Compiler loses ability to infer Root of KeyPath when it is Optional and Sendable

[iwt-dahlborn]

Description

When you require a keypath to be Sendable using & Sendable and then make that Optional the compiler loses the ability to infer the root and the KeyPath always needs to be fully qualified to compile.

Reproduction

import Foundation

struct SendableTest: Sendable {
    var text: String = "Hello, World!"
}

struct Storage {
    let keyPath: KeyPath<SendableTest, String> & Sendable
    let optionalKeyPath: (KeyPath<SendableTest, String> & Sendable)?
    let nonSendableKeyPath: KeyPath<SendableTest, String>
    let nonSendableOptionalKeyPath: KeyPath<SendableTest, String>?
}

let storage = Storage(
    keyPath: \.text,
    // Cannot convert value of type 'KeyPath<_, String>'
    // to expected argument type 'KeyPath<SendableTest, String>'
    optionalKeyPath: \.text,
    nonSendableKeyPath: \.text,
    nonSendableOptionalKeyPath: \.text
)

Expected behavior

The Root should be inferable even if the sendable keypath is optional.

Environment

Tested with:
swift-driver version: 1.115 Apple Swift version 6.0 (swiftlang-6.0.0.9.10 clang-1600.0.26.2)
swift-driver version: 1.115 Apple Swift version 6.0 (swiftlang-6.0.0.9.11 clang-1600.0.26.2)

Additional information

No response

[stephencelis]

@xedin I believe you were aware of a similar issue with appending sendable key paths, so I wonder if this should be grouped in the same issue bucket.