table.insert does not call the __len metamethod

[wmoors]

mt = {__len = function(t) print('__len called') return rawlen(t) end}
t = setmetatable({},mt)

table.insert(t, 1)

The provided snippet will show __len called when ran in lua 5.2.
luajit does not, which caused me quite a headache. :)

I built LuaJIT 2.1.0-beta3 with -DLUAJIT_ENABLE_LUA52COMPAT

[wmoors]

for completeness...

print(#t)

will print __len called both in lua5.2 and luajit 2.1.0-beta3

[MikePall]

To be compatible with Lua 5.2 this needs changes in both the interpreter and the JIT compiler.

Alternatively, table.insert could be implemented in internal bytecode, just like table.remove. The difficult part is an efficient check for the number of arguments. A fixarg Lua function cannot differentiate f(t,x) from f(t,x,nil), but a vararg function would be very inefficient. Tricky.

There's a performance impact, too.

[Biep]

The bug in Lua has been described here. It seems trivial to correct, if one has access to the code.

I too have spent many hours chasing this bug, believing it was mine.