added news for PR 7260 (#7263)

Author: Mukulyadav2004

NEWS.md

@@ -8,6 +8,10 @@
 
 1. `data.table(x=1, <expr>)`, where `<expr>` is an expression resulting in a 1-column matrix without column names, will eventually have names `x` and `V2`, not `x` and `V1`, consistent with `data.table(x=1, <expr>)` where `<expr>` results in an atomic vector, for example `data.table(x=1, cbind(1))` and `data.table(x=1, 1)` will both have columns named `x` and `V2`. In this release, the matrix case continues to be named `V1`, but the new behavior can be activated by setting `options(datatable.old.matrix.autoname)` to `FALSE`. See point 5 under Bug Fixes for more context; this change will provide more internal consistency as well as more consistency with `data.frame()`.
 
+### BREAKING CHANGE
+
+1. `dcast()` now errors when `fun.aggregate` returns length != 1 (consistent with documentation), regardless of `fill`, [#6629](https://github.com/Rdatatable/data.table/issues/6629). Previously, when `fill` was not `NULL`, `dcast` warned and returned an undefined result. This change has been planned since 1.16.0 (25 Aug 2024).
+
 ### NEW FEATURES
 
 1. New `sort_by()` method for data.tables, [#6662](https://github.com/Rdatatable/data.table/issues/6662). It uses `forder()` to improve upon the data.frame method and also matches `DT[order(...)]` behavior with respect to locale. Thanks @rikivillalba for the suggestion and PR.