The types of thrown errors will be restricted in some manner, because thrown errors are of existential type any Error (which is disallowed by the prior item).

https://github.com/kubamracek/swift-evolution/blob/embedded-swift/visions/embedded-swift.md#embedded-swift-language-subset (edited)

swift-evolution/visions/embedded-swift.md at embedded-swift · kubam...

This maintains proposals for changes and user-visible enhancements to the Swift Programming Language. - kubamracek/swift-evolution