TIL there is a scalacheck. ./test/scalacheck/scala/collection/IndexOfSliceTest.scala

sbt:root> scalacheck/testOnly scala.collection.IndexOfSliceTest

Edit: not SliceOfToast

As sjrd pointed out on the ticket, it's desirable for indexOfSlice to return an index for slice, not an index for an element via apply.

The fix is for the case where source seq and slice do not both have known lengths.

If the source seq has a known length, we can distinguish from == length and from == length + 1. Use drop to skip.

Otherwise, we count the prefix elements up to from.

I dozed briefly with my head on the keyboard and had a dream in which tpolecat commanded me to remove any usages of return where possible.

The added commit "returns them", as in returning product to Costco. Also rebased, as it incurs only one CI run.

I will leave the PR unsquashed for a short period for review purposes.

I feel like we need to agree on, and spec, out-of-bounds handing before fixing individual issues. scala/bug#12795 (comment)

The sjrd justification is that it returns a number >= the given start index.

scala> xs.indexOfSlice(Nil, -11)
val res10: Int = 0   // was -11

scala> xs.indexOfSlice(Nil, 11)
val res11: Int = -1

I suspect that only operations requiring an existing element ("the index of an existing element") should throw, paradigmatically, apply.