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LuaItemStack::set_mapper() has an UpgradeFilter parameter that says it can be nil to remove a filter in the description, but isn't marked as optional in the actual documentation. This trickles down to a LuaLS warning, either missing-parameter or param-type-mismatch.

Thanks for noting this, fixed for the next release (1.1.102). Same fix was done for the filter in LuaItemStack:set_tile_filter().