Reimplementation of Implicitly Unwrapped Optionals

A new implementation of implicitly unwrapped optionals (IUOs) landed in the Swift compiler earlier this year and is available to try in recent Swift snapshots. This completes the implementation of SE-0054 - Abolish ImplicitlyUnwrappedOptional Type. This is an important change to the language that eliminated some inconsistencies in type checking and clarified the rule of how these values are to be treated so that it is consistent and easy to reason about. For more information, see the motivation section of that proposal.

→ swift.org/blog/iuo/

Swift 4.1 reimplements implicitly unwrapped optionals (IUOs) so they act as regular optionals with a flag that allows automatic unwrapping when needed, rather than as a distinct type.

This change makes type checking more consistent, shows diagnostics as T? instead of T!, and fixes inconsistencies from earlier versions. Some code may need updates for compatibility, such as removing coercions to T! or handling cases like calling map on an IUO value.

The update reduces special cases in the compiler, which should lead to fewer bugs. Developers should prefer explicit unwrapping with if let or guard let, and use force-unwrapping only when safe.