Test that reproduces compiler error

gistfile1.txt

// RUN: %target-run-simple-swift                                                                
// REQUIRES: executable_test                                                                    import StdlibUnittest
#if os(macOS)
import Darwin.C
#else
import Glibc
#endif

var OptionalTests = TestSuite("Optional")

typealias OptionalGrad = Optional<Float>.TangentVector
extension Optional: Differentiable where Wrapped: Differentiable {
    public struct TangentVector:  Differentiable,
                                  AdditiveArithmetic {
        public var value: Wrapped.TangentVector?
        public init(value: Wrapped.TangentVector?) {
            self.value = value
        }
        public static var zero: Self {
            get { Self(value: Wrapped.TangentVector.zero) }
        }

        public static func + (lhs: Self, rhs: Self) -> Self {
            return self.zero
        }

        public static func - (lhs: Self, rhs: Self) -> Self {
            return self.zero
        }
        public typealias TangentVector = Self
    }
    public mutating func move(along direction: TangentVector) -> () {
        return //self = self.zeroTangentVector //self?.move(along: direction.value!)                }

    public var zeroTangentVector: TangentVector {
        return TangentVector.zero //TangentVector(value: .zero)                                 
    }
}

OptionalTests.test("Optional Dense") {

    func optIdentity( _ f1: Optional<Float>) -> Optional<Float>{
        return f1
    }
    let backprop = pullback(at: 5, in: optIdentity)
    expectEqual(1,1)

}

runAllTests()