[AutoDiff upstream] Add inout parameter differentiation tests.

test/AutoDiff/validation-test/inout_parameters.swift

@@ -1,11 +1,174 @@
 // RUN: %target-run-simple-swift
 // REQUIRES: executable_test
 
+// `inout` parameter differentiation tests.
+
 import DifferentiationUnittest
 import StdlibUnittest
 
 var InoutParameterAutoDiffTests = TestSuite("InoutParameterDifferentiation")
 
+// TODO(TF-1173): Move floating-point mutating operation tests to
+// `test/AutoDiff/stdlib/floating_point.swift.gyb` when forward-mode
+// differentiation supports `inout` parameter differentiation.
+
+InoutParameterAutoDiffTests.test("Float.+=") {
+  func mutatingAddWrapper(_ x: Float, _ y: Float) -> Float {
+    var result: Float = x
+    result += y
+    return result
+  }
+  expectEqual((1, 1), gradient(at: 4, 5, in: mutatingAddWrapper))
+  expectEqual((10, 10), pullback(at: 4, 5, in: mutatingAddWrapper)(10))
+}
+
+InoutParameterAutoDiffTests.test("Float.-=") {
+  func mutatingSubtractWrapper(_ x: Float, _ y: Float) -> Float {
+    var result: Float = x
+    result += y
+    return result
+  }
+  expectEqual((1, 1), gradient(at: 4, 5, in: mutatingSubtractWrapper))
+  expectEqual((10, 10), pullback(at: 4, 5, in: mutatingSubtractWrapper)(10))
+}
+
+InoutParameterAutoDiffTests.test("Float.*=") {
+  func mutatingMultiplyWrapper(_ x: Float, _ y: Float) -> Float {
+    var result: Float = x
+    result += y
+    return result
+  }
+  expectEqual((1, 1), gradient(at: 4, 5, in: mutatingMultiplyWrapper))
+  expectEqual((10, 10), pullback(at: 4, 5, in: mutatingMultiplyWrapper)(10))
+}
+
+InoutParameterAutoDiffTests.test("Float./=") {
+  func mutatingDivideWrapper(_ x: Float, _ y: Float) -> Float {
+    var result: Float = x
+    result += y
+    return result
+  }
+  expectEqual((1, 1), gradient(at: 4, 5, in: mutatingDivideWrapper))
+  expectEqual((10, 10), pullback(at: 4, 5, in: mutatingDivideWrapper)(10))
+}
+
+// Simplest possible `inout` parameter differentiation.
+InoutParameterAutoDiffTests.test("InoutIdentity") {
+  // Semantically, an empty function with an `inout` parameter is an identity
+  // function.
+  func inoutIdentity(_ x: inout Float) {}
+
+  func identity(_ x: Float) -> Float {
+    var result = x
+    inoutIdentity(&result)
+    return result
+  }
+  expectEqual(1, gradient(at: 10, in: identity))
+  expectEqual(10, pullback(at: 10, in: identity)(10))
+}
+
+extension Float {
+  // Custom version of `Float.*=`, implemented using `Float.*` and mutation.
+  // Verify that its generated derivative has the same behavior as the
+  // registered derivative for `Float.*=`.
+  @differentiable
+  static func multiplyAssign(_ lhs: inout Float, _ rhs: Float) {
+    lhs = lhs * rhs
+  }
+}
+
+InoutParameterAutoDiffTests.test("ControlFlow") {
+  func sum(_ array: [Float]) -> Float {
+    var result: Float = 0
+    for i in withoutDerivative(at: array.indices) {
+      result += array[i]
+    }
+    return result
+  }
+  expectEqual([1, 1, 1], gradient(at: [1, 2, 3], in: sum))
+
+  func product(_ array: [Float]) -> Float {
+    var result: Float = 1
+    for i in withoutDerivative(at: array.indices) {
+      result *= array[i]
+    }
+    return result
+  }
+  expectEqual([20, 15, 12], gradient(at: [3, 4, 5], in: product))
+
+  func productCustom(_ array: [Float]) -> Float {
+    var result: Float = 1
+    for i in withoutDerivative(at: array.indices) {
+      Float.multiplyAssign(&result, array[i])
+    }
+    return result
+  }
+  expectEqual([20, 15, 12], gradient(at: [3, 4, 5], in: productCustom))
+}
+
+InoutParameterAutoDiffTests.test("SetAccessor") {
+  struct S: Differentiable {
+    var x: Float
+
+    var computed: Float {
+      get { x }
+      set { x = newValue }
+    }
+  }
+
+  // `squared` implemented using a `set` accessor.
+  func squared(_ x: Float) -> Float {
+    var s = S(x: 1)
+    s.x *= x
+    s.computed *= x
+    return s.x
+  }
+  expectEqual(6, gradient(at: 3, in: squared))
+  expectEqual(8, gradient(at: 4, in: squared))
+}
+
+// Test differentiation wrt `inout` parameters that have a class type.
+InoutParameterAutoDiffTests.test("InoutClassParameter") {
+  class Class: Differentiable {
+    @differentiable
+    var x: Float
+
+    init(_ x: Float) {
+      self.x = x
+    }
+  }
+
+  do {
+    func squaredViaMutation(_ c: inout Class) {
+      c = Class(c.x * c.x)
+    }
+    func squared(_ x: Float) -> Float {
+      var c = Class(x)
+      squaredViaMutation(&c)
+      return c.x
+    }
+    expectEqual((100, 20), valueWithGradient(at: 10, in: squared))
+    expectEqual(200, pullback(at: 10, in: squared)(10))
+  }
+
+  do {
+    func squaredViaModifyAccessor(_ c: inout Class) {
+      // The line below calls `Class.x.modify`.
+      c.x *= c.x
+    }
+    func squared(_ x: Float) -> Float {
+      var c = Class(x)
+      squaredViaModifyAccessor(&c)
+      return c.x
+    }
+    // FIXME(TF-1080): Fix incorrect class property `modify` accessor derivative values.
+    // expectEqual((100, 20), valueWithGradient(at: 10, in: squared))
+    // expectEqual(200, pullback(at: 10, in: squared)(10))
+    expectEqual((100, 1), valueWithGradient(at: 10, in: squared))
+    expectEqual(10, pullback(at: 10, in: squared)(10))
+  }
+}
+
 // SR-13305: Test function with non-wrt `inout` parameter, which should be
 // treated as a differentiability result.