[AutoDiff] Re-enable a reflection test.

rdar://75916878 is no longer reproducible.

Author: rxwei

test/AutoDiff/validation-test/function_type_metadata.swift

@@ -61,20 +61,18 @@ if #available(macOS 11.3, iOS 14.5, tvOS 14.5, watchOS 7.4, *) {
   }
 }
 
-// FIXME(rdar://75916878): Investigate why reflecting differentiable function
-// types that contain generic parameters will lose '@differentiable' annotation.
-// FunctionTypeMetadataTests.test("Reflect generic differentiable function type") {
-//   func testGeneric<T: Differentiable>(_ type: T.Type) {
-//     expectEqual(
-//       """
-//       @differentiable(reverse) (\(String(reflecting: type))) -> \
-//       \(String(reflecting: type))
-//       """,
-//       String(reflecting: (@differentiable(reverse) (T) -> T).self))
-//   }
-//   testGeneric(Double.self)
-//   testGeneric([Float].self)
-//   testGeneric(Float?.self)
-// }
+FunctionTypeMetadataTests.test("Reflect generic differentiable function type") {
+  func testGeneric<T: Differentiable>(_ type: T.Type) {
+    expectEqual(
+      """
+      @differentiable(reverse) (\(String(reflecting: type))) -> \
+      \(String(reflecting: type))
+      """,
+      String(reflecting: (@differentiable(reverse) (T) -> T).self))
+  }
+  testGeneric(Double.self)
+  testGeneric([Float].self)
+  testGeneric(Float?.self)
+}
 
 runAllTests()