Remove previous hack for SR-5206

[itaiferber]

What's in this pull request?
Better resolves SR-5206 until the introduction of conditional conformance.

As a temporary workaround for SR-5206, certain Foundation types which had custom behavior in JSONEncoder and JSONDecoder were granted special knowledge of those types internally in order to preserve strategies on encode/decode.

This replaces that special knowledge with a more general-purpose fix that works for all types and all encoders/decoders.

Thanks to @norio-nomura for thinking of the approach on this.

[itaiferber]

@swift-ci Please test macOS

[itaiferber]

@swift-ci Please smoke test Linux

[swift-ci]

Build failed
Jenkins build - Swift Test OS X Platform
Git Commit - fbdcbee
Test requested by - @itaiferber

[itaiferber]

Mm. These fileprivate extensions are visible in accessibility checking, so are changing the errors that you'd get when you fail to inherit init(from:) from a superclass. Changing the parameter names to these initializers should prevent this.

[itaiferber]

@swift-ci Please test

[swift-ci]

Build failed
Jenkins build - Swift Test Linux Platform
Git Commit - fbdcbee
Test requested by - @itaiferber

[swift-ci]

Build failed
Jenkins build - Swift Test OS X Platform
Git Commit - fbdcbee
Test requested by - @itaiferber

[norio-nomura]

@itaiferber Can this change go to Swift 4.0.1?

[itaiferber]

@norio-nomura Unfortunately, the bar for inclusion into the next patch release is incredibly high, and this isn't considered a high enough priority item for inclusion. However, I believe I can get this pushed into the release after that.

[norio-nomura]

@itaiferber Thank you for your reply and information.🙏
I will look forward to that release.

[nerdsupremacist]

Do we know if Xcode 9.1 will ship with a toolchain that includes this fix?

[itaiferber]

@mathiasquintero It should, yes.

[jrose-apple]

Ah, that's not correct. Xcode 9.1 contains the targeted fixes that went to swift-4.0-branch after the 4.0 release.

[itaiferber]

@mathiasquintero @jrose-apple Sorry! Misread that as "Swift 4.1" instead of "Xcode 9.1" — skimmed too quick. Jordan has the answer.

[nerdsupremacist]

@jrose-apple @itaiferber but wasn't this later cherrypicked to the 4.0 branch?

[jrose-apple]

Heh, I missed that too, but that cherry-pick was very recent. We can't promise that that'll make it into Xcode 9.1 rather than a possible later point release before we're ready for Swift 4.1.

[nerdsupremacist]

Here's hoping for 4.0.2 rather than 4.1 ;)