[Pitch] Make Collection Super-Convenient and Retire Sequence

[dabrahams (Dave Abrahams)]

@dabrahams

Well, I tried, but at the moment I can’t even get past step 1. Just adding this , which should be benign, causes the standard library to not compile .

Well, I found a workaround, so the work is currently unblocked. Until SR-7605 is fixed, we will have the compiler deducing Index = DefaultIndex<Self> for any Collection without an explicit nested Index type. After it is fixed, though, the picture changes slightly: either we make Sequence a refinement of Collection that supplies DefaultIndex<Self> as a default, or Sequence = Collection and we have to migrate existing Sequence instances to include typealias Index = DefaultIndex<Self>.