[Test] Add test for SR-8666 / rdar://problem/43893945.

This test case crashed at runtime in Swift 4.2, but is now fixed. Add it
so that we don't regress.

Author: DougGregor

validation-test/Runtime/sr8666.swift

@@ -0,0 +1,57 @@
+// RUN: %target-run-simple-swift
+// REQUIRES: executable_test
+
+private protocol AnyChangeTracker {
+    var myVariable: Any? { get }
+    var isModified: Bool { get }
+}
+
+protocol ChangeTrackerType {
+    associatedtype Value
+    var originalValue: Value { get }
+    var value: Value { get set }
+    var isModified: Bool { get }
+}
+
+extension ChangeTrackerType where Value: Equatable {
+    var isModified: Bool {
+	return value != originalValue
+    }
+}
+
+struct ChangeTracker<T: Equatable>: ChangeTrackerType {
+    let originalValue: T
+    var value: T
+
+    init(value: T) {
+	originalValue = value
+	self.value = value
+    }
+}
+
+extension ChangeTracker: AnyChangeTracker where Value: OptionalType, Value.Wrapped: Equatable {
+    var myVariable: Any? {
+	return value
+    }
+}
+
+protocol OptionalType {
+    associatedtype Wrapped
+    var value: Wrapped? { get }
+}
+
+extension Optional: OptionalType {
+    var value: Wrapped? {
+	return self
+    }
+}
+
+let s: Any = ChangeTracker<String?>(value: "Foo")
+
+guard let s = s as? AnyChangeTracker else {
+  fatalError("Does not comply to AnyChangeTracker")
+}
+
+let myVar = String(describing: s.myVariable ?? "nil")
+assert(myVar == "Optional(\"Foo\")")
+