__tostring and string.format

[HyperHacker <hyperhacker@...>]

Something that always bugged me:
> t={}
> setmetatable(t, {__tostring = function(t) return "xxx" end})
> print(t)
xxx
> print(('%s'):format(t))
stdin:1: bad argument #1 to 'format' (string expected, got table)

Why does string.format not call tostring() on arguments given as %s?
(and for that matter tonumber() for %d/%u, which in turn could call a
__tonumber metamethod...)

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