[Sema] [AutoDiff] Synthesize more @_fieldwiseProductSpace attributes.

[dan-zheng]

When all members have all Differentiable associated types equal to Self,
a single member AllDifferentiableVariables struct is synthesized.
Later, TangentVector and CotangentVector typealiases are generated that
forward to it.

This patch proactively generates TangentVector and CotangentVector aliases
with the @_fieldwiseProductSpace attribute.

[dan-zheng]

@swift-ci Please test tensorflow

[rxwei]

What if, in a type whose members’ tangent/cotangent/alldiffable equals the original type, the user explicitly declares ‘typealias TangentVector = AllDifferentiableVariables’? Will synthesis still run?

[dan-zheng]

What if, in a type whose members’ tangent/cotangent/alldiffable equals the original type, the user explicitly declares ‘typealias TangentVector = AllDifferentiableVariables’? Will synthesis still run?

Not currently, but it's a simple fix to do so.
Since CI mysteriously failed and needs to run again, I'll push the fix in this PR.

---

EDIT: Sorry, I was mistaken. The following simple program fails:

// test.swift
public struct Foo : Differentiable {
  public var a: Float
  typealias TangentVector = AllDifferentiableVariables
}

test.swift:3:29: error: reference to invalid associated type 'AllDifferentiableVariables' of type 'Foo'
  typealias TangentVector = AllDifferentiableVariables
                            ^
test.swift:1:15: error: type 'Foo' does not conform to protocol 'Differentiable'
public struct Foo : Differentiable {
              ^
Swift.Differentiable:2:20: note: protocol requires nested type 'TangentVector'; do you want to add it?
    associatedtype TangentVector where Self.TangentVector == Self.AllDifferentiableVariables.TangentVector, Self.AllDifferentiableVariables.TangentVector == Self.CotangentVector.CotangentVector, Self.CotangentVector.TangentVector == Self.TangentVector.AllDifferentiableVariables.CotangentVector
                   ^
Swift.Differentiable:3:20: note: protocol requires nested type 'CotangentVector'; do you want to add it?
    associatedtype CotangentVector : Differentiable where Self.CotangentVector == Self.AllDifferentiableVariables.CotangentVector, Self.AllDifferentiableVariables.CotangentVector == Self.CotangentVector.TangentVector, Self.CotangentVector.CotangentVector == Self.TangentVector.AllDifferentiableVariables.TangentVector
                   ^
Swift.Differentiable:4:20: note: protocol requires nested type 'AllDifferentiableVariables'; do you want to add it?
    associatedtype AllDifferentiableVariables : Differentiable where Self.AllDifferentiableVariables == Self.AllDifferentiableVariables.AllDifferentiableVariables, Self.CotangentVector.AllDifferentiableVariables == Self.CotangentVector.AllDifferentiableVariables.AllDifferentiableVariables
                   ^

Referencing AllDifferentiableVariables in the source file seems problematic. We could try to work around that later, if important.

[dan-zheng]

@swift-ci Please test tensorflow

[dan-zheng]

macOS CI fails mysteriously, twice in a row now.
https://ci-external.swift.org/job/swift-PR-TensorFlow-macOS/640/console

@shahmishal Do you have any idea what's going on? Thanks!

[shahmishal]

Looks like it lost connection:

Caused: java.io.IOException: Unexpected termination of the channel
	at hudson.remoting.SynchronousCommandTransport$ReaderThread.run(SynchronousCommandTransport.java:73)
Caused: java.io.IOException: Backing channel 'macOS-tensorflow' is disconnected.

[dan-zheng]

@swift-ci Please test tensorflow

[rxwei]

The current synthesis behavior is fine. Not running synthesis is better than running and forgetting to add @fieldwise.