duplicates:

• SR-5220 Expose API to retrieve string representation of KeyPath

Issue Description:

Hi,

I've created a KVO wrapper in past that uses string based key path. Swift 4.0 introduces `KeyPath` class that provides a lot more type safety when working with key paths.

I can't seem to find any normal public API that exposes string keypath from KeyPath object.

The only API I could find is this `_kvcKeyPathString` weird looking property. It's underscored (so if I understand this correctly, even though it's public, it's intended for private use only) and doesn't contain any comments.
https://github.com/apple/swift/blob/main/stdlib/public/core/KeyPath.swift#L124

• Is there some official public API that provides me with string keypath that is compatible with this objc API
  ````

• (void)addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context;
  ```

• Can I count on `_kvcKeyPathString` being backwards compatible when using Swift 4.x versions?

• Is `_kvcKeyPathString` guaranteed to always return a value when working with objc classes only?

• Is some new API being planned that exposes this functionality?

• Why is the proper interpretation of `_`?