[tex-k] In TeX: The Program, section 88, interactively deleting outer macros can cause confusing errors

[Karl Berry]

Hi Tyge - back on your report in
https://tug.org/pipermail/tex-k/2024-March/004021.html:

    Interactively deleting a control sequence token corresponding to an
    outer macro while in a state that forbids outer macros, such as the
    replacement text of a macro definition, can cause a confusing error
    message. See for example the following run:

      This is TeX, Version 3.141592653
      **\outer\def\someoutermacro{}

      *\edef\a{\undefinedmacro\someoutermacro}
      ! Undefined control sequence.
      <*> \edef\a{\undefinedmacro
                                 \someoutermacro}
      ? 1
      Runaway definition?

Agreed. Added at https://tug.org/texmfbug/newbug.html#B142outer.

Copying the text I wrote there: The simplest fix DRF found was take
advantage of the fact that OK_to_interrupt has been set to
false during the critical moment and omit the \outer check (this is
in the check_outer_validity procedure):

@x [24.336] l.7152 - allow interactive deletion of \outer token
begin if scanner_status<>normal then
@y
begin if OK_to_interrupt and(scanner_status<>normal) then
@z

This is applied in Web2c tex.ch now (so, for all engines except LuaTeX,
which will probably follow).

You had written:

    A possible fix would be to temporarily set `scanner_status` to
    `normal` in section 88 before deleting tokens and restoring it
    afterwards as is similarly done for `align_state`.

DRF looked into this too, copied below. But the above fix is a lot
simpler. Let me know if you see problems ... --thanks, karl.

From: DRF

Yes, you can do the suggested scanner_status save/restore, but it
requires a little bit of a hack, as normal hasn't been defined yet:

@x 1881
@!s1,@!s2,@!s3,@!s4:integer;
@y
@!s1,@!s2,@!s3,@!s4,@!s5:integer;
@z

@x 1990
begin s1:=cur_tok; s2:=cur_cmd; s3:=cur_chr; s4:=align_state;
align_state:=1000000; OK_to_interrupt:=false;
@y
begin s1:=cur_tok; s2:=cur_cmd; s3:=cur_chr; s4:=align_state;
s5:=scanner_status;
align_state:=1000000; scanner_status:=0; {|normal|} OK_to_interrupt:=false;
@z

@x 1999
cur_tok:=s1; cur_cmd:=s2; cur_chr:=s3; align_state:=s4; OK_to_interrupt:=true;
@y
cur_tok:=s1; cur_cmd:=s2; cur_chr:=s3; align_state:=s4; scanner_status:=s5;
OK_to_interrupt:=true;
@z