New release breaks in ConTeXt MkIV (LuaTeX)

[ghost]

The previous release worked fine with ConTeXt (LuaTeX). However, now the following input:

\input expl3-generic
\starttext ... \stoptext

Gives the following message:

tex error       > tex error on line 43 in file <path/to/l3backend>/l3backend-luatex.def: ! Undefined control sequence

\__file_mismatched_dependency_error:nn ...oup_begin: \cs_set:Npn \\{\MessageBreak }\cs_set_eq:NN \ \c_space_tl \@latex@error 
                                                                                                                             {Mismatched LaTeX support files detected.\\Loading '#2' aborted!\tl_if_exist:NT \c__kernel_expl_date_tl {\\\\The L3 progra...
l.43   }
      

33           {l3backend-luatex.def}
34       }
35       {
36         \cs_if_exist_use:cF { @latex@error } { \errmessage }
37           {
38             Mismatched~LaTeX~support~files~detected. \MessageBreak
39             Loading~aborted!
40           }
41           { \use:c { @ehd } }
42         \tex_endinput:D
43 >>    }
44     \cs_new_eq:NN \__kernel_backend_literal:e \tex_special:D
45     \cs_new_protected:Npn \__kernel_backend_literal:n #1
46       { \__kernel_backend_literal:e { \exp_not:n {#1} } }
47     \cs_generate_variant:Nn \__kernel_backend_literal:n { x }
48     \cs_new_protected:Npn \__kernel_backend_literal_pdf:n #1
49       {
50         \tex_pdfextension:D literal
51           { \exp_not:n {#1} }
52       }
53     \cs_generate_variant:Nn \__kernel_backend_literal_pdf:n { x }

[ghost]

I've just played a bit and I see the following works:

\unprotect
\def\@latex@error#1#2{}
\protect 

\input expl3-generic

\ExplSyntaxOn
\cs_new_protected:Npn \myuppercase #1
{\text_uppercase:n{#1}}
\ExplSyntaxOff
\starttext  
\myuppercase{aéreos août}
\stoptext 

I've tested random examples from StackExchange and they seem to work fine in ConTeXt.

Some commands as \@latex@error (the apparent culprit), \MessageBreak or \@ehd are only defined on LaTeX. I could copypaste them from LaTeX as a temporary fix, but I'm not sure that's a good idea. In the meanwhile, is it safe to silence the error message?

[PhelypeOleinik]

The problem was that when I wrote the version-check code for expl3 I forgot to define the token list to track the current expl3 date in the generic loader. I pushed a fix to this, so loading in generic mode should work properly in the next release.

---

About your workaround:

Redefining an error message command (such as \@latex@error) is never the right solution, as the error happens anyhow: you just don't see the pretty-printed message telling you what happened (though here the situation is really not good: the error message isn't printed at all, due to the missing \@latex@error, so your workaround is not entirely bad).

However defining \def\@latex@error#1#2{} doesn't solve the problem, as the loading of the backend files would be interrupted anyhow, so backend-dependent operations (drawing, colour, transformation, etc.) would be broken:

\unprotect
\def\@latex@error#1#2{}
\protect

\input expl3-generic

\starttext
\ExplSyntaxOn
\hbox_set:Nn \l_tmpa_box { x }
\box_scale:Nnn \l_tmpa_box { 2 } { 2 }
x\box_use:N \l_tmpa_box
\ExplSyntaxOff
\stoptext

A less worse workaround would be to define \c__kernel_expl_date_tl to match the current expl3 date in your distribution:

\unprotect
\def\c__kernel_expl_date_tl{2020-09-03}
\protect

so the backend files will load cleanly.