Define Complex.zeroTangentVectorInitializer.

[dan-zheng]

The Differentiable.zeroTangentVectorInitializer protocol requirement is a closure producing a zero tangent vector, capturing minimal information from self.

• Documentation
• Default synthesis

Fixes the Complex: Differentiable conformance for master toolchains:

$ swift --version
Apple Swift version 5.3-dev (LLVM f516ac602c, Swift 09ea5fd1f7)
Target: x86_64-apple-darwin19.3.0

$ swift build
swift-numerics/Sources/ComplexModule/Differentiable.swift:15:1: error: implementation of 'Differentiable' cannot be automatically synthesized in an extension in a different file to the type
extension Complex: Differentiable
^
swift-numerics/Sources/ComplexModule/Complex.swift:48:15: note: type declared here
public struct Complex<RealType> where RealType: Real {
              ^

Necessary after swiftlang/swift#31823, which removed the temporary default implementation of Differentiable.zeroTangentVectorInitializer.

[dan-zheng]

I chose not to use a memberwise implementation of zeroTangentVectorInitializer:

  public var zeroTangentVectorInitializer: () -> Self {
    { [realTanInit = real.zeroTangentVectorInitializer,
       imaginaryTanInit = imaginary.zeroTangentVectorInitializer] in
      Complex(realTanInit(), imaginaryTanInit())
    }
  }

I don't believe it's necessary for Complex.

[dan-zheng]

cc @rxwei

[rxwei]

Why was this broken? I thought we specifically made the patch that added zeroTangentVectorInitializer synthesis be source compatible.

[dan-zheng]

zeroTangentVectorInitializer synthesis is source-compatible, except that cross-file synthesis is disallowed for all existing derived conformances. I think the diagnostic makes that clear.

We could consider lifting the same-file synthesis restriction just for Differentiable.zeroTangentVectorInitializer. I'm not sure if that has negative implications.

[dan-zheng]

@stephentyrone: could you please take a look?
This fix helps unblock apple/swift-numerics users using Swift master toolchains.