#t calls __len(t, t)?

[Rett Berg]

I'm very confused by this:

$ lua -v
Lua 5.4.8  Copyright (C) 1994-2025 Lua.org, PUC-Rio

$ lua

> t = setmetatable({}, {__len=function(...) print(...) end})
table: 0x556e5dc1ad50
> #t
table: 0x556e5dc1ad50   table: 0x556e5dc1ad50
nil

It seems like #t is passing t twice?

[Luiz Henrique de Figueiredo]

> It seems like #t is passing t twice?

Yes. For treating binary and unary metamethods uniformly.
Arithmetic negation works the same way:

t = setmetatable({}, {__unm=function(...) print(...) end})
print(-t)

[Roberto Ierusalimschy]

https://www.lua.org/manual/5.4/manual.html#2.4

[...]
For the unary operators (negation, length, and bitwise NOT), the
metamethod is computed and called with a dummy second operand, equal
to the first one. This extra operand is only to simplify Lua's
internals (by making these operators behave like a binary operation)
and may be removed in future versions. For most uses this extra
operand is irrelevant.

-- Roberto

[Sainan]

May I suggest a patch?

> luaV_objlen
-luaT_callTMres(L, tm, rb, rb, ra);
+luaT_callTMres(L, tm, rb, 0, ra);

> luaV_execute
-Protect(luaT_trybinTM(L, rb, rb, ra, TM_UNM));
+Protect(luaT_trybinTM(L, rb, 0, ra, TM_UNM));
...
-Protect(luaT_trybinTM(L, rb, rb, ra, TM_BNOT));
+Protect(luaT_trybinTM(L, rb, 0, ra, TM_BNOT));

> luaT_callTMres
-setobj2s(L, func + 2, p2); /* 2nd argument */
-L->top.p += 3;
+L->top.p += 2;
+if (p2) {
+ setobj2s(L, func + 2, p2); /* 2nd argument */
+ L->top.p += 1;
+}

-- Sainan

[Rett Berg]

Thanks everyone, glad to know it was a known behavior and may be removed.

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