Define Complex.zeroTangentVectorInitializer.

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

Fixes the `Complex: Differentiable` conformance for master toolchains.

Author: dan-zheng

Sources/ComplexModule/Differentiable.swift

@@ -15,6 +15,11 @@ import _Differentiation
 extension Complex: Differentiable
 where RealType: Differentiable, RealType.TangentVector == RealType {
   public typealias TangentVector = Self
+
+  @inlinable
+  public var zeroTangentVectorInitializer: () -> Self {
+    { Complex.zero }
+  }
 }
 
 extension Complex

Tests/ComplexTests/DifferentiableTests.swift

@@ -57,6 +57,10 @@ final class DifferentiableTests: XCTestCase {
     XCTAssertEqual(φDivide(Complex(1, 0)), Complex(0.25, -0.25))
     XCTAssertEqual(φDivide(Complex(0, 1)), Complex(0.25, 0.25))
   }
+
+  func testZeroTangentVectorInitializer() {
+    XCTAssertEqual(Complex<Float>(-5, 5).zeroTangentVector, Complex(0, 0))
+  }
 }
 
 #endif