-15. `dcast()` now issues a warning when `fun.aggregate` is used but not provided by the user. `fun.aggregate` defaults to `length` in this case; previously, only a message was issued, but often relying on this default signals unexpected duplicates in the data, so a stricter class of signal was deemed more appropriate, [#5386](https://github.com/Rdatatable/data.table/issues/5386). The warning is classed as `dt_missing_fun_aggregate_warning`, allowing for more targeted handling in user code. Thanks @MichaelChirico for the suggestion and @Nj221102 for the fix.
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