[AutoDiff] Conform Array's and Optional's TangentVector to CustomReflectable

Resolves rdar://88542240.

Author: rxwei

stdlib/public/Differentiation/ArrayDifferentiation.swift

@@ -110,6 +110,12 @@ where Element: Differentiable {
   }
 }
 
+extension Array.DifferentiableView: CustomReflectable {
+  public var customMirror: Mirror {
+    return base.customMirror
+  }
+}
+
 /// Makes `Array.DifferentiableView` additive as the product space.
 ///
 /// Note that `Array.DifferentiableView([])` is the zero in the product spaces

stdlib/public/Differentiation/OptionalDifferentiation.swift

@@ -57,3 +57,9 @@ extension Optional: Differentiable where Wrapped: Differentiable {
     }
   }
 }
+
+extension Optional.TangentVector: CustomReflectable {
+  public var customMirror: Mirror {
+    return value.customMirror
+  }
+}

test/AutoDiff/stdlib/optional.swift

@@ -76,4 +76,15 @@ OptionalDifferentiationTests.test("Optional.TangentVector operations") {
   }
 }
 
+OptionalDifferentiationTests.test("Optional.TangentVector reflection") {
+  let tan = Optional<Float>.TangentVector(42)
+  let children = Array(Mirror(reflecting: tan).children)
+  expectEqual(1, children.count)
+  // We test `==` first because `as?` will flatten optionals.
+  expectTrue(type(of: children[0].value) == Float.self)
+  if let child = expectNotNil(children[0].value as? Float) {
+    expectEqual(42, child)
+  }
+}
+
 runAllTests()

test/AutoDiff/validation-test/array.swift

@@ -454,4 +454,15 @@ ArrayAutoDiffTests.test("Array.DifferentiableView.move") {
   expectEqual(z, [])
 }
 
+ArrayAutoDiffTests.test("Array.DifferentiableView reflection") {
+  let tan = [Float].DifferentiableView([41, 42])
+  let children = Array(Mirror(reflecting: tan).children)
+  expectEqual(2, children.count)
+  if let child1 = expectNotNil(children[0].value as? Float),
+     let child2 = expectNotNil(children[1].value as? Float) {
+    expectEqual(41, child1)
+    expectEqual(42, child2)
+  }
+}
+
 runAllTests()