sbt/notes/0.13.13/ops_deprecation.md

Fixes with compatibility implications

• Deprecate old operators.

Deprecate old operators

The no-longer-documented operators <<=, <+=, and <++= are deprecated, and will be removed in sbt 1.0.

For <<=, the suggested migration would be to use either := or ~= operators. The RHS of <<= takes an Initialize[_] expression, which can be converted to := style by wrapping the expression in parenthesis, and calling .value at the end. For example:

key := (key.dependsOn(compile in Test)).value

For <+= and <++=, use += { x.value } and ++= { x.value }.