fix(lua): revert vim.tbl_extend behavior change and document it

Problem: vim.tbl_deep_extend had an undocumented feature where arrays
(integer-indexed tables) were not merged but compared literally (used
for merging default and user config, where one list should overwrite the
other completely). Turns out this behavior was relied on in quite a
number of plugins (even though it wasn't a robust solution even for that
use case, since lists of tables (e.g., plugin specs) can be array-like
as well).

Solution: Revert the removal of this special feature. Check for
list-like (contiguous integer indices) instead, as this is closer to the
intent. Document this behavior.

1 files changed, 3 insertions, 4 deletions

diff --git a/test/functional/lua/vim_spec.lua b/test/functional/lua/vim_spec.lua
index 7f10dcd8da..599b688bf4 100644
--- a/test/functional/lua/vim_spec.lua
+++ b/test/functional/lua/vim_spec.lua
@@ -1071,12 +1071,11 @@ describe('lua stdlib', function()
     ]])
     )
 
-    -- Fix github issue #23654
     ok(exec_lua([[
-      local a = { sub = { [1] = 'a' } }
-      local b = { sub = { b = 'a' } }
+      local a = { sub = { 'a', 'b' } }
+      local b = { sub = { 'b', 'c' } }
       local c = vim.tbl_deep_extend('force', a, b)
-      return vim.deep_equal(c, { sub = { [1] = 'a', b = 'a' } })
+      return vim.deep_equal(c, { sub = { 'b', 'c' } })
     ]]))
 
     matches('invalid "behavior": nil', pcall_err(exec_lua, [[return vim.tbl_deep_extend()]]))