Add regression test for https://bugs.swift.org/browse/SR-12531

Author: slavapestov

test/Generics/sr12531.swift

@@ -0,0 +1,23 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol AnyPropertyProtocol {
+    associatedtype Root = Any
+    associatedtype Value = Any
+    associatedtype KP: AnyKeyPath
+    var key: KP { get }
+    var value: Value { get }
+}
+
+protocol PartialPropertyProtocol: AnyPropertyProtocol
+where KP: PartialKeyPath<Root> {
+}
+
+protocol PropertyProtocol: PartialPropertyProtocol
+where KP: WritableKeyPath<Root, Value> {
+}
+
+extension Dictionary where Value: AnyPropertyProtocol {
+    subscript<R, V, P>(key: Key, path: WritableKeyPath<R, V>) -> P? where P: PropertyProtocol, P.Root == R, P.Value == V {
+        return self[key] as? P
+    }
+}