

  @documentencoding UTF-8

  @node ß
  @unnumberedsubsec ß

  @bye

makes xetex from TeXLive 2015 abort with

  ! Extra @else.
  @txiescapepdf ...hisisundefined @xdef #1{#1}@else 
                                                    @xdef #1{@pdfescapestring 
...

  @setpdfdestname ...{#1}@txiescapepdf @pdfdestname 
                                                    }
  @pdfmkdest #1->@setpdfdestname {#1}
                                     @safewhatsit {@pdfdest name{@pdfdestname 
...

  @setref #1#2->@pdfmkdest {#1}
                               @iflinks {@requireauxfile @atdummies @def 
@valu...

  @donoderef ...empty @else @setref {@lastnode }{#1}
                                                    @global @let @lastnode 
=@e...

  @sectionheading ...nlevel }{#1}{#4}@donoderef {#3}
                                                    @nobreak @vbox 
{@hyphenpen...
  ...
  l.6 @unnumberedsubsec ß

Later TeXLive versions of xetex work fine.

The commit that introduced this problem is

  commit 4f92cb18e72f748217b1d2475884848a68ccf682
  Author: Gavin Smith <gavinsmith0123@gmail.com>
  Date:   Fri May 8 19:51:40 2020 +0100

      * doc/texinfo.tex (\indexnofonts):
        Give many commands a definition that removes the {} after use.
        Sebastian Urban reported that @TeX{} appeared as "TeX{}" in a
        PDF outline rather than "TeX".

Applying

  diff --git a/tex/texinfo.tex b/tex/texinfo.tex
  index 391414187d..c731d1a152 100644
  --- a/tex/texinfo.tex
  +++ b/tex/texinfo.tex
  @@ -5248,7 +5248,7 @@ $$%
 
   % Give the control sequence a definition that removes the {} that follows 
   % its use, e.g. @AA{} -> AA
  -\def\indexnofontsdef#1#2{\def#1##1{#2}}%
  +\def\indexnofontsdef#1#2{\def#1{#2}}%
 
as a temporary solution works for us.  However, it would be nice if we
could use an unpatched `texinfo.tex` file...

Normally, I wouldn't bother with such old versions of TeXLive and
suggest to simply upgrade.  However, Ubuntu's long-term release 16.04
comes with TeXLive 2015; this release is still supported until 2024.


    Werner