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The addfield() subroutine of table.concat is (in 5.3.x and 5.4.x):

static void addfield (lua_State *L, luaL_Buffer *b, lua_Integer i) {
  lua_geti(L, 1, i);
  if (!lua_isstring(L, -1))
    luaL_error(L, "invalid value (%s) at index %d in table for 'concat'",
                  luaL_typename(L, -1), i);

But this error message is wrong, because %d expects an "int" value, not
a lua_Integer.

This shows up most clearly on 32-bit ARM (which is what the IRC user who
reported it was using); under the rules for that platform, a 64-bit int
as the 4th arg after 3 pointers ends up on the stack, because register
alignment rules prevent it going into r3; but pushvfstring is looking
for it in the place where r3 would have been stored since it thinks it
is only 32 bits. So on that platform I get, for example:

> table.concat({{}, {}}, ',')
stdin:1: invalid value (table) at index 539390232 in table for 'concat'

It wouldn't surprise me if there were other format-string errors of this
kind, though I've not checked yet.

(Original report on the freenode #lua IRC channel by "no-n", analysis is