Re: __update metamethod — intercept wr ites to existing keys (patch for 5.5.0)

It seems that in Lua 5.5, the `isrealasize` is no longer in use, and the 
corresponding comments should be able to be removed.

```
src/ltable.c:** dummy bit must be exchanged: The 'isrealasize' is not 
related
src/ltm.h:** the table is using the dummy node; bit 7 is used for 
'isrealasize'.)
```

在2026年4月8日星期三 UTC+8 18:23:46<chen chen> 写道：

Hello,

I've been thinking about the gap between `__newindex` (new keys only) and 
the need to intercept updates to existing keys. The proxy-table workaround 
works but has notable overhead. I put together a small patch (~95 lines) 
for Lua 5.5.0 to explore what a native solution might look like — I'd love 
to hear if this direction has been considered or if there are concerns I'm 
missing.

The patch adds an `__update` metamethod that fires when assigning to an 
existing key (including deletion via nil), complementing `__newindex`. 
Same-value writes are skipped as a fast-path optimization.

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