[Test] Ensure that a class using the Observable macro builds a proper interface

Author: DougGregor

test/ModuleInterface/Observable.swift

@@ -0,0 +1,22 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-emit-module-interface(%t/Library.swiftinterface) %s -module-name Library -plugin-path %swift-host-lib-dir/plugins -disable-availability-checking
+// RUN: %target-swift-typecheck-module-from-interface(%t/Library.swiftinterface) -module-name Library -disable-availability-checking
+// RUN: %FileCheck %s < %t/Library.swiftinterface
+
+import _Observation
+
+// CHECK-NOT: @Observable
+// CHECK-NOT: @ObservationIgnored
+// CHECK-NOT: @ObservationTracked
+
+@Observable
+public class SomeClass {
+  public var field = 3
+  @ObservationIgnored public var ignored = 4
+}
+
+public func requiresObservable<T: Observable>(_: T) { }
+
+@inlinable func useObservable(sc: SomeClass) {
+  requiresObservable(sc)
+}