Revert "Don't warn about numeric coercion in set(j=)"

This reverts commit f6c1385.

Author: MichaelChirico

NEWS.md

@@ -133,8 +133,6 @@ rowwiseDT(
 
 8. `measurev()` was implemented and documented in v1.15.0, for use within `melt()`, and it is now exported (dependent packages can now use without a NOTE from CRAN check).
 
-9. Using a double vector in `set()`'s `j=` no longer throws a warning about preferring integer, [#6594](https://github.com/Rdatatable/data.table/issues/6594). While it may improve efficiency to use integer, there's no guarantee it's an improvement and the difference is likely to be minimal. The coercion will still be reported under `datatable.verbose=TRUE`. For package/production use cases, static analyzers such as `lintr::implicit_integer_linter()` can also report when numeric literals should be rewritten as integer literals.
-
 # data.table [v1.16.0](https://github.com/Rdatatable/data.table/milestone/30)  (25 August 2024)
 
 ## BREAKING CHANGES

src/assign.c

@@ -425,8 +425,7 @@ SEXP assign(SEXP dt, SEXP rows, SEXP cols, SEXP newcolnames, SEXP values)
   } else {
     if (isReal(cols)) {
       cols = PROTECT(coerceVector(cols, INTSXP)); protecti++;
-      if (verbose)
-        Rprintf(_("Coerced j from numeric to integer. Please pass integer for efficiency; e.g., 2L rather than 2"));
+      warning(_("Coerced j from numeric to integer. Please pass integer for efficiency; e.g., 2L rather than 2"));
     }
     if (!isInteger(cols))
       error(_("j is type '%s'. Must be integer, character, or numeric is coerced with warning."), type2char(TYPEOF(cols)));