[SR-14021] Functions accepting multiple inout parameters aren't differentiable

[porterchild]

Previous ID	SR-14021
Radar	None
Original Reporter	@porterchild
Type	New Feature

Additional Detail from JIRA

Votes	1
Component/s	Compiler
Labels	New Feature, AutoDiff
Assignee	None
Priority	Medium

md5: aec1df3f108a4e0b962fbcb0167a08a4

is blocked by:

• TF-1250 Support differentiation of functions with multiple results

Issue Description:

Declaring a differentiable function with more than one inout parameter results in the compiler error:

"Cannot differentiate functions with both an 'inout' parameter and a result"

[typesanitizer]

cc @dan-zheng

[philipturner]

I found something interesting when narrowing the reproducer found in #57331 (formerly SR-14989). If you make setA only differentiable with respect to a, then it triggers an incorrect error like the one described in this issue. This should be fixed in #41174.

Reproducer:

import _Differentiation

protocol P: Differentiable {
  @differentiable(reverse, wrt: self)
  var a: Double { get set }
}

struct Holder<ConcreteP: P>: Differentiable {
  var p: ConcreteP

  @differentiable(reverse, wrt: a)
  mutating func setA(a: Double) -> Double {
    fatalError()
  }
}

Built output:

(base) philipturner@m1-max-mbp bug % swiftc file.swift
file.swift:12:4: error: cannot differentiate functions with both an 'inout' parameter and a result
  @differentiable(reverse, wrt: a)
   ^