[AutoDiff upstream] Test differentiable collection higher-order functions.

Test `Array.differentiableMap(_:)` and `Array.differentiableReduce(_:_:)`.

Author: dan-zheng

test/AutoDiff/stdlib/collection_higher_order_functions.swift

@@ -0,0 +1,33 @@
+// RUN: %target-run-simple-swift
+// REQUIRES: executable_test
+
+import _Differentiation
+import StdlibUnittest
+
+// Test differentiable collection higher order functions:
+// `differentiableMap(_:)` and `differentiableReduce(_:_:)`.
+
+var CollectionHOFTests = TestSuite("CollectionHigherOrderFunctions")
+
+let array: [Float] = [1, 2, 3, 4, 5]
+
+CollectionHOFTests.test("differentiableMap(_:)") {
+  func double(_ array: [Float]) -> [Float] {
+    array.differentiableMap { $0 * $0 }
+  }
+  expectEqual([], pullback(at: array, in: double)([]))
+  expectEqual([0], pullback(at: array, in: double)([0]))
+  expectEqual([2], pullback(at: array, in: double)([1]))
+  expectEqual([2, 4, 6, 8, 10], pullback(at: array, in: double)([1, 1, 1, 1, 1]))
+}
+
+CollectionHOFTests.test("differentiableReduce(_:_:)") {
+  func product(_ array: [Float]) -> Float {
+    array.differentiableReduce(1) { $0 * $1 }
+  }
+  expectEqual([1], gradient(at: [0], in: product))
+  expectEqual([1], gradient(at: [1], in: product))
+  expectEqual([120, 60, 40, 30, 24], gradient(at: array, in: product))
+}
+
+runAllTests()