[BUG] Being provided some equal recursive structures, equality operator never stops comparison

[ZyX]

Attached script, being sourced causes vim to hang up forever. Test command:
    vim -u NONE -c 'so bug.vim'

let l=[]
call add(l, l)
let dict4={"l": l}
call add(dict4.l, dict4)
echo (dict4 ==# deepcopy(dict4))
qa!

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