これです https://github.com/apple/swift-evolution/blob/main/proposals/0328-structural-opaque-result-types.md

swift-evolution/0328-structural-opaque-result-types.md at main · ap...

This maintains proposals for changes and user-visible enhancements to the Swift Programming Language. - swift-evolution/0328-structural-opaque-result-types.md at main · apple/swift-evolution