var a: Float = 0
enum T {
  @differentiable
  func foo(x: Float) -> Float {
    a = x
    a = a + x
    return a
  }
}

/Users/rxwei/Desktop/diffattrcrash.swift:13:12: note: expression is not differentiable
    return a
           ^

protocol P {
  @differentiable
  func foo(x: Float) -> Float
}
var a: Float = 0
enum T : P {
  @differentiable
  func foo(x: Float) -> Float {
    a = x
    a = a + x
    return a
  }
}

➜  swift git:(tensorflow) ✗ $SWIFT_XCODE_BUILD_PATH/bin/swiftc ~/Desktop/diffattrcrash.swift
/Users/rxwei/Desktop/diffattrcrash.swift:13:12: note: expression is not differentiable
    return a
           ^
Assertion failed: (it != FunctionOrder.end() && "no order number for SIL function definition?"), function getFunctionOrder, file /Users/rxwei/Development/Swift/swift-tf-source/swift/lib/IRGen/IRGenModule.h, line 397.
Stack dump:
...