This is much-needed functionality! One important implementation concern, though, is how to deal with backward deployment to older runtimes where tuples do not have any protocol conformances. There are couple of possibilities I can think of:
- The tuple-equatable conformance is only available when targeting an OS with Swift 5.2 or later. This wouldn't require any backward deployment hackery, but would likely require us to design and implement the general feature of how conditionally available protocol conformances work in the language.
- We use the backward compatibility library to backward-deploy this functionality into binaries targeting older OSes. This would allow users to take advantage of the functionality without restrictions*, but would require that we actually have enough hooks in the 5.0 and 5.1 runtimes that can be replaced to understand tuple conformances. In theory, it seems like this is possible, since
swift_conformsToProtocolought to be hookable, and we could provide copies of the runtime data structures for the conformance in the compatibility library, but it would require implementation effort to make sure this is actually possible.
(*And the hooks have limitations of their own—they only take effect when linked into the main binary, not dylibs, so dynamic libraries that want to be distributed for use with older Swift toolchains that don't have the compatibility library would still not be able to take advantage of the new functionality.)