[SR-14117] stdlib: support differentiation of `Array.subscript._modify`

[dan-zheng]

Previous ID	SR-14117
Radar	None
Original Reporter	@dan-zheng
Type	Sub-task

Additional Detail from JIRA

Votes	1
Component/s	Compiler
Labels	Sub-task, AutoDiff
Assignee	None
Priority	Medium

md5: df2740eef47719be1f5be6f070efa03d

Parent-Task:

• SR-14113 Support _read and _modify accessor differentiation

is blocked by:

• TF-1193 Support @derivative derivative registration for specific storage accessors

Issue Description:

This makes it possible to differentiate idiomatic array element assignments: array[i] = newValue.

This is the final user code that we want to support.

[swift-ci]

Comment by Akshaan Kakar (JIRA)

Hey @dan-zheng, I'd like to take a shot at this issue. Do you think it's a reasonable first issue to take on? If so, is it alright if I assign it to myself?

[dan-zheng]

Hi akshaan (JIRA User): this actually isn't quite suitable as a first issue.

It's blocked because derivative registration for modify accessors (e.g. @derivative(of: subscript._modify)) isn't supported yet. We haven't thought through a general way to support that. In the meantime, we're considering adding special-case differentiation support for array subscript assignments.