FFI: Fix ffi.metatype() for non-raw types.

Reported by 999pingGG.

(cherry picked from commit 4c35a42)

This patch is a follow-up to the previous commit. Since the child id is
used for the index in the lookup table, it has no effect, and metatype
methods are not overloaded. This patch fixes the behaviour by using the
correct index.

Sergey Kaplun:
* added the description and the test for the problem

Part of tarantool/tarantool#9924

Author: Mike Pall

src/lib_ffi.c

@@ -776,7 +776,7 @@ LJLIB_CF(ffi_metatype)
   if (!(ctype_isstruct(ct->info) || ctype_iscomplex(ct->info) ||
 	ctype_isvector(ct->info)))
     lj_err_arg(L, 1, LJ_ERR_FFI_INVTYPE);
-  tv = lj_tab_setinth(L, t, -(int32_t)id);
+  tv = lj_tab_setinth(L, t, -(int32_t)ctype_typeid(cts, ct));
   if (!tvisnil(tv))
     lj_err_caller(L, LJ_ERR_PROTMT);
   settabV(L, tv, mt);

test/tarantool-tests/lj-861-ctype-attributes.test.lua

@@ -2,12 +2,14 @@ local tap = require('tap')
 
 -- Test file to demonstrate LuaJIT incorrect behaviour during
 -- parsing and working with ctypes with attributes.
--- See also: https://github.com/LuaJIT/LuaJIT/issues/861.
+-- See also:
+-- * https://github.com/LuaJIT/LuaJIT/issues/861,
+-- * https://github.com/LuaJIT/LuaJIT/issues/1005.
 
 local test = tap.test('lj-861-ctype-attributes')
 local ffi = require('ffi')
 
-test:plan(5)
+test:plan(6)
 
 local EXPECTED_ALIGN = 4
 
@@ -37,8 +39,15 @@ test:is(ffi.sizeof('struct test_parsing_sizeof'), EXPECTED_ALIGN,
 test:is(ffi.sizeof('struct test_parsing_alignof'), EXPECTED_ALIGN,
         'correct alignof during C parsing')
 
-local ok, _ = pcall(ffi.metatype, 's_aligned', {})
+local EXPECTED_TOSTR = '__tostring overloaded'
+local ok, obj = pcall(ffi.metatype, 's_aligned', {
+  __tostring = function()
+    return EXPECTED_TOSTR
+  end,
+})
 
 test:ok(ok, 'ffi.metatype is called at the structure with attributes')
+test:is(tostring(obj()), EXPECTED_TOSTR,
+        '__tostring is overloaded for the structure with attributes')
 
 test:done(true)