Strange change in load/loadstring in Lua-5.4.4

[Domingo Alvarez Duarte <mingodad@...>]

Hello !

While trying to change Lua-5.4.4 to LJS 
(https://github.com/mingodad/ljs) I found an intriguing change in 
behavior/bug between Lua-5.3.6 and Lua-5.4.4, in LJS I decided to forbid 
shadowing variables in the same scope and it works as intended in LJS 
from LjsJit, Lua-(5.1.5 .. 5.3.6) but applying a similar check to 
Lua-5.4.4 find a duplicated variable name where it shouldn't (see code 
shown bellow).

Somehow this line 'local res = loadstring("a=1")' brings the 'a' 
variable name to visibility for the parser in Lua-5.4.4 but not for 
previous Lua versions.

It seems like the parser is parsing the string passed to 'load' when 
parsing the script instead of at runtime, or something else that I'm not 
seeing through the code.

Can someone shed some light on this issue ?

====

if loadstring == null then
    loadstring = load;
end
function bug()
    do
      local a = 2;
    end
    print(a);
    local res = loadstring("a=1"); --somehow this brings 'a' visible 
internally
    --res();
    print(a);
    --Lua-5.4.4 find the next 'a' declaration as duplicated
    --local a = 3; -- but if we comment this line 'a' is printed as 
'nil' bellow
    print(res, a);
end

print(a);
bug();
print(a);

====

====

/*
** Create a new local variable with the given 'name'. Return its index
** in the function.
*/
static int new_localvar (LexState *ls, TString *name) {
  lua_State *L = ls->L;
  FuncState *fs = ls->fs;
  Dyndata *dyd = ls->dyd;
  Vardesc *var;

  /* START Check for duplicated varibale names in local scope */
  int vidx, nactvar_n;
  vidx = fs->bl ? fs->bl->nactvar + fs->firstlocal : fs->firstlocal;
  //nactvar_n = dyd->actvar.n;
  nactvar_n = fs->nactvar;
  if(nactvar_n > fs->ndebugvars) nactvar_n = fs->ndebugvars;
  for (; vidx < nactvar_n; ++vidx) {
      Vardesc *vd = dyd->actvar.arr+vidx;
    LocVar *lv = &fs->f->locvars[vd->vd.ridx];
    if (lv && name == lv->varname) {
      /* allow '_' duplicates */
      const char *sname = getstr(name);
      if(tsslen(name) == 1 && sname[0] == '_') break;
      luaX_syntaxerror(ls, luaO_pushfstring(ls->L,
             "Name [%s] already declared", sname));
    }
  }
  /* END Check for duplicated varibale names in local scope */

  checklimit(fs, dyd->actvar.n + 1 - fs->firstlocal,
                 MAXVARS, "local variables");
  luaM_growvector(L, dyd->actvar.arr, dyd->actvar.n + 1,
                  dyd->actvar.size, Vardesc, USHRT_MAX, "local variables");
  var = &dyd->actvar.arr[dyd->actvar.n++];
  var->vd.kind = VDKREG;  /* default */
  var->vd.name = name;
  return dyd->actvar.n - 1 - fs->firstlocal;
}

====

Cheers !