luacode can drop brace groups

[davidcarlisle]

Usually in luacode { and } are passed through to Lua if not used as the argument to a TeX command, however if an entire segment to end of line is a brace group the braces are dropped in #1\@nil due to TeX delimited argument parsing rules.

at https://tex.stackexchange.com/a/762809/1090 I suggest a fix of adding and removing a token so that the delimited argument is never a single brace group.

\documentclass{standalone}% compile with lualatex
\usepackage{luacode}
\makeatletter
\show\luacode@strip@sp@look
\renewcommand \luacode@strip@sp@look {%
  \expandafter\ifx\luacode@spaceother\@let@token
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi{%
    \afterassignment\luacode@strip@sp@peek
    \let\@let@token=
  }{%
    \luacode@strip@sp@def!%
  }}

\def \luacode@strip@sp@def #1\@nil{%
  \expandafter\def\expandafter\luacode@curr\expandafter{\@gobble#1}}

\makeatother
\begin{document}

\begin{luacode}
local f = function(x, y, opt)
    opt = opt or {}
    opt.sg = opt.sg or '+'
    if opt.sg == '+' then return x+y
    else
        return x-y
    end
end

local x = 
  f(2,
    2,
    {sg='+'}
    ) -- < this causes an error with luacode, but not luacode*
tex.print("Result ="..x)
\end{luacode}
\end{document}