swift-mirror/test/AutoDiff/compiler_crashers_fixed/Inputs/tf1202-differentiability-witness-dead-function-elimination.swift

import _Differentiation

@inlinable
@differentiable(where T: Differentiable)
public func identity<T>(_ x: T) -> T { x }

public func foo<T: Differentiable>(_ f: @differentiable (T) -> T = identity) -> T {
  fatalError()
}