The return types for stride(from: to: by:) and stride(from: through: by:), StrideTo and StrideThrough respectively, are only Sequences, not Collections. This was caused by a bug in conditional conformance, as mentioned in the preamble to the commented-out Collection-conformance code. Since SR-6474 is now fixed, can we now make them collections?
Added as SR-13492 ("Make StrideTo and StrideThrough conform to (RandomAccess)Collection").